the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jun 2023
Surface Rupture Kinematics of the 2020 Mw6.6 Masbate (Philippines) Earthquake determined from Optical and Radar Data
Abstract. Optical correlation, interferometry, and field investigation of laterally offset features were undertaken to analyze the kinematics of the 2020 Mw6.6 Masbate earthquake. Coseismic displacement fields from optical correlation show a maximum displacement of 0.61 m corresponding to Mw6.64 geodetic moment magnitude and a lone asperity in Cataingan. Post-seismic deformation from interferometry highlights a maximum 0.14 m sinistral displacement equivalent to a Mw6.15 post-seismic moment magnitude, with coincident afterslip and coseismic slip distributions. The measured slip decreased towards the north, suggesting the presence of a slip barrier where stress can accumulate. Slip measurements and rupture length estimates characterize the Masbate segment as capable of producing unusually long ruptures with significant offsets despite the presence of creep. Post-seismic interferograms resolved the rupture far better than optical correlation, which was degraded due to high amplitude noise from sensor and environmental sources. Nevertheless, the resultant surface rupture morphology, as observed in optical correlation outputs and interferograms, demonstrated the presence of two transtensional basins in the north and south of the province, interlinked by a stepover of the respective Riedel shear zones. This review of the 2020 Mw6.6 Masbate earthquake reveals new insights into the seismic hazard and seismotectonic setting of Masbate province in Central Philippines.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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RC1: 'Comment on egusphere-2023-978', Gordon Woo, 02 Jul 2023
This is a welcome contribution to the seismological literature for the Philippines. The use of optical and radar data for the 2020 Mw6.6 Masbate earthquake has elucidated the surface rupture kinematics quite well. From a seismic hazard perspective, rupture dynamics are of considerable interest and importance. The data acquired for the 2020 Masbate earthquake provide a good basis for a computational stochastic model of the rupture dynamics. Recognizing the inherent stochasticity in the rupture process, what actually happened on 18 August 2020 is just one realization of what might have transpired (see Mignan and Woo, Seismological Research Letters, 2018).
Even if the authors do not undertake any stochastic modeling of the Masbate earthquake, it would be insightful if they could address in their paper some key outstanding questions. What is the probability that the earthquake might have been larger than Mw6.6? This is termed a downward counterfactual (see Woo, Frontiers in Earth Science, 2019). Suppose that a stochastic model of the Masbate earthquake were to be undertaken. Out of the possible scenarios generated, would any have approached Mw7? In other words, how surprising would such an outcome have been in August 2020? Response to these questions would elevate the practical significance of this interesting seismological paper.
Citation: https://doi.org/10.5194/egusphere-2023-978-RC1 -
AC1: 'Reply on RC1', Khelly Shan Sta. Rita, 14 Jul 2023
We would like to express our sincere gratitude for dedicating your time to reviewing our paper and recognizing the study's value in the Philippine context. We also extend our appreciation for introducing us to the concept of downward counterfactual thought and providing us with relevant references. Your comments and questions have been received with a positive outlook, and we are committed to addressing each of them in a thorough manner.
- What is the probability that the earthquake might have been larger than Mw6.6?
Response: Our preferred estimate of Mw6.64 indicates a marginally larger magnitude than the instrumental value. However, given the rupture length estimates, and the lack of data in the offshore sections, the downward counterfactual approach can suggest a larger geodetic moment magnitude. Estimating such can be undertaken upon the availability of detailed information at depth of the fault plane and the submerged portions. As such, we recognized and acknowledged the downward counterfactual thinking in-text between lines 511 to 516 as follows:
"The Mw estimates from the maximum and average displacements range between Mw6.50 to Mw6.66, and are similar to the instrumental 6.6 moment magnitude. All rupture length estimates returned high Mw values between Mw6.73 to Mw6.97 which agrees with the 2003 earthquake observations, wherein the fault rupture is longer than expected relative to the magnitude. However, a downward counterfactual approach (Mignan and Woo, 2018; Woo 2019) given the fault uncertainties at depth, suggests the probability of the larger Mw estimates. Nevertheless, comparing the individually calculated Mw values show that the moment magnitude estimates from maximum displacement provide the closest fit with instrumentally determined moment. The accepted Mw6.64 from the maximum displacement is equivalent to a seismic moment of 1.15x1019 N-m."
- Suppose that a stochastic model of the Masbate earthquake were to be undertaken, out of the possible scenarios generated, would any have approached Mw7? In other words, how surprising would such an outcome have been in August 2020?
Response: The highest Mw estimate derived from the rupture lengths is Mw6.97, suggesting a potential scenario that approaches Mw7.0. However, it is essential to consider the available historical seismicity spanning from 1800 to the present (SEASEE, 1985; Besana and Ando, 2005). The record indicates that significant seismic events produced by the Masbate segment have been moderate in magnitude and have not reached Mw7.0. This observation is likely attributed to the presence of interseismic creep, which enables a constant release of stress along the segment. The Masbate segment acts as a relay (Bacolcol, 2003), which transfers stress onto the other segments of the Philippine Fault. Consequently, we regard the Mw6.97 estimate as implausible in light of this information. Furthermore, there is a concurrence among the instrumental measurements of various monitoring agencies.
References:
Bacolcol, T. C. (2003). Etude geodesique de la faille Philippine dans les Visayas [These de doctorat, Paris 6]. https://www.theses.fr/2003PA066404
Besana, G. M., & Ando, M. (2005). The central Philippine Fault Zone: Location of great earthquakes, slow events and creep activity. Earth, Planets, and Space, 57(10), 987–994. https://doi.org/10.1186/BF03351877
SEASEE. (1985). Series on seismology Philippines: Vol. IV (Southeast Asia Association of Seismology and Earthquake Engineering). Government Printing Office.
Woo, G. (2019). Downward counterfactual search for extreme events. Frontiers in Earth Science, 7, 340. https://doi.org/10.3389/feart.2019.00340
Woo, G., & Mignan, A. (2018). Counterfactual analysis of runaway earthquakes. Seismological Research Letters, 89(6), 2266–2273. https://doi.org/10.1785/0220180138
Citation: https://doi.org/10.5194/egusphere-2023-978-AC1
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AC1: 'Reply on RC1', Khelly Shan Sta. Rita, 14 Jul 2023
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RC2: 'Comment on egusphere-2023-978', Anonymous Referee #2, 21 Aug 2023
Summary:
Rita et al present optical image correlation, InSAR time series, and field measurements of lateral co- and post-seismic deformation resulting from the Mw6.6 Masbate, Phillipines, earthquake in 2020. The manuscript is written well, easy to follow, and has good figures. The authors use standard techniques to report on critical parameters for a relatively understudied earthquake on the Philippine Fault, making this an important contribution to the body of work on the Phillipine Fault. There are no major issues with the work or analysis, but if the authors want to make the work more impactful, they could expand on the insights for seismic hazard that result from this study (as mentioned in the abstract and conclusion).
General Comments:
Field data – There are few details on the field offset measurements compared to the detailed description of the insar and optical processing, and no uncertainties are plotted. How were field measurements made? How are uncertainties determined? Please show uncertainties in the figures that include field data points. The field measurements follow the optical measurements extremely closely. This is surprising because field measurements tend to be more variable than optical measurements, and because the field measurements were made two years after the earthquake. The authors suggest that geomorphic change can explain this, but geomorphic change is likely to make field measurements more variable, not less. Furthermore, optical image correlation includes far-field distributed deformation where it exists, so it is expected that optical measurements might be larger than field measurements in places. What else can explain why the field measurements are so similar to the optical measurements? Finally, the table of the field measurements is not referenced in the main text.
Post-seismic deformation – The values calculated for post-seismic deformation (min/avg/max, Mw, etc) are all minimums because the post-seismic insar time series begins two days after the mainshock. This point is not made clear until late in the discussion. It should be made clear in the Data and Methods section, and every place one of those values is discussed it must be stated that it is a minimum, including in the abstract and conclusion.
Supplementary Material – The supplement is detailed and contains a lot of relevant information, but it is not referenced in the main text. Add references to the supplement where appropriate throughout the main text.
Diverging colorbars (red-white-blue) – Many figures use the diverging red-white-blue colorbar, which is a great choice for visualization of diverging data such as optical pixel correlation and unwrapped interferograms. However, many of these figures have asymmetric scales, and it’s unclear if 0 is white. The purpose of a diverging colorbar is to highlight a break in data (such as from negative to positive in pixel correlation results), and making that break within one side (not at white) negates that purpose. Make sure that all diverging colorbars have white = 0.
Line Comments:
Lines 3-4 – State average displacement in addition to max for optical and insar results.
Line 10 – Present tense “demonstrates”
Line 12 – Vague. What are the insights gained from this work for seismic hazard in Masbate?
Line 21 – If these preliminary assessments are not part of the work presented in this study, use past tense (showed or imaged).
Line 24 – Unclear what “alongside the reduced coherence due to the interval” means in this context.
Lines 26-27 – How different are the expected insar and seismic models for slip compared to the expected slip calculated from moment based on the Wells and Coppersmith equations? Since the data that those equations are based on are fairly cloudy (i.e., they do not exactly follow a line – see figures 10 and 11 in Wells and Coppersmith 1994), there is a sizeable range allowed for the slip for any given magnitude. Suggest add more details or delete this sentence.
Line 28 – Delete “the novel”. Optical image correlation methods are well established.
Line 32 – Delete “relatively”. State information about the 2003 event. Not all readers will be familiar with it. What magnitude? On what fault?
Lines 42-43 – Consider using consistent units for tectonic rates (i.e., both in cm or both in mm)
Line 48 – Delete “extant”
Figure 1 caption – Delete “Bathymetry data from GEBCO” in the description for (C). No bathymetry data are shown in panel C. Earthquake “epicenters” not earthquake “centers”.
Lines 70-77 – Consider if this detailed geologic information is necessary to include to understand the results or discussion.
Line 105 – “Ground motion” refers to ground shaking. “Surface displacement” is more appropriate here.
Line 109 – Delete “this study”
Line 110-111 – Not sure what is meant by this sentence. Consider deleting.
Line 145 – Replace “was’ with “were”
Line 158 – Which dimension is profile width and which is length? What is profile spacing? Is there space between the profiles or is every section of fault considered in a profile?
Line 165-166 – The moving mean may have minimal smoothing, but the size of the profile width and profile spacing both contribute to smoothing the overall slip distribution (e.g., see comparison of hand-measured vs optical-measured offsets for the same earthquakes in Figure 9 in https://doi.org/10.1002/esp.5294)
Table 2 – Consider replacing “granules” with “scenes”
Line 193 – Fieldwork occurred almost two years after the earthquake! There could be significant geomorphic or human modification of offset features in the interim, and it’s likely that many offsets and evidence of surface rupture have already been erased given the small magnitude of surface slip. This should be discussed somewhere.
Line 200 – Please provide more details on the field measurements. How were offset measurements made in the field? How is uncertainty characterized?
Line 209 – Peaks in what dataset?
Line 214 – Unclear what is meant by “days without earthquakes are closer to each other”. Please rephrase.
Table 3 – Consider if this table is necessary since the focal mechanisms are shown in Figure 4, and this information is easier to understand from the visual focal mechanisms than numbers in a table.
Line 221 – Delete “(3)”
Figure 4 – Great figure!
Table 4 – Consider if this table is necessary. Some of this information is stated in the text in section 4.1.1, and it could all be included there.
Section 4.2.1 – Add references to Figure 5 within this paragraph.
Figure 5 – Add north arrow and legend for line colors to panels A and B. For the color scales in panel A and B is white 0? If not, adjust color scale so that white is 0. Consider if there is a better color scale for panels C, D, and F because rainbow scales are difficult to perceive (e.g., https://www.scientificamerican.com/article/end-of-the-rainbow-new-map-scale-is-more-readable-by-people-who-are-color-blind/ and https://hess.copernicus.org/articles/25/4549/2021/hess-25-4549-2021.html ). Label east and west blocks in panels D, E, F, G. In explanation for E: “negative Y axis”
Figure 6 – The peak ~0.60 cm value seems anomalously high. Are there profiles adjacent to this profile (or could more profiles be analyzed) that can corroborate the peak value? Is the “overall trendline” the weighted moving mean discussed in the text or calculated a different way? Please explain.
Line 273 – “which may be due to noise” --> this signal could also be due to distributed deformation. It is well known that far-field signals capture distributed deformation over 10s-100s of meters from the primary fault trace. What is the evidence for or against noise versus distributed deformation?
Line 281 – From Figure 6, it looks like the peak profile measurement is 60.6 cm, not the maximum value of the moving mean, but this sentence says that’s the peak of the moving mean. Check values, and please give average and maximum of both the measurements and the moving mean. Also, state if uncertainties are 1 or 2 sigma.
Figure 7 – Explanation for B should include “LOS”; explanation for D should include “unwrapped”. In panel D, is white on the colorbar set to 0? Because it seems like most of the deformation is occurring on the dashed fault and not on the primary fault, but that could be an artifact of the visualization and colorbar. For diverging colorbars, always make sure that 0 is white.
Figure 8 – Check that the flight and look directions are correct. They do not seem perpendicular to each other as they should be for ascending and descending scenes. Add north arrows and line legend to panels A and B. Consider replacing the rainbow colorbar (see comment for Figure 5)
Figure 9 – What are the parameters for making the weighted moving mean? It seems like the moving mean should follow the two higher measurements at ~9 km more closely.
Line 345 – delete “down”
Figure 10 – Label panels D-H with the amount of displacement measured at each site.
Figure 11 – The datapoints are difficult to see. Plot in a contrasting dark color such as blue or black. Please show uncertainty error bars for each measurement. The points at the right end of the rupture could be represented with a distribution and error bars rather than the slip distribution going steeply up in a very short distance, which is unrealistic.
Line 371-372 – What figure is the “solid green line” in reference to?
Figure 12 – Field data points are difficult to see and lack uncertainty. Plot field data in darker color (blue or black) and with error bars.
Lines 425-428 – “Our measured…field offset distribution” --> Surprising! The opposite might be expected. Short wavelength variability in field measurements is generally much larger than variability from optical image correlation (https://doi.org/10.1002/esp.5294 and references therein), and geomorphic and human modification of offset features would likely exacerbate this issue. Are there any other possibilities that could explain the excellent agreement between the field and optical displacement measurements?
Line 427 – subject-verb agreement issue (ruptures…is)
Line 432 – Delete “merely” and “results”.
Lines 471-473 – The 2-day lag between the earthquake and the first interferogram used to calculate “post-seismic” slip means that all estimates of post-seismic slip are minimums. This point could be made earlier and repeated when the magnitude of post-seismic slip is discussed.
Line 482 – This is a strike-slip fault, not normal or thrust. Please add appropriate references for strike-slip post-seismic deformation.
Line 497 – Why is surface rupture length assumed to be symmetric around peak displacement? Are there studies to support this assumption? Though many strike-slip surface ruptures are roughly elliptical, they are often at least somewhat asymmetric (as is stated a few lines later)
Section 5.3.4 – It might be useful to also calculate Mo and Mw from rupture area, slip, and rigidity parameters (using a range for parameters that are unconstrained) to compare to the estimated Mw from the Wells and Coppersmith equations. They should be similar, but if they are very different that might indicate useful information about the rupture.
Line 522 – At least 18%! That’s a minimum given the 2-day window lacking in the post-seismic insar time series stack. State that it is a minimum.
Line 561 – Again, 18% and Mw 6.15 are minimums due to the 2-day delay. Must be stated that these are minimums.
Lines 566-568 – Again, the opposite might be expected…that weathering and erosion create more variability in field measurements. What are other reasons this could be true?
Lines 582-584 – The final sentence is a nice sentiment, but how exactly this study improves resilience to hazards hasn’t been addressed in the text, so it seems out of place in the conclusion. Consider either adding to the discussion how this work addresses hazard vulnerability or changing the final sentence.
Citation: https://doi.org/10.5194/egusphere-2023-978-RC2 -
AC3: 'Reply on RC2', Khelly Shan Sta. Rita, 29 Sep 2023
We would like to express our sincere gratitude to the reviewer for dedicating their time to reviewing our paper and providing us with valuable feedback. We appreciate the reviewer's recognition of the study's contribution to improving the understanding of the Philippine Fault, as well as their suggestion to expand on the insights for seismic hazards to further the study’s impact. We have taken the reviewer's comments and suggestions into careful consideration, and we are committed to addressing them in a thorough manner.
- Field data - There are few details on the field offset measurements compared to the detailed description of the InSAR and optical processing, and no uncertainties are plotted. How were field measurements made? How are uncertainties determined? Please show uncertainties in the figures that include field data points. The field measurements follow the optical measurements extremely closely. This is surprising because field measurements tend to be more variable than optical measurements, and because the field measurements were made two years after the earthquake. The authors suggest that geomorphic change can explain this, but geomorphic change is likely to make field measurements more variable, not less. Furthermore, optical image correlation includes far-field distributed deformation where it exists, so it is expected that optical measurements might be larger than field measurements in places. What else can explain why the field measurements are so similar to the optical measurements? Finally, the table of the field measurements is not referenced in the main text.
Response: We appreciate the reviewer’s comments. However, we stand by our findings and the most that we can do is to add the field measurement uncertainties in the field measurements from the inherent fuzziness of offset features coupled with probable human errors. We will be applying an arbitrary 5-10 cm uncertainty for the field measures to account for the previously mentioned factors. We cannot alter either our field measurements or the image correlation results. As for the variability of slip distribution, there are areas where variability is present and was discussed in the manuscript. In addition, we calculated a coefficient of variability (Cv) = 0.36 (Reitman et al, 2021), which shows that our data is well within the global range. Lastly, per reviewer #3 (Dr. Austin Elliot), our “remote sensing results accurately capture ground displacements and provide greater along-strike coverage. The similarity between the image correlation results and field measurements is a good point rather than a bad point, and emphasizes the strength and usefulness of the method.”
- Post-seismic deformation - The values calculated for post-seismic deformation (min/avg/max, Mw, etc) are all minimums because the post-seismic insar time series begins two days after the mainshock. This point is not made clear until late in the discussion. It should be made clear in the Data and Methods section, and every place one of those values is discussed it must be stated that it is a minimum, including in the abstract and conclusion.
Response: Thank you very much for pointing this out. We will make it clear early on that the values are minimum values of post-seismic deformation.
- Supplementary Material – The supplement is detailed and contains a lot of relevant information, but it is not referenced in the main text. Add references to the supplement where appropriate throughout the main text.
Response: Thank you. We will include the references to the supplements accordingly.
- Diverging colorbars (red-white-blue) – Many figures use the diverging red-white-blue colorbar, which is a great choice for visualization of diverging data such as optical pixel correlation and unwrapped interferograms. However, many of these figures have asymmetric scales, and it’s unclear if 0 is white. The purpose of a diverging colorbar is to highlight a break in data (such as from negative to positive in pixel correlation results), and making that break within one side (not at white) negates that purpose. Make sure that all diverging colorbars have white = 0.
Response: This is acknowledged. We will modify the color scales to a sequential one for the figures in-text.
For the specific comments, our responses are as follows:
Lines 3-4 – State average displacement in addition to max for optical and insar results.
Response: We will add this accordingly.Line 10 – Present tense “demonstrates”
Response: Thank you for pointing it out. We will modify it.Line 12 – Vague. What are the insights gained from this work for seismic hazard in Masbate?
Response: The findings and the methods we used for this study in Masbate leads us and future scientists to further analyze the geometry, kinematics, mechanics, and dynamics of fault movement in the area for improved seismic hazard mitigation. Specifically, the comprehensive slip data and observed unusually long rupture lengths along the fault measured through the optical correlation method agree with previous characterizations by Besana and Ando (2005), indicating significant movement despite the presence of creep. These sentences are also added in the conclusions.Line 21 – If these preliminary assessments are not part of the work presented in this study, use past tense (showed or imaged).
Response: Thank you for pointing it out. We will update accordingly.Line 24 – Unclear what “alongside the reduced coherence due to the interval” means in this context.
Response: This refers to the phase coherence of the interferograms produced by Tiongson and Ramirez (2022). The 12-day interval that they used will have less coherence compared to a shorter 6-day interval. We preferentially employed the shorter interval knowing the vegetative cover of the study area.Lines 26-27 – How different are the expected insar and seismic models for slip compared to the expected slip calculated from moment based on the Wells and Coppersmith equations? Since the data that those equations are based on are fairly cloudy (i.e., they do not exactly follow a line – see figures 10 and 11 in Wells and Coppersmith 1994), there is a sizeable range allowed for the slip for any given magnitude. Suggest add more details or delete this sentence.
Response: The three previous studies that we cited resulted in the following: 3-30 cm LOS displacement (PHIVOLCS, 2020), >15 cm LOS displacement (Tiongson and Ramirez, 2022), and >1.0 m peak ground displacement (Simborio et al, 2022). If we convert these values to moment magnitudes, the values would be Mw 5.6 to 6.4 (PHIVOLCS, 2020), Mw 6.2 (Tiongson and Ramirez, 2022), and Mw 6.8 (Simborio et al., 2022). Looking at the range of peak displacements from Wells and Coppersmith (1994), a Mw 6.6 earthquake should produce displacements between ~0.45 to ~1 m, considering a 95% confidence interval. This was our basis for saying that the previous studies have under- and overestimated results.Line 28 – Delete “the novel”. Optical image correlation methods are well established.
Response: Thank you. This will be removed.Line 32 – Delete “relatively”. State information about the 2003 event. Not all readers will be familiar with it. What magnitude? On what fault?
Response: This is noted. We will identify the magnitude and include the responsible fault segment.Lines 42-43 – Consider using consistent units for tectonic rates (i.e., both in cm or both in mm)
Response: Thank you. We will standardize the values.Line 48 – Delete “extant”
Response: This is acknowledged. Thank you.Figure 1 caption – Delete “Bathymetry data from GEBCO” in the description for (C). No bathymetry data are shown in panel C. Earthquake “epicenters” not earthquake “centers”.
Response: Thank you for pointing it out. This will be deleted.Lines 70-77 – Consider if this detailed geologic information is necessary to include to understand the results or discussion.
Response: Thank you for the suggestion. We briefly explored some possible lithological controls on the rupture process in the discussions, which makes these lines relevant.Line 105 – “Ground motion” refers to ground shaking. “Surface displacement” is more appropriate here.
Response: Thank you. We will update it.Line 109 – Delete “this study”
Response: Thank you. We will remove it.Line 110-111 – Not sure what is meant by this sentence. Consider deleting.
Response: Noted. It will be deleted.Line 145 – Replace “was’ with “were”
Response: Thank you. It will be updated.Line 158 – Which dimension is profile width and which is length? What is profile spacing? Is there space between the profiles or is every section of fault considered in a profile?
Response: The profile width is the wider dimension (~1.0 km) across the fault and vice versa. Every section of the fault is included in the profiling. A sample configuration is shown in Supplement S1.Line 165-166 – The moving mean may have minimal smoothing, but the size of the profile width and profile spacing both contribute to smoothing the overall slip distribution (e.g., see comparison of hand-measured vs optical-measured offsets for the same earthquakes in Figure 9 in https://doi.org/10.1002/esp.5294)
Response: Thank you for the clarification.Table 2 – Consider replacing “granules” with “scenes”
Response: Thank you. This will be updated.Line 193 – Fieldwork occurred almost two years after the earthquake! There could be significant geomorphic or human modification of offset features in the interim, and it’s likely that many offsets and evidence of surface rupture have already been erased given the small magnitude of surface slip. This should be discussed somewhere.
Response: Thank you for pointing it out. We will include a clarifying paragraph in this section as follows “Due to the two-year interval between the earthquake and fieldwork, geomorphic or human modifications of offset features may have occurred. Additionally, many offsets and evidence of surface rupture may have been erased given the small magnitude of surface slip. The significance of these changes can be assessed by correlating field data with other measurement techniques to determine the slip.”Line 200 – Please provide more details on the field measurements. How were offset measurements made in the field? How is uncertainty characterized?
Response: Only 1 measurement was in most locations. Due to this, the primary source of uncertainty would be human error and the inherent fuzziness of the features that we measured. From experience, we can apply an arbitrary 5-10 cm uncertainty for the field measurements.Line 209 – Peaks in what dataset?
Response: These refer to the peaks of Figures 3a and 3b. We will add “peaks of the frequency plots” for clarity.Line 214 – Unclear what is meant by “days without earthquakes are closer to each other”. Please rephrase.
Response: Thank you. We will replace it with “similar to the recent background activity from 2019 to early 2020”Table 3 – Consider if this table is necessary since the focal mechanisms are shown in Figure 4, and this information is easier to understand from the visual focal mechanisms than numbers in a table.
Response: Thank you. We agree that this can be unnecessary and can be removed.Line 221 – Delete “(3)”
Response: Thank you, this will be removed.Figure 4 – Great figure!
Response: Thank you. Much appreciated.Table 4 – Consider if this table is necessary. Some of this information is stated in the text in section 4.1.1, and it could all be included there.
Response: Thank you and we agree that this is repetitive.Section 4.2.1 – Add references to Figure 5 within this paragraph.
Response: We will add the appropriate reference.Figure 5 – Add north arrow and legend for line colors to panels A and B. For the color scales in panel A and B is white 0? If not, adjust color scale so that white is 0. Consider if there is a better color scale for panels C, D, and F because rainbow scales are difficult to perceive (e.g., https://www.scientificamerican.com/article/end-of-the-rainbow-new-map-scale-is-more-readable-by-people-who-are-color-blind/ and https://hess.copernicus.org/articles/25/4549/2021/hess-25-4549-2021.html). Label east and west blocks in panels D, E, F, G. In explanation for E: “negative Y axis”
Response: This is noted. We will replace it with a single-hue sequential color scale. The north arrow and scale bar will be moved to panel A to immediately assist the observer. East and west notations will be added to panels D, E, F, and G.Figure 6 – The peak ~0.60 cm value seems anomalously high. Are there profiles adjacent to this profile (or could more profiles be analyzed) that can corroborate the peak value? Is the “overall trendline” the weighted moving mean discussed in the text or calculated a different way? Please explain.
Response: The ~0.60 cm value wouldn’t be an anomaly. Multiple image pairs agreed to this value as indicated by the overlapping circles. The value was excluded by the weighted moving mean due to smoothing. And yes, the weighted moving mean refers to the “overall trendline”, we will replace the legend label for clarity.Line 273 – “which may be due to noise” --> this signal could also be due to distributed deformation. It is well known that far-field signals capture distributed deformation over 10s-100s of meters from the primary fault trace. What is the evidence for or against noise versus distributed deformation?
Response: Thank you for the explanation. It is challenging to discern between the two due to the dataset and land cover. However, the location of box 32 has a rugged topography, which may indicate the trend may be from noise rather than distributed deformation.Line 281 – From Figure 6, it looks like the peak profile measurement is 60.6 cm, not the maximum value of the moving mean, but this sentence says that’s the peak of the moving mean. Check values, and please give average and maximum of both the measurements and the moving mean. Also, state if uncertainties are 1 or 2 sigma.
Response: The values discussed in this paragraph (lines 276-282) refer to the individual data points and not the weighted moving mean. We preferred to take the peak value from the individual data points, while the average value is from the moving mean. If necessary, we can mention the maximum and average of both approaches.Figure 7 – Explanation for B should include “LOS”; explanation for D should include “unwrapped”. In panel D, is white on the colorbar set to 0? Because it seems like most of the deformation is occurring on the dashed fault and not on the primary fault, but that could be an artifact of the visualization and colorbar. For diverging colorbars, always make sure that 0 is white.
Response: This is duly noted. The captions are updated. The colors will be replaced with a sequential scale.Figure 8 – Check that the flight and look directions are correct. They do not seem perpendicular to each other as they should be for ascending and descending scenes. Add north arrows and line legend to panels A and B. Consider replacing the rainbow colorbar (see comment for Figure 5)
Response: Thank you for pointing it out as they may have been affected by the frame rotation, this will be corrected. As for the colors, they will be replaced with a sequential color scale. North arrows will be added to all panels.Figure 9 – What are the parameters for making the weighted moving mean? It seems like the moving mean should follow the two higher measurements at ~9 km more closely.
Response: The smoothing factor used here is 0.2.Line 345 – delete “down”
Response: This will be removed.Figure 10 – Label panels D-H with the amount of displacement measured at each site.
Response: Thank you. This will be added.Figure 11 – The datapoints are difficult to see. Plot in a contrasting dark color such as blue or black. Please show uncertainty error bars for each measurement. The points at the right end of the rupture could be represented with a distribution and error bars rather than the slip distribution going steeply up in a very short distance, which is unrealistic.
Response: Thank you. The colors will be changed. The values at the right end will be represented as suggested, and the error bars will be added similar to the previously noted items.Line 371-372 – What figure is the “solid green line” in reference to?
Response: Thank you for pointing it out. This was meant to be “gray” instead of green, and it will be corrected.Figure 12 – Field data points are difficult to see and lack uncertainty. Plot field data in darker color (blue or black) and with error bars.
Response: Thank you. The colors will be changed. The error bars will be added similar to the previously noted items.Lines 425-428 – “Our measured…field offset distribution” --> Surprising! The opposite might be expected. Short wavelength variability in field measurements is generally much larger than variability from optical image correlation (https://doi.org/10.1002/esp.5294 and references therein), and geomorphic and human modification of offset features would likely exacerbate this issue. Are there any other possibilities that could explain the excellent agreement between the field and optical displacement measurements?
Response: We appreciate the reviewer’s comments. However, we stand by our findings and we cannot alter either our field measurements or the image correlation results. As for the variability of slip distribution, there are areas where variability is present and was discussed in the manuscript. We calculated a coefficient of variability (Cv) = 0.36 (Reitman et al, 2021), which shows that our data is well within the global range. Lastly, per reviewer #3 (Dr. Austin Elliot), our “remote sensing results accurately capture ground displacements and provide greater along-strike coverage. The similarity between the image correlation results and field measurements is a good point rather than a bad point, and emphasizes the strength and usefulness of the method.”Line 427 – subject-verb agreement issue (ruptures…is)
Response: Thank you for noticing. We will correct it.Line 432 – Delete “merely” and “results”.
Response: Thank you. We will remove them.Lines 471-473 – The 2-day lag between the earthquake and the first interferogram used to calculate “post-seismic” slip means that all estimates of post-seismic slip are minimums. This point could be made earlier and repeated when the magnitude of post-seismic slip is discussed.
Response: This is duly noted. We will add “However, due to a two-day observation lag between the earthquake and the earliest Sentinel-1 pass, all our measurements herein are minimum values.” in the results section and mention it accordingly when appropriate.Line 482 – This is a strike-slip fault, not normal or thrust. Please add appropriate references for strike-slip post-seismic deformation.
Response: Thank you. We will add/replace them with more relevant citations.Line 497 – Why is surface rupture length assumed to be symmetric around peak displacement? Are there studies to support this assumption? Though many strike-slip surface ruptures are roughly elliptical, they are often at least somewhat asymmetric (as is stated a few lines later)
Response: Due to the lack of submarine data, one of the approaches was to interpolate by symmetry. Nevertheless, we made sure to emphasize that the assumption should be taken with a grain of salt and provided other estimations of the rupture length.Section 5.3.4 – It might be useful to also calculate Mo and Mw from rupture area, slip, and rigidity parameters (using a range for parameters that are unconstrained) to compare to the estimated Mw from the Wells and Coppersmith equations. They should be similar, but if they are very different that might indicate useful information about the rupture.
Response: Our earliest drafts included the other rupture parameters mentioned in this comment. However, since we can only access the government earthquake data to a limited extent, and that inversion is outside our scope, we opted to exclude the other rupture parameters in presenting the results. We can revisit this when we gain access to the raw earthquake data and upon the conduct of geodetic inversion in our next papers.Line 522 – At least 18%! That’s a minimum given the 2-day window lacking in the post-seismic InSAR time series stack. State that it is a minimum.
Response: Thank you for pointing it out. We will update this and other relevant phrases accordingly.Line 561 – Again, 18% and Mw 6.15 are minimums due to the 2-day delay. Must be stated that these are minimums.
Response: Thank you for pointing it out. We will update this and other relevant phrases accordingly.Lines 566-568 – Again, the opposite might be expected…that weathering and erosion create more variability in field measurements. What are other reasons this could be true?
Response: Thank you for the comment. However, these are our results and reviewer 3 noted the notion of our observations that our results accurately capture the field movements.Lines 582-584 – The final sentence is a nice sentiment, but how exactly this study improves resilience to hazards hasn’t been addressed in the text, so it seems out of place in the conclusion. Consider either adding to the discussion how this work addresses hazard vulnerability or changing the final sentence.
Response: According to scientific writing guidelines (Bouchrika, 2021; Sacred Heart University, n.d.), a good paper writes about the larger purpose of the study and puts the objectives of the work into context, which future investigators may pursue. We have added the following in the conclusions: “The findings and the methods we used for this study in Masbate lead us and future scientists to further analyze the geometry, kinematics, mechanics, and dynamics of fault movement in the area for improved seismic hazard mitigation. Specifically, the comprehensive slip data and observed unusually long rupture lengths along the fault measured through the optical correlation method agree with previous characterizations by Besana and Ando (2005), indicating significant movement despite the presence of creep.”References:
Besana, G. M., & Ando, M. (2005, Oct). The Central Philippine Fault Zone: Location of great earthquakes, slow events, and creep activity. Earth Planets Space, 57, 987–994. doi: 10.1186/BF03351877
Bouchrika, I. (2021). How to write a conclusion for a research paper: Effective tips and strategies. https://research.com/research/how-to-write-a-conclusion-for-a-research-paper
PHIVOLCS. (2020). 2020 Masbate Earthquake (Tech. Rep.). Retrieved from https://www.phivolcs.dost.gov.ph/index.php/2-uncategorised/10490 -2020-masbate-earthquake
Reitman, N.G., Mueller, K.J. & Tucker, G.E. (2022) Surface slip variability on strike-slip faults. Earth Surface Processes and Landforms, 47(4), 908–935. Available from: https://doi.org/10.1002/esp.5294
Sacred Heart University. (n.d.). Organizing academic research papers: 9. the conclusion. Research Guides. https://library.sacredheart.edu/c.php?g=29803&p=185935
Simborio, T., Yokoi, T., & Hara, T. (2022). Strong ground motion simulation of the 2020 Masbate, Philippines earthquake (Mw6.6) using Empirical Green’s Function method (Master’s thesis). Retrieved from https://iisee.kenken.go.jp/syndb/data/20230117f70f6a30.pdf (Synopsis of IISEE-GRIPS)
Tiongson, S. F., & Ramirez, R. (2022). Mapping of ground surface deformations and its associated damage using SAR interferometry: a case study of the 2020 Masbate earthquake. In (Vol. 347, p. 03014).
Citation: https://doi.org/10.5194/egusphere-2023-978-AC3
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AC3: 'Reply on RC2', Khelly Shan Sta. Rita, 29 Sep 2023
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RC3: 'Comment on egusphere-2023-978', Austin Elliott, 29 Aug 2023
Rita Et al. review comments:
The authors present remote sensing analyses of deformation from a significant earthquake in the Philippines in 2020, using both InSAR and optical image pixel-tracking to define ground displacements and fault offsets from both the earthquake and postseismic slip. The paper presents geometric parameters of the earthquake rupture’s surface trace, slip distribution, and aseismic slip along the fault, while it also provides some analytical and interpretive insight into the capabilities and value of each remote sensing technique in the challenging tropical forested terrain. With some minor edits to the text and the persistent clarification of a few repeated concepts, this paper will be a good contribution setting the stage for the application of these techniques in other Philippine earthquakes.
General/repeating comments:
The distinction between co-seismic and post-seismic deformation is a key aspect of this project, yet no truly purely coseismic imaging was possible. At best the remote sensing data include two days of afterslip (the period when that rate is presumably highest) and in general the measurements are made on data that include months of afterslip. Care should be taken to very explicitly distinguish these results; multiple time windows might provide an opportunity to define the afterslip rate decay and thus estimate how much may have accumulated in the first two days, thus providing a better estimate of coseismic slip. Otherwise what you have is just total slip and a minimum estimate of postseismic slip, and it should be presented as such.
In a couple places the authors refer to a presumed slip barrier, but it is unclear what exactly this interpretation involves or implies or is based on, and given the legitimacy of scientific debate over rupture barriers/boundaries, etc., these terms should only be introduced confidently with clear explanation. In this case it is unclear to me how a gradually tapering slip distribution implicates a “slip barrier” as opposed to simply being a manifestation of a slip profile across a finite crack. The authors have not explored the possibility that it results from an increase in the depth at which slip is concentrated, with less reaching the surface the farther north it goes. These ideas should be discussed if you wish to invoke a “slip barrier” model/conclusion; otherwise, as it is only mentioned briefly twice and is not what the paper hinges on, it may be better simply omitted.
I have noted in my line-by-line comments a few places where phrasing could be clarified and word choice reconsidered. I particularly urge the authors to use “left-lateral” throughout in place of sinistral.
The figures are very well drafted and nicely explanatory; the tables are appropriate, I have no edits to recommend there! Below are specific comments to help with revision:
Line-by-line comments:
3 - should the second instance of “displacement” in this sentence rather say “offset” as it is referring to the relative total displacement across the fault rather than the net displacement of either side, which is what the displacement fields show. Also can it be described with kinematics (i.e., left-lateral) as the postseismic offset is?
3 - rather than “in Cataingan”, the location of this asperity should be described relative to the fault rupture or epicenter; rather than “lone” one might instead say “single” as this result does not necessarily require that this represents the only asperity present on the fault.
4 - “left lateral” is less jargon-y than “sinistral” and may make for a clearer alternative phrase; repeating comment regarding using “offset” instead of “displacement” when describing the relative displacement across the fault
6 - does tapering slip require a slip barrier? Any rupture will inherently have slip taper toward the crack tips; consider rephrasing this to remove “barrier” or insert text that explains what about the tapering slip suggests a barrier rather than just a rupture tip
7 - the rupture length has not yet been mentioned—have you derived it from the remote sensing data? If so state it in an additional sentence in the abstract here to justify this statement.
14 - what magnitude was the strong aftershock? The specific magnitude is relevant to understand whether it would be likely to produce further ground deformation or was below that threshold.
17 - hypocenter, since the depth is listed
21 - add a word to modify “butterfly” for clarity, such as “butterfly-shaped”
22 - double-check the grammar of this sentence, perhaps a simple correction would be to move “descending track” to just before “line-of-sight” and add another “LOS” after ascending track or before “ground deformation”
25 - spectral analysis “of seismic waveforms”* should be specified for clarity, as remote sensing and displacement fields can also involve spectral analysis
27 - insert “compared to the slip predicted from global scaling relationships based on moment magnitude”
45 - consider using “left-lateral” instead of “sinistral”
47 - ASEAN should perhaps be spelled out on the first instance
49 - why convex? difficult way to describe a 2D fault trace, as it must be relative to one side… would be concave on the other side…. Perhaps just “curved” would be better? or curving
55 - segment “… of the PF…”
63 - left lateral
64 - “extensional” instead of “extension”
66 - this sentence should be rewritten for clarity; firstly, the relationship of a fault and the stress regime is perhaps more two-way causal. The fault is present, so what is the stress regime altered *from*? It’s unclear exactly how you mean the stress state is influenced by the fault, perhaps “the SSF influences the local stress regime due to multiple bends and varying orientation”. “in between the junction of the SSF and PF” doesn’t make sense as a local for either of the possible subjects it refers to, which is also unclear—the SSF itself or the seismic profiles?
84 - Survey, not Service; consider citation specifically of ANSS ComCat (https://earthquake.usgs.gov/data/comcat/)
85 - epicenters, rather than centers
90-91 - presumably these offsets were measured over a period that included the postseismic slip and so co- vs post-seismic slip magnitudes cannot be deconvolved; if accurate, this should be stated here to explain the subsequent conclusion that the excess slip/moment may have been the post-seismic component
95 - hairline fractures… along the fault? Specify whether these were clearly tectonic or possibly had some other origin
98 - rather than presumed to *cause* the subsequent earthquake, it would be more accurate in a current understanding to say “presumed to have reduced the time elapsed before the 2020 earthquake”
106 - would be good convention to specify the spatial resolution of the PlanetLabs images
145-147 - this sentence could be rewritten for a bit more clarity. Something like, “the rasters chosen to extract profiles and calculate across-fault offsets were selected based on…”
148 - specify when using “offset”: “pixel-offset” or “ground-offset”. Again I suggest using “displacement” to refer to absolute position change, and “offset” to refer to relative change across the fault (generally 2x displacement magnitudes). Thus the rasters you have are displacement rasters. They might also reasonably be called “pixel offset” rasters but note that offset of pixels from their pre-event positions is different from our domain-specific use of “offset” to refer to separation of features across the fault, which is generally double the value of displacement on either side and is a closer approximation of a different conceptual quantity: the fault slip.
155 - “profiling” hasn’t been methodologically defined yet here and may thus be an ambiguous term to use without that defintion. You may wish to define it in a prior sentence, or explicitly say, “taking fault-perpendicular profiles (transects) of displacement data in order to measure the net fault-parallel offset”
191 - presumably this means decorrelation in the coseismic interferogram due to higher magnitudes of deformation; you may wish to state that explicitly for clarity.
193 - across the ground rupture instead of “of” the ground rupture
224 - may be worth noting relative horizontal location uncertainties; I see USGS is 7km which may explain the discrepancy
257 - left lateral
279 - describe the distribution along strike more accurately. “followed by an increase” is a temporal term… quantify slip values with numbers and specify relative location along strike using geographical terms like northwest or southeast.
280 - not followed by, that’s a time term; “south of the 60 cm high, displacement declines southward toward the shoreline”
288 - left-lateral
341 - are “subparallel” and “at a low angle” meant to contrast? They sound synonymous; perhaps omit “and at a low angle” or quantify the range of observed strikes in addition to the mean
342 - “the northern terminus was observed” - does this mean the end of the crack tip? Because a measurement of 0.5 cm may be the last resolvable measurement of offset along the surface rupture but care should be taken to explain whether it simply provides a minimum northward constraint on where surface rupture is observed or whether the actual end of the crack is observed and no others can be discovered north of it
345 - remove “down”
350, 355 - “the rupture is continuous southward” is a more neutral description of its characteristics; “the rupture continued” would be how to describe its coseismic propagation, which presumably was northward from the hypocenter
351 - can omit “for”
379 - nothing you have measured is definitively coseismic, correct? In fact it is presumed all of these methods capture co- and post-seismic displacement without being able to fully discern them
383-384 - this sentence is meant to speifically apply to the postseismic interferograms, correct? Specify.
384 - rather than qualitatively say “low”, quantify the uncertainty
391 - “higher”* uncertainties; again, quantify
412 - inverse, rather than indirect
431-432 - I do not believe that after all this your null hypothesis would be that the remote sensing results are “merely arbitrary”. You would perhaps rather say that the agreement with field measurements “indicate that remote sensing results accurately capture ground displacements and provide greater along-strike coverage”
451-454 - this is a slightly unclear sentence, as phrased. Rephrase to make clearer inferences and conclusions based specifically on well articulated observations. Consider changing to the active voice to help explain what observations lead to which inferences. “mostly vertical rupture propagation” doesn’t make sense for a long strike-slip rupture; what does “a southwesterly component” mean or refer to? Is this sentence describing fault attitude? Subvertical with a slight southwest dip? The subject as written is “rupture propagation” which doesn’t make sense in this context.
458 - how does the decreasing slip trend “mark” or indicate the presence of a specific barrier? Declining slip is expected for any secular crack; in fact an anomalous barrier may be expected to make slip decline more abruptly rather than smoothly? This conclusion bears further exploration and/or explanation in your text. Such a gradual, low magnitude and low gradient decline in slip actually suggests an anomalously low stress/strain concentration toward the northern end of the rupture…
467 - The relationship described here seems inverted: those various processes are accommodated by postseismic deformation; postseismic deformation is caused by one or all of those processes.
468 - I do not believe you have presented a coseismic slip distribution. At best you capture the earthquake and the first two days of postseismic (presumably the fastest!), or, more realistically since coseismic profiles were only derived from optical images, the first month (!) of afterslip. You say as much in the subsequent sentences. This sentence warrants a more in depth and logical explanation or walk-through. First you must define how you assess the component of the measured slip which is coseismic versus postseismic; then explain how they relate (proportionally and along strike), and then explain how that means or suggests that postseismic slip is stress driven (mustn’t it be? Is this a novel conclusion?) Your subsequent paragraphs may begin to do this, but that means this sentence is preemptive.
501 - “slip distributions” rather than “profiles”
557 - “the peak coseismic offset is equivalent to a moment magnitude…” doesn’t make sense or is stated incorrectly. Based on scaling relationships the peak slip is consistent with that magnitude. Also rememnber that you have no strictly coseismic displacement measurements
Citation: https://doi.org/10.5194/egusphere-2023-978-RC3 -
AC2: 'Reply on RC3', Khelly Shan Sta. Rita, 29 Sep 2023
We would like to express our sincere gratitude to the reviewer for his favorable and thoughtful comments and suggestions. We appreciate the reviewer’s support for promulgating the techniques used in the study throughout the Philippines. We have taken the reviewer's comments and suggestions into careful consideration, and we are committed to addressing them in a thorough manner.
- The distinction between co-seismic and post-seismic deformation is a key aspect of this project, yet no truly purely coseismic imaging was possible. At best the remote sensing data include two days of afterslip (the period when that rate is presumably highest) and in general, the measurements are made on data that include months of afterslip. Care should be taken to very explicitly distinguish these results; multiple time windows might provide an opportunity to define the afterslip rate decay and thus estimate how much may have accumulated in the first two days, thus providing a better estimate of coseismic slip. Otherwise what you have is just total slip and a minimum estimate of postseismic slip, and it should be presented as such.
Response: Following the paper of Elliot et al (2020), we chose to address this comment by adding the words “may be inferred to represent the coseismic slip” whenever necessary in our manuscript. This addresses the lack of a “truly purely coseismic slip imaging”. For the second point, we will be emphasizing that the post-seismic measurements are minimum estimates, in accordance with this comment and to a similar point raised by reviewer #2.
- In a couple places, the authors refer to a presumed slip barrier, but it is unclear what exactly this interpretation involves or implies or is based on, and given the legitimacy of scientific debate over rupture barriers/boundaries, etc., these terms should only be introduced confidently with clear explanation. In this case, it is unclear to me how a gradually tapering slip distribution implicates a “slip barrier” as opposed to simply being a manifestation of a slip profile across a finite crack. The authors have not explored the possibility that it results from an increase in the depth at which slip is concentrated, with less reaching the surface the farther north it goes. These ideas should be discussed if you wish to invoke a “slip barrier” model/conclusion; otherwise, as it is only mentioned briefly twice and is not what the paper hinges on, it may be better simply omitted.
Response: Thank you and we agree that further research is needed to define this in the study area. In line with this, we will omit this point to avoid scientific inaccuracies.
For the specific comments, the following are our responses:
3 - should the second instance of “displacement” in this sentence rather say “offset” as it is referring to the relative total displacement across the fault rather than the net displacement of either side, which is what the displacement fields show. Also can it be described with kinematics (i.e., left-lateral) as the postseismic offset is?
Response: Thank you for the comment and noticing. We will replace the particular word with “offset”. We believe that adding the movement sense of the fault will add an important context in the abstract, and we will be adding it as well.3 - rather than “in Cataingan”, the location of this asperity should be described relative to the fault rupture or epicenter; rather than “lone” one might instead say “single” as this result does not necessarily require that this represents the only asperity present on the fault.
Response: Much appreciated. This will be modified.4 - “left lateral” is less jargon-y than “sinistral” and may make for a clearer alternative phrase; repeating comment regarding using “offset” instead of “displacement” when describing the relative displacement across the fault
Response: Thank you. We will update each mention of the word accordingly.6 - Does tapering slip require a slip barrier? Any rupture will inherently have slip taper toward the crack tips; consider rephrasing this to remove “barrier” or insert text that explains what about the tapering slip suggests a barrier rather than just a rupture tip
Response: Thank you for the clarifications. We will avoid invoking the slip barrier concept.7 - the rupture length has not yet been mentioned—have you derived it from the remote sensing data? If so, state it in an additional sentence in the abstract here to justify this statement.
Response: Thank you. We will include the parameter source in this line.14 - what magnitude was the strong aftershock? The specific magnitude is relevant to understand whether it would be likely to produce further ground deformation or was below that threshold.
Response: We will update and specify the magnitude.17 - hypocenter, since the depth is listed
Response: Thank you. We will correct it.21 - add a word to modify “butterfly” for clarity, such as “butterfly-shaped”
Response: This will be included.22 - double-check the grammar of this sentence, perhaps a simple correction would be to move “descending track” to just before “line-of-sight” and add another “LOS” after ascending track or before “ground deformation”
Response: Thank you for the suggestion. We will correct it.25 - spectral analysis “of seismic waveforms”* should be specified for clarity, as remote sensing and displacement fields can also involve spectral analysis
Response: Thank you for the clarification. We will add the phrase.27 - insert “compared to the slip predicted from global scaling relationships based on moment magnitude”
Response: Much appreciated. We will add this for clarity.45 - consider using “left-lateral” instead of “sinistral”
Response: All instances of “sinistral” have been modified.47 - ASEAN should perhaps be spelled out on the first instance
Response: This is noted. We will spell this out.49 - why convex? difficult way to describe a 2D fault trace, as it must be relative to one side… would be concave on the other side…. Perhaps just “curved” would be better? or curving
Response: Thank you for this. This notation is to indicate the direction of the curve since a simple “curve” inherently lacks direction. Also, the term is adapted from the cited author.55 - segment “… of the PF…”
Response: Much appreciated. We will include this for clarification.63 - left lateral
Response: All instances of “sinistral” have been modified for simplicity.64 - “extensional” instead of “extension”
Response: Thank you for this. It will be corrected.66 - this sentence should be rewritten for clarity; firstly, the relationship of a fault and the stress regime is perhaps more two-way causal. The fault is present, so what is the stress regime altered *from*? It’s unclear exactly how you mean the stress state is influenced by the fault, perhaps “the SSF influences the local stress regime due to multiple bends and varying orientation”. “in between the junction of the SSF and PF” doesn’t make sense as a local for either of the possible subjects it refers to, which is also unclear—the SSF itself or the seismic profiles?
Response: Much appreciated. We propose modifying it to “The SSF influences the local stress regime expressed by alternating extensive and compressive structures, as it interacts with the PF.”84 - Survey, not Service; consider citation specifically of ANSS ComCat (https://earthquake.usgs.gov/data/comcat/)
Response: Thank you for pointing it out. We will correct it and add the citation.85 - epicenters, rather than centers
Response: Thank you. This will be corrected.90-91 - presumably these offsets were measured over a period that included the postseismic slip and so co- vs post-seismic slip magnitudes cannot be deconvolved; if accurate, this should be stated here to explain the subsequent conclusion that the excess slip/moment may have been the post-seismic component
Response: The mentioned measurements and statements on excess slip/moment were from the cited authors. Per their papers, the field investigation began on 18 Feb. 2003, a mere 3 days after the 15 Feb. 2023 event.95 - hairline fractures… along the fault? Specify whether these were clearly tectonic or possibly had some other origin.
Response: The original authors suggested that these may indicate the presence of post-seismic slip. As for their location of occurrence, these fractures were located on an asphalt pavement along the southern terminus of the 2003 rupture. The possible origin of these were discussed in the succeeding sentences 96-97.98 - rather than presumed to *cause* the subsequent earthquake, it would be more accurate in a current understanding to say “presumed to have reduced the time elapsed before the 2020 earthquake”
Response: Thank you for this suggestion. We will modify this accordingly.106 - would be good convention to specify the spatial resolution of the PlanetLabs images.
Response: Thank you for this. We will add this here.145-147 - this sentence could be rewritten for a bit more clarity. Something like, “the rasters chosen to extract profiles and calculate across-fault offsets were selected based on…”
Response: Thank you. We will modify this.148 - specify when using “offset”: “pixel-offset” or “ground-offset”. Again I suggest using “displacement” to refer to absolute position change, and “offset” to refer to relative change across the fault (generally 2x displacement magnitudes). Thus the rasters you have are displacement rasters. They might also reasonably be called “pixel offset” rasters but note that offset of pixels from their pre-event positions is different from our domain-specific use of “offset” to refer to separation of features across the fault, which is generally double the value of displacement on either side and is a closer approximation of a different conceptual quantity: the fault slip.
Response: This is well noted. Thank you for the clarification. We will update the terminologies.155 - “profiling” hasn’t been methodologically defined yet here and may thus be an ambiguous term to use without that definition. You may wish to define it in a prior sentence, or explicitly say, “taking fault-perpendicular profiles (transects) of displacement data in order to measure the net fault-parallel offset”.
Response: Thank you. We will add an introductory sentence “Measurements were acquired by taking profiles across the fault using displacement data to determine the net fault-parallel offset.” to discuss profiling.191 - presumably this means decorrelation in the coseismic interferogram due to higher magnitudes of deformation; you may wish to state that explicitly for clarity.
Response: Thank you for this one. We will add this to clarify.193 - across the ground rupture instead of “of” the ground rupture
Response: Thank you for pointing it out. We will correct it.224 - may be worth noting relative horizontal location uncertainties; I see USGS is 7km which may explain the discrepancy
Response: Thank you. We have gone over the reference files again and the uncertainty ranges are unavailable. However, the discrepancies aren’t surprising knowing that PHIVOLCS would have a denser network of instruments for the study area.257 - left lateral
Response: Thank you. All instances of “sinistral” will be replaced.279 - describe the distribution along strike more accurately. “followed by an increase” is a temporal term… quantify slip values with numbers and specify relative location along strike using geographical terms like northwest or southeast.
Response: Thank you for this. We will modify this in the revisions.280 - not followed by, that’s a time term; “south of the 60 cm high, displacement declines southward toward the shoreline”
Response: Thank you for pointing it out. We will correct this.288 - left-lateral
Response: Thank you. All instances of “sinistral” will be replaced.341 - are “subparallel” and “at a low angle” meant to contrast? They sound synonymous; perhaps omit “and at a low angle” or quantify the range of observed strikes in addition to the mean
Response: Thank you for this. We agree that it can become redundant.342 - “the northern terminus was observed” - does this mean the end of the crack tip? Because a measurement of 0.5 cm may be the last resolvable measurement of offset along the surface rupture but care should be taken to explain whether it simply provides a minimum northward constraint on where surface rupture is observed or whether the actual end of the crack is observed and no others can be discovered north of it
Response: This is noted. We will modify it with “onshore surface rupture” and add “minimum northward constraint”.345 - remove “down”
Response: Thank you. We will take it out.350, 355 - “the rupture is continuous southward” is a more neutral description of its characteristics; “the rupture continued” would be how to describe its coseismic propagation, which presumably was northward from the hypocenter
Response: Thank you. We will improve this.351 - can omit “for”
Response: Revised.379 - nothing you have measured is definitively coseismic, correct? In fact it is presumed all of these methods capture co- and post-seismic displacement without being able to fully discern them
Response: Similar to the approach of Elliot et al (2020) in the Tajikistan paper, we will add “may be inferred to represent the coseismic slip” whenever necessary in our manuscript.383-384 - this sentence is meant to specifically apply to the postseismic interferograms, correct? Specify.
Response: Thank you for pointing it out. We will specify this.384 - rather than qualitatively say “low”, quantify the uncertainty
Response: This is noted. We will provide either the average uncertainty or the range.391 - “higher”* uncertainties; again, quantify
Response: This is noted. We will provide either the average uncertainty or the range.412 - inverse, rather than indirect
Response: Thank you. This is noted and will be revised.431-432 - I do not believe that after all this your null hypothesis would be that the remote sensing results are “merely arbitrary”. You would perhaps rather say that the agreement with field measurements “indicate that remote sensing results accurately capture ground displacements and provide greater along-strike coverage”
Response: Thank you very much for this one. We really appreciate the suggestion and will incorporate this phrase.451-454 - this is a slightly unclear sentence, as phrased. Rephrase to make clearer inferences and conclusions based specifically on well articulated observations. Consider changing to the active voice to help explain what observations lead to which inferences. “mostly vertical rupture propagation” doesn’t make sense for a long strike-slip rupture; what does “a southwesterly component” mean or refer to? Is this sentence describing fault attitude? Subvertical with a slight southwest dip? The subject as written is “rupture propagation” which doesn’t make sense in this context.
Response: Thank you for this one. We will remove the rupture propagation idea and instead focus on the slip variability aspect to equate to fault complexity.458 - how does the decreasing slip trend “mark” or indicate the presence of a specific barrier? Declining slip is expected for any secular crack; in fact an anomalous barrier may be expected to make slip decline more abruptly rather than smoothly? This conclusion bears further exploration and/or explanation in your text. Such a gradual, low magnitude and low gradient decline in slip actually suggests an anomalously low stress/strain concentration toward the northern end of the rupture…
Response: Thank you and we agree after further deliberation. This will no longer be included.467 - The relationship described here seems inverted: those various processes are accommodated by postseismic deformation; postseismic deformation is caused by one or all of those processes.
Response: Thank you for pointing it out. We will use “by” instead of “as” to correct the conceptual relationship.468 - I do not believe you have presented a coseismic slip distribution. At best you capture the earthquake and the first two days of postseismic (presumably the fastest!), or, more realistically since coseismic profiles were only derived from optical images, the first month (!) of afterslip. You say as much in the subsequent sentences. This sentence warrants a more in depth and logical explanation or walk-through. First you must define how you assess the component of the measured slip which is coseismic versus postseismic; then explain how they relate (proportionally and along strike), and then explain how that means or suggests that postseismic slip is stress driven (mustn’t it be? Is this a novel conclusion?) Your subsequent paragraphs may begin to do this, but that means this sentence is preemptive.
Response: Similar to the responses above, we will add that we infer our measurements to be coeval to the coseismic slip. In addition, its agreement with the field data effectively gives us a combined dataset for this particular period. The most we can add is the improved description of its relationship with the minimum post-seismic dataset, and we are grateful for the suggestions on describing such.501 - “slip distributions” rather than “profiles”
Response: Thank you. We will modify this accordingly.557 - “the peak coseismic offset is equivalent to a moment magnitude…” doesn’t make sense or is stated incorrectly. Based on scaling relationships the peak slip is consistent with that magnitude. Also remember that you have no strictly coseismic displacement measurements
Response: We can insert “the preferred (or inferred) coseismic offset” to emphasize our inference.References:
Elliott, A., Elliott, J., Hollingsworth, J., Kulikova, G., Parsons, B., & Walker, R. (2020, 02). Satellite imaging of the 2015 M7.2 earthquake in the Central Pamir, Tajikistan, elucidates a sequence of shallow strike-slip ruptures of the Sarez-Karakul fault. Geophysical Journal International, 221(3), 1696-1718. Retrieved from https://doi.org/10.1093/gji/ggaa090 doi: 10.1093/gji/ggaa090
Citation: https://doi.org/10.5194/egusphere-2023-978-AC2
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AC2: 'Reply on RC3', Khelly Shan Sta. Rita, 29 Sep 2023
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-978', Gordon Woo, 02 Jul 2023
This is a welcome contribution to the seismological literature for the Philippines. The use of optical and radar data for the 2020 Mw6.6 Masbate earthquake has elucidated the surface rupture kinematics quite well. From a seismic hazard perspective, rupture dynamics are of considerable interest and importance. The data acquired for the 2020 Masbate earthquake provide a good basis for a computational stochastic model of the rupture dynamics. Recognizing the inherent stochasticity in the rupture process, what actually happened on 18 August 2020 is just one realization of what might have transpired (see Mignan and Woo, Seismological Research Letters, 2018).
Even if the authors do not undertake any stochastic modeling of the Masbate earthquake, it would be insightful if they could address in their paper some key outstanding questions. What is the probability that the earthquake might have been larger than Mw6.6? This is termed a downward counterfactual (see Woo, Frontiers in Earth Science, 2019). Suppose that a stochastic model of the Masbate earthquake were to be undertaken. Out of the possible scenarios generated, would any have approached Mw7? In other words, how surprising would such an outcome have been in August 2020? Response to these questions would elevate the practical significance of this interesting seismological paper.
Citation: https://doi.org/10.5194/egusphere-2023-978-RC1 -
AC1: 'Reply on RC1', Khelly Shan Sta. Rita, 14 Jul 2023
We would like to express our sincere gratitude for dedicating your time to reviewing our paper and recognizing the study's value in the Philippine context. We also extend our appreciation for introducing us to the concept of downward counterfactual thought and providing us with relevant references. Your comments and questions have been received with a positive outlook, and we are committed to addressing each of them in a thorough manner.
- What is the probability that the earthquake might have been larger than Mw6.6?
Response: Our preferred estimate of Mw6.64 indicates a marginally larger magnitude than the instrumental value. However, given the rupture length estimates, and the lack of data in the offshore sections, the downward counterfactual approach can suggest a larger geodetic moment magnitude. Estimating such can be undertaken upon the availability of detailed information at depth of the fault plane and the submerged portions. As such, we recognized and acknowledged the downward counterfactual thinking in-text between lines 511 to 516 as follows:
"The Mw estimates from the maximum and average displacements range between Mw6.50 to Mw6.66, and are similar to the instrumental 6.6 moment magnitude. All rupture length estimates returned high Mw values between Mw6.73 to Mw6.97 which agrees with the 2003 earthquake observations, wherein the fault rupture is longer than expected relative to the magnitude. However, a downward counterfactual approach (Mignan and Woo, 2018; Woo 2019) given the fault uncertainties at depth, suggests the probability of the larger Mw estimates. Nevertheless, comparing the individually calculated Mw values show that the moment magnitude estimates from maximum displacement provide the closest fit with instrumentally determined moment. The accepted Mw6.64 from the maximum displacement is equivalent to a seismic moment of 1.15x1019 N-m."
- Suppose that a stochastic model of the Masbate earthquake were to be undertaken, out of the possible scenarios generated, would any have approached Mw7? In other words, how surprising would such an outcome have been in August 2020?
Response: The highest Mw estimate derived from the rupture lengths is Mw6.97, suggesting a potential scenario that approaches Mw7.0. However, it is essential to consider the available historical seismicity spanning from 1800 to the present (SEASEE, 1985; Besana and Ando, 2005). The record indicates that significant seismic events produced by the Masbate segment have been moderate in magnitude and have not reached Mw7.0. This observation is likely attributed to the presence of interseismic creep, which enables a constant release of stress along the segment. The Masbate segment acts as a relay (Bacolcol, 2003), which transfers stress onto the other segments of the Philippine Fault. Consequently, we regard the Mw6.97 estimate as implausible in light of this information. Furthermore, there is a concurrence among the instrumental measurements of various monitoring agencies.
References:
Bacolcol, T. C. (2003). Etude geodesique de la faille Philippine dans les Visayas [These de doctorat, Paris 6]. https://www.theses.fr/2003PA066404
Besana, G. M., & Ando, M. (2005). The central Philippine Fault Zone: Location of great earthquakes, slow events and creep activity. Earth, Planets, and Space, 57(10), 987–994. https://doi.org/10.1186/BF03351877
SEASEE. (1985). Series on seismology Philippines: Vol. IV (Southeast Asia Association of Seismology and Earthquake Engineering). Government Printing Office.
Woo, G. (2019). Downward counterfactual search for extreme events. Frontiers in Earth Science, 7, 340. https://doi.org/10.3389/feart.2019.00340
Woo, G., & Mignan, A. (2018). Counterfactual analysis of runaway earthquakes. Seismological Research Letters, 89(6), 2266–2273. https://doi.org/10.1785/0220180138
Citation: https://doi.org/10.5194/egusphere-2023-978-AC1
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AC1: 'Reply on RC1', Khelly Shan Sta. Rita, 14 Jul 2023
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RC2: 'Comment on egusphere-2023-978', Anonymous Referee #2, 21 Aug 2023
Summary:
Rita et al present optical image correlation, InSAR time series, and field measurements of lateral co- and post-seismic deformation resulting from the Mw6.6 Masbate, Phillipines, earthquake in 2020. The manuscript is written well, easy to follow, and has good figures. The authors use standard techniques to report on critical parameters for a relatively understudied earthquake on the Philippine Fault, making this an important contribution to the body of work on the Phillipine Fault. There are no major issues with the work or analysis, but if the authors want to make the work more impactful, they could expand on the insights for seismic hazard that result from this study (as mentioned in the abstract and conclusion).
General Comments:
Field data – There are few details on the field offset measurements compared to the detailed description of the insar and optical processing, and no uncertainties are plotted. How were field measurements made? How are uncertainties determined? Please show uncertainties in the figures that include field data points. The field measurements follow the optical measurements extremely closely. This is surprising because field measurements tend to be more variable than optical measurements, and because the field measurements were made two years after the earthquake. The authors suggest that geomorphic change can explain this, but geomorphic change is likely to make field measurements more variable, not less. Furthermore, optical image correlation includes far-field distributed deformation where it exists, so it is expected that optical measurements might be larger than field measurements in places. What else can explain why the field measurements are so similar to the optical measurements? Finally, the table of the field measurements is not referenced in the main text.
Post-seismic deformation – The values calculated for post-seismic deformation (min/avg/max, Mw, etc) are all minimums because the post-seismic insar time series begins two days after the mainshock. This point is not made clear until late in the discussion. It should be made clear in the Data and Methods section, and every place one of those values is discussed it must be stated that it is a minimum, including in the abstract and conclusion.
Supplementary Material – The supplement is detailed and contains a lot of relevant information, but it is not referenced in the main text. Add references to the supplement where appropriate throughout the main text.
Diverging colorbars (red-white-blue) – Many figures use the diverging red-white-blue colorbar, which is a great choice for visualization of diverging data such as optical pixel correlation and unwrapped interferograms. However, many of these figures have asymmetric scales, and it’s unclear if 0 is white. The purpose of a diverging colorbar is to highlight a break in data (such as from negative to positive in pixel correlation results), and making that break within one side (not at white) negates that purpose. Make sure that all diverging colorbars have white = 0.
Line Comments:
Lines 3-4 – State average displacement in addition to max for optical and insar results.
Line 10 – Present tense “demonstrates”
Line 12 – Vague. What are the insights gained from this work for seismic hazard in Masbate?
Line 21 – If these preliminary assessments are not part of the work presented in this study, use past tense (showed or imaged).
Line 24 – Unclear what “alongside the reduced coherence due to the interval” means in this context.
Lines 26-27 – How different are the expected insar and seismic models for slip compared to the expected slip calculated from moment based on the Wells and Coppersmith equations? Since the data that those equations are based on are fairly cloudy (i.e., they do not exactly follow a line – see figures 10 and 11 in Wells and Coppersmith 1994), there is a sizeable range allowed for the slip for any given magnitude. Suggest add more details or delete this sentence.
Line 28 – Delete “the novel”. Optical image correlation methods are well established.
Line 32 – Delete “relatively”. State information about the 2003 event. Not all readers will be familiar with it. What magnitude? On what fault?
Lines 42-43 – Consider using consistent units for tectonic rates (i.e., both in cm or both in mm)
Line 48 – Delete “extant”
Figure 1 caption – Delete “Bathymetry data from GEBCO” in the description for (C). No bathymetry data are shown in panel C. Earthquake “epicenters” not earthquake “centers”.
Lines 70-77 – Consider if this detailed geologic information is necessary to include to understand the results or discussion.
Line 105 – “Ground motion” refers to ground shaking. “Surface displacement” is more appropriate here.
Line 109 – Delete “this study”
Line 110-111 – Not sure what is meant by this sentence. Consider deleting.
Line 145 – Replace “was’ with “were”
Line 158 – Which dimension is profile width and which is length? What is profile spacing? Is there space between the profiles or is every section of fault considered in a profile?
Line 165-166 – The moving mean may have minimal smoothing, but the size of the profile width and profile spacing both contribute to smoothing the overall slip distribution (e.g., see comparison of hand-measured vs optical-measured offsets for the same earthquakes in Figure 9 in https://doi.org/10.1002/esp.5294)
Table 2 – Consider replacing “granules” with “scenes”
Line 193 – Fieldwork occurred almost two years after the earthquake! There could be significant geomorphic or human modification of offset features in the interim, and it’s likely that many offsets and evidence of surface rupture have already been erased given the small magnitude of surface slip. This should be discussed somewhere.
Line 200 – Please provide more details on the field measurements. How were offset measurements made in the field? How is uncertainty characterized?
Line 209 – Peaks in what dataset?
Line 214 – Unclear what is meant by “days without earthquakes are closer to each other”. Please rephrase.
Table 3 – Consider if this table is necessary since the focal mechanisms are shown in Figure 4, and this information is easier to understand from the visual focal mechanisms than numbers in a table.
Line 221 – Delete “(3)”
Figure 4 – Great figure!
Table 4 – Consider if this table is necessary. Some of this information is stated in the text in section 4.1.1, and it could all be included there.
Section 4.2.1 – Add references to Figure 5 within this paragraph.
Figure 5 – Add north arrow and legend for line colors to panels A and B. For the color scales in panel A and B is white 0? If not, adjust color scale so that white is 0. Consider if there is a better color scale for panels C, D, and F because rainbow scales are difficult to perceive (e.g., https://www.scientificamerican.com/article/end-of-the-rainbow-new-map-scale-is-more-readable-by-people-who-are-color-blind/ and https://hess.copernicus.org/articles/25/4549/2021/hess-25-4549-2021.html ). Label east and west blocks in panels D, E, F, G. In explanation for E: “negative Y axis”
Figure 6 – The peak ~0.60 cm value seems anomalously high. Are there profiles adjacent to this profile (or could more profiles be analyzed) that can corroborate the peak value? Is the “overall trendline” the weighted moving mean discussed in the text or calculated a different way? Please explain.
Line 273 – “which may be due to noise” --> this signal could also be due to distributed deformation. It is well known that far-field signals capture distributed deformation over 10s-100s of meters from the primary fault trace. What is the evidence for or against noise versus distributed deformation?
Line 281 – From Figure 6, it looks like the peak profile measurement is 60.6 cm, not the maximum value of the moving mean, but this sentence says that’s the peak of the moving mean. Check values, and please give average and maximum of both the measurements and the moving mean. Also, state if uncertainties are 1 or 2 sigma.
Figure 7 – Explanation for B should include “LOS”; explanation for D should include “unwrapped”. In panel D, is white on the colorbar set to 0? Because it seems like most of the deformation is occurring on the dashed fault and not on the primary fault, but that could be an artifact of the visualization and colorbar. For diverging colorbars, always make sure that 0 is white.
Figure 8 – Check that the flight and look directions are correct. They do not seem perpendicular to each other as they should be for ascending and descending scenes. Add north arrows and line legend to panels A and B. Consider replacing the rainbow colorbar (see comment for Figure 5)
Figure 9 – What are the parameters for making the weighted moving mean? It seems like the moving mean should follow the two higher measurements at ~9 km more closely.
Line 345 – delete “down”
Figure 10 – Label panels D-H with the amount of displacement measured at each site.
Figure 11 – The datapoints are difficult to see. Plot in a contrasting dark color such as blue or black. Please show uncertainty error bars for each measurement. The points at the right end of the rupture could be represented with a distribution and error bars rather than the slip distribution going steeply up in a very short distance, which is unrealistic.
Line 371-372 – What figure is the “solid green line” in reference to?
Figure 12 – Field data points are difficult to see and lack uncertainty. Plot field data in darker color (blue or black) and with error bars.
Lines 425-428 – “Our measured…field offset distribution” --> Surprising! The opposite might be expected. Short wavelength variability in field measurements is generally much larger than variability from optical image correlation (https://doi.org/10.1002/esp.5294 and references therein), and geomorphic and human modification of offset features would likely exacerbate this issue. Are there any other possibilities that could explain the excellent agreement between the field and optical displacement measurements?
Line 427 – subject-verb agreement issue (ruptures…is)
Line 432 – Delete “merely” and “results”.
Lines 471-473 – The 2-day lag between the earthquake and the first interferogram used to calculate “post-seismic” slip means that all estimates of post-seismic slip are minimums. This point could be made earlier and repeated when the magnitude of post-seismic slip is discussed.
Line 482 – This is a strike-slip fault, not normal or thrust. Please add appropriate references for strike-slip post-seismic deformation.
Line 497 – Why is surface rupture length assumed to be symmetric around peak displacement? Are there studies to support this assumption? Though many strike-slip surface ruptures are roughly elliptical, they are often at least somewhat asymmetric (as is stated a few lines later)
Section 5.3.4 – It might be useful to also calculate Mo and Mw from rupture area, slip, and rigidity parameters (using a range for parameters that are unconstrained) to compare to the estimated Mw from the Wells and Coppersmith equations. They should be similar, but if they are very different that might indicate useful information about the rupture.
Line 522 – At least 18%! That’s a minimum given the 2-day window lacking in the post-seismic insar time series stack. State that it is a minimum.
Line 561 – Again, 18% and Mw 6.15 are minimums due to the 2-day delay. Must be stated that these are minimums.
Lines 566-568 – Again, the opposite might be expected…that weathering and erosion create more variability in field measurements. What are other reasons this could be true?
Lines 582-584 – The final sentence is a nice sentiment, but how exactly this study improves resilience to hazards hasn’t been addressed in the text, so it seems out of place in the conclusion. Consider either adding to the discussion how this work addresses hazard vulnerability or changing the final sentence.
Citation: https://doi.org/10.5194/egusphere-2023-978-RC2 -
AC3: 'Reply on RC2', Khelly Shan Sta. Rita, 29 Sep 2023
We would like to express our sincere gratitude to the reviewer for dedicating their time to reviewing our paper and providing us with valuable feedback. We appreciate the reviewer's recognition of the study's contribution to improving the understanding of the Philippine Fault, as well as their suggestion to expand on the insights for seismic hazards to further the study’s impact. We have taken the reviewer's comments and suggestions into careful consideration, and we are committed to addressing them in a thorough manner.
- Field data - There are few details on the field offset measurements compared to the detailed description of the InSAR and optical processing, and no uncertainties are plotted. How were field measurements made? How are uncertainties determined? Please show uncertainties in the figures that include field data points. The field measurements follow the optical measurements extremely closely. This is surprising because field measurements tend to be more variable than optical measurements, and because the field measurements were made two years after the earthquake. The authors suggest that geomorphic change can explain this, but geomorphic change is likely to make field measurements more variable, not less. Furthermore, optical image correlation includes far-field distributed deformation where it exists, so it is expected that optical measurements might be larger than field measurements in places. What else can explain why the field measurements are so similar to the optical measurements? Finally, the table of the field measurements is not referenced in the main text.
Response: We appreciate the reviewer’s comments. However, we stand by our findings and the most that we can do is to add the field measurement uncertainties in the field measurements from the inherent fuzziness of offset features coupled with probable human errors. We will be applying an arbitrary 5-10 cm uncertainty for the field measures to account for the previously mentioned factors. We cannot alter either our field measurements or the image correlation results. As for the variability of slip distribution, there are areas where variability is present and was discussed in the manuscript. In addition, we calculated a coefficient of variability (Cv) = 0.36 (Reitman et al, 2021), which shows that our data is well within the global range. Lastly, per reviewer #3 (Dr. Austin Elliot), our “remote sensing results accurately capture ground displacements and provide greater along-strike coverage. The similarity between the image correlation results and field measurements is a good point rather than a bad point, and emphasizes the strength and usefulness of the method.”
- Post-seismic deformation - The values calculated for post-seismic deformation (min/avg/max, Mw, etc) are all minimums because the post-seismic insar time series begins two days after the mainshock. This point is not made clear until late in the discussion. It should be made clear in the Data and Methods section, and every place one of those values is discussed it must be stated that it is a minimum, including in the abstract and conclusion.
Response: Thank you very much for pointing this out. We will make it clear early on that the values are minimum values of post-seismic deformation.
- Supplementary Material – The supplement is detailed and contains a lot of relevant information, but it is not referenced in the main text. Add references to the supplement where appropriate throughout the main text.
Response: Thank you. We will include the references to the supplements accordingly.
- Diverging colorbars (red-white-blue) – Many figures use the diverging red-white-blue colorbar, which is a great choice for visualization of diverging data such as optical pixel correlation and unwrapped interferograms. However, many of these figures have asymmetric scales, and it’s unclear if 0 is white. The purpose of a diverging colorbar is to highlight a break in data (such as from negative to positive in pixel correlation results), and making that break within one side (not at white) negates that purpose. Make sure that all diverging colorbars have white = 0.
Response: This is acknowledged. We will modify the color scales to a sequential one for the figures in-text.
For the specific comments, our responses are as follows:
Lines 3-4 – State average displacement in addition to max for optical and insar results.
Response: We will add this accordingly.Line 10 – Present tense “demonstrates”
Response: Thank you for pointing it out. We will modify it.Line 12 – Vague. What are the insights gained from this work for seismic hazard in Masbate?
Response: The findings and the methods we used for this study in Masbate leads us and future scientists to further analyze the geometry, kinematics, mechanics, and dynamics of fault movement in the area for improved seismic hazard mitigation. Specifically, the comprehensive slip data and observed unusually long rupture lengths along the fault measured through the optical correlation method agree with previous characterizations by Besana and Ando (2005), indicating significant movement despite the presence of creep. These sentences are also added in the conclusions.Line 21 – If these preliminary assessments are not part of the work presented in this study, use past tense (showed or imaged).
Response: Thank you for pointing it out. We will update accordingly.Line 24 – Unclear what “alongside the reduced coherence due to the interval” means in this context.
Response: This refers to the phase coherence of the interferograms produced by Tiongson and Ramirez (2022). The 12-day interval that they used will have less coherence compared to a shorter 6-day interval. We preferentially employed the shorter interval knowing the vegetative cover of the study area.Lines 26-27 – How different are the expected insar and seismic models for slip compared to the expected slip calculated from moment based on the Wells and Coppersmith equations? Since the data that those equations are based on are fairly cloudy (i.e., they do not exactly follow a line – see figures 10 and 11 in Wells and Coppersmith 1994), there is a sizeable range allowed for the slip for any given magnitude. Suggest add more details or delete this sentence.
Response: The three previous studies that we cited resulted in the following: 3-30 cm LOS displacement (PHIVOLCS, 2020), >15 cm LOS displacement (Tiongson and Ramirez, 2022), and >1.0 m peak ground displacement (Simborio et al, 2022). If we convert these values to moment magnitudes, the values would be Mw 5.6 to 6.4 (PHIVOLCS, 2020), Mw 6.2 (Tiongson and Ramirez, 2022), and Mw 6.8 (Simborio et al., 2022). Looking at the range of peak displacements from Wells and Coppersmith (1994), a Mw 6.6 earthquake should produce displacements between ~0.45 to ~1 m, considering a 95% confidence interval. This was our basis for saying that the previous studies have under- and overestimated results.Line 28 – Delete “the novel”. Optical image correlation methods are well established.
Response: Thank you. This will be removed.Line 32 – Delete “relatively”. State information about the 2003 event. Not all readers will be familiar with it. What magnitude? On what fault?
Response: This is noted. We will identify the magnitude and include the responsible fault segment.Lines 42-43 – Consider using consistent units for tectonic rates (i.e., both in cm or both in mm)
Response: Thank you. We will standardize the values.Line 48 – Delete “extant”
Response: This is acknowledged. Thank you.Figure 1 caption – Delete “Bathymetry data from GEBCO” in the description for (C). No bathymetry data are shown in panel C. Earthquake “epicenters” not earthquake “centers”.
Response: Thank you for pointing it out. This will be deleted.Lines 70-77 – Consider if this detailed geologic information is necessary to include to understand the results or discussion.
Response: Thank you for the suggestion. We briefly explored some possible lithological controls on the rupture process in the discussions, which makes these lines relevant.Line 105 – “Ground motion” refers to ground shaking. “Surface displacement” is more appropriate here.
Response: Thank you. We will update it.Line 109 – Delete “this study”
Response: Thank you. We will remove it.Line 110-111 – Not sure what is meant by this sentence. Consider deleting.
Response: Noted. It will be deleted.Line 145 – Replace “was’ with “were”
Response: Thank you. It will be updated.Line 158 – Which dimension is profile width and which is length? What is profile spacing? Is there space between the profiles or is every section of fault considered in a profile?
Response: The profile width is the wider dimension (~1.0 km) across the fault and vice versa. Every section of the fault is included in the profiling. A sample configuration is shown in Supplement S1.Line 165-166 – The moving mean may have minimal smoothing, but the size of the profile width and profile spacing both contribute to smoothing the overall slip distribution (e.g., see comparison of hand-measured vs optical-measured offsets for the same earthquakes in Figure 9 in https://doi.org/10.1002/esp.5294)
Response: Thank you for the clarification.Table 2 – Consider replacing “granules” with “scenes”
Response: Thank you. This will be updated.Line 193 – Fieldwork occurred almost two years after the earthquake! There could be significant geomorphic or human modification of offset features in the interim, and it’s likely that many offsets and evidence of surface rupture have already been erased given the small magnitude of surface slip. This should be discussed somewhere.
Response: Thank you for pointing it out. We will include a clarifying paragraph in this section as follows “Due to the two-year interval between the earthquake and fieldwork, geomorphic or human modifications of offset features may have occurred. Additionally, many offsets and evidence of surface rupture may have been erased given the small magnitude of surface slip. The significance of these changes can be assessed by correlating field data with other measurement techniques to determine the slip.”Line 200 – Please provide more details on the field measurements. How were offset measurements made in the field? How is uncertainty characterized?
Response: Only 1 measurement was in most locations. Due to this, the primary source of uncertainty would be human error and the inherent fuzziness of the features that we measured. From experience, we can apply an arbitrary 5-10 cm uncertainty for the field measurements.Line 209 – Peaks in what dataset?
Response: These refer to the peaks of Figures 3a and 3b. We will add “peaks of the frequency plots” for clarity.Line 214 – Unclear what is meant by “days without earthquakes are closer to each other”. Please rephrase.
Response: Thank you. We will replace it with “similar to the recent background activity from 2019 to early 2020”Table 3 – Consider if this table is necessary since the focal mechanisms are shown in Figure 4, and this information is easier to understand from the visual focal mechanisms than numbers in a table.
Response: Thank you. We agree that this can be unnecessary and can be removed.Line 221 – Delete “(3)”
Response: Thank you, this will be removed.Figure 4 – Great figure!
Response: Thank you. Much appreciated.Table 4 – Consider if this table is necessary. Some of this information is stated in the text in section 4.1.1, and it could all be included there.
Response: Thank you and we agree that this is repetitive.Section 4.2.1 – Add references to Figure 5 within this paragraph.
Response: We will add the appropriate reference.Figure 5 – Add north arrow and legend for line colors to panels A and B. For the color scales in panel A and B is white 0? If not, adjust color scale so that white is 0. Consider if there is a better color scale for panels C, D, and F because rainbow scales are difficult to perceive (e.g., https://www.scientificamerican.com/article/end-of-the-rainbow-new-map-scale-is-more-readable-by-people-who-are-color-blind/ and https://hess.copernicus.org/articles/25/4549/2021/hess-25-4549-2021.html). Label east and west blocks in panels D, E, F, G. In explanation for E: “negative Y axis”
Response: This is noted. We will replace it with a single-hue sequential color scale. The north arrow and scale bar will be moved to panel A to immediately assist the observer. East and west notations will be added to panels D, E, F, and G.Figure 6 – The peak ~0.60 cm value seems anomalously high. Are there profiles adjacent to this profile (or could more profiles be analyzed) that can corroborate the peak value? Is the “overall trendline” the weighted moving mean discussed in the text or calculated a different way? Please explain.
Response: The ~0.60 cm value wouldn’t be an anomaly. Multiple image pairs agreed to this value as indicated by the overlapping circles. The value was excluded by the weighted moving mean due to smoothing. And yes, the weighted moving mean refers to the “overall trendline”, we will replace the legend label for clarity.Line 273 – “which may be due to noise” --> this signal could also be due to distributed deformation. It is well known that far-field signals capture distributed deformation over 10s-100s of meters from the primary fault trace. What is the evidence for or against noise versus distributed deformation?
Response: Thank you for the explanation. It is challenging to discern between the two due to the dataset and land cover. However, the location of box 32 has a rugged topography, which may indicate the trend may be from noise rather than distributed deformation.Line 281 – From Figure 6, it looks like the peak profile measurement is 60.6 cm, not the maximum value of the moving mean, but this sentence says that’s the peak of the moving mean. Check values, and please give average and maximum of both the measurements and the moving mean. Also, state if uncertainties are 1 or 2 sigma.
Response: The values discussed in this paragraph (lines 276-282) refer to the individual data points and not the weighted moving mean. We preferred to take the peak value from the individual data points, while the average value is from the moving mean. If necessary, we can mention the maximum and average of both approaches.Figure 7 – Explanation for B should include “LOS”; explanation for D should include “unwrapped”. In panel D, is white on the colorbar set to 0? Because it seems like most of the deformation is occurring on the dashed fault and not on the primary fault, but that could be an artifact of the visualization and colorbar. For diverging colorbars, always make sure that 0 is white.
Response: This is duly noted. The captions are updated. The colors will be replaced with a sequential scale.Figure 8 – Check that the flight and look directions are correct. They do not seem perpendicular to each other as they should be for ascending and descending scenes. Add north arrows and line legend to panels A and B. Consider replacing the rainbow colorbar (see comment for Figure 5)
Response: Thank you for pointing it out as they may have been affected by the frame rotation, this will be corrected. As for the colors, they will be replaced with a sequential color scale. North arrows will be added to all panels.Figure 9 – What are the parameters for making the weighted moving mean? It seems like the moving mean should follow the two higher measurements at ~9 km more closely.
Response: The smoothing factor used here is 0.2.Line 345 – delete “down”
Response: This will be removed.Figure 10 – Label panels D-H with the amount of displacement measured at each site.
Response: Thank you. This will be added.Figure 11 – The datapoints are difficult to see. Plot in a contrasting dark color such as blue or black. Please show uncertainty error bars for each measurement. The points at the right end of the rupture could be represented with a distribution and error bars rather than the slip distribution going steeply up in a very short distance, which is unrealistic.
Response: Thank you. The colors will be changed. The values at the right end will be represented as suggested, and the error bars will be added similar to the previously noted items.Line 371-372 – What figure is the “solid green line” in reference to?
Response: Thank you for pointing it out. This was meant to be “gray” instead of green, and it will be corrected.Figure 12 – Field data points are difficult to see and lack uncertainty. Plot field data in darker color (blue or black) and with error bars.
Response: Thank you. The colors will be changed. The error bars will be added similar to the previously noted items.Lines 425-428 – “Our measured…field offset distribution” --> Surprising! The opposite might be expected. Short wavelength variability in field measurements is generally much larger than variability from optical image correlation (https://doi.org/10.1002/esp.5294 and references therein), and geomorphic and human modification of offset features would likely exacerbate this issue. Are there any other possibilities that could explain the excellent agreement between the field and optical displacement measurements?
Response: We appreciate the reviewer’s comments. However, we stand by our findings and we cannot alter either our field measurements or the image correlation results. As for the variability of slip distribution, there are areas where variability is present and was discussed in the manuscript. We calculated a coefficient of variability (Cv) = 0.36 (Reitman et al, 2021), which shows that our data is well within the global range. Lastly, per reviewer #3 (Dr. Austin Elliot), our “remote sensing results accurately capture ground displacements and provide greater along-strike coverage. The similarity between the image correlation results and field measurements is a good point rather than a bad point, and emphasizes the strength and usefulness of the method.”Line 427 – subject-verb agreement issue (ruptures…is)
Response: Thank you for noticing. We will correct it.Line 432 – Delete “merely” and “results”.
Response: Thank you. We will remove them.Lines 471-473 – The 2-day lag between the earthquake and the first interferogram used to calculate “post-seismic” slip means that all estimates of post-seismic slip are minimums. This point could be made earlier and repeated when the magnitude of post-seismic slip is discussed.
Response: This is duly noted. We will add “However, due to a two-day observation lag between the earthquake and the earliest Sentinel-1 pass, all our measurements herein are minimum values.” in the results section and mention it accordingly when appropriate.Line 482 – This is a strike-slip fault, not normal or thrust. Please add appropriate references for strike-slip post-seismic deformation.
Response: Thank you. We will add/replace them with more relevant citations.Line 497 – Why is surface rupture length assumed to be symmetric around peak displacement? Are there studies to support this assumption? Though many strike-slip surface ruptures are roughly elliptical, they are often at least somewhat asymmetric (as is stated a few lines later)
Response: Due to the lack of submarine data, one of the approaches was to interpolate by symmetry. Nevertheless, we made sure to emphasize that the assumption should be taken with a grain of salt and provided other estimations of the rupture length.Section 5.3.4 – It might be useful to also calculate Mo and Mw from rupture area, slip, and rigidity parameters (using a range for parameters that are unconstrained) to compare to the estimated Mw from the Wells and Coppersmith equations. They should be similar, but if they are very different that might indicate useful information about the rupture.
Response: Our earliest drafts included the other rupture parameters mentioned in this comment. However, since we can only access the government earthquake data to a limited extent, and that inversion is outside our scope, we opted to exclude the other rupture parameters in presenting the results. We can revisit this when we gain access to the raw earthquake data and upon the conduct of geodetic inversion in our next papers.Line 522 – At least 18%! That’s a minimum given the 2-day window lacking in the post-seismic InSAR time series stack. State that it is a minimum.
Response: Thank you for pointing it out. We will update this and other relevant phrases accordingly.Line 561 – Again, 18% and Mw 6.15 are minimums due to the 2-day delay. Must be stated that these are minimums.
Response: Thank you for pointing it out. We will update this and other relevant phrases accordingly.Lines 566-568 – Again, the opposite might be expected…that weathering and erosion create more variability in field measurements. What are other reasons this could be true?
Response: Thank you for the comment. However, these are our results and reviewer 3 noted the notion of our observations that our results accurately capture the field movements.Lines 582-584 – The final sentence is a nice sentiment, but how exactly this study improves resilience to hazards hasn’t been addressed in the text, so it seems out of place in the conclusion. Consider either adding to the discussion how this work addresses hazard vulnerability or changing the final sentence.
Response: According to scientific writing guidelines (Bouchrika, 2021; Sacred Heart University, n.d.), a good paper writes about the larger purpose of the study and puts the objectives of the work into context, which future investigators may pursue. We have added the following in the conclusions: “The findings and the methods we used for this study in Masbate lead us and future scientists to further analyze the geometry, kinematics, mechanics, and dynamics of fault movement in the area for improved seismic hazard mitigation. Specifically, the comprehensive slip data and observed unusually long rupture lengths along the fault measured through the optical correlation method agree with previous characterizations by Besana and Ando (2005), indicating significant movement despite the presence of creep.”References:
Besana, G. M., & Ando, M. (2005, Oct). The Central Philippine Fault Zone: Location of great earthquakes, slow events, and creep activity. Earth Planets Space, 57, 987–994. doi: 10.1186/BF03351877
Bouchrika, I. (2021). How to write a conclusion for a research paper: Effective tips and strategies. https://research.com/research/how-to-write-a-conclusion-for-a-research-paper
PHIVOLCS. (2020). 2020 Masbate Earthquake (Tech. Rep.). Retrieved from https://www.phivolcs.dost.gov.ph/index.php/2-uncategorised/10490 -2020-masbate-earthquake
Reitman, N.G., Mueller, K.J. & Tucker, G.E. (2022) Surface slip variability on strike-slip faults. Earth Surface Processes and Landforms, 47(4), 908–935. Available from: https://doi.org/10.1002/esp.5294
Sacred Heart University. (n.d.). Organizing academic research papers: 9. the conclusion. Research Guides. https://library.sacredheart.edu/c.php?g=29803&p=185935
Simborio, T., Yokoi, T., & Hara, T. (2022). Strong ground motion simulation of the 2020 Masbate, Philippines earthquake (Mw6.6) using Empirical Green’s Function method (Master’s thesis). Retrieved from https://iisee.kenken.go.jp/syndb/data/20230117f70f6a30.pdf (Synopsis of IISEE-GRIPS)
Tiongson, S. F., & Ramirez, R. (2022). Mapping of ground surface deformations and its associated damage using SAR interferometry: a case study of the 2020 Masbate earthquake. In (Vol. 347, p. 03014).
Citation: https://doi.org/10.5194/egusphere-2023-978-AC3
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AC3: 'Reply on RC2', Khelly Shan Sta. Rita, 29 Sep 2023
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RC3: 'Comment on egusphere-2023-978', Austin Elliott, 29 Aug 2023
Rita Et al. review comments:
The authors present remote sensing analyses of deformation from a significant earthquake in the Philippines in 2020, using both InSAR and optical image pixel-tracking to define ground displacements and fault offsets from both the earthquake and postseismic slip. The paper presents geometric parameters of the earthquake rupture’s surface trace, slip distribution, and aseismic slip along the fault, while it also provides some analytical and interpretive insight into the capabilities and value of each remote sensing technique in the challenging tropical forested terrain. With some minor edits to the text and the persistent clarification of a few repeated concepts, this paper will be a good contribution setting the stage for the application of these techniques in other Philippine earthquakes.
General/repeating comments:
The distinction between co-seismic and post-seismic deformation is a key aspect of this project, yet no truly purely coseismic imaging was possible. At best the remote sensing data include two days of afterslip (the period when that rate is presumably highest) and in general the measurements are made on data that include months of afterslip. Care should be taken to very explicitly distinguish these results; multiple time windows might provide an opportunity to define the afterslip rate decay and thus estimate how much may have accumulated in the first two days, thus providing a better estimate of coseismic slip. Otherwise what you have is just total slip and a minimum estimate of postseismic slip, and it should be presented as such.
In a couple places the authors refer to a presumed slip barrier, but it is unclear what exactly this interpretation involves or implies or is based on, and given the legitimacy of scientific debate over rupture barriers/boundaries, etc., these terms should only be introduced confidently with clear explanation. In this case it is unclear to me how a gradually tapering slip distribution implicates a “slip barrier” as opposed to simply being a manifestation of a slip profile across a finite crack. The authors have not explored the possibility that it results from an increase in the depth at which slip is concentrated, with less reaching the surface the farther north it goes. These ideas should be discussed if you wish to invoke a “slip barrier” model/conclusion; otherwise, as it is only mentioned briefly twice and is not what the paper hinges on, it may be better simply omitted.
I have noted in my line-by-line comments a few places where phrasing could be clarified and word choice reconsidered. I particularly urge the authors to use “left-lateral” throughout in place of sinistral.
The figures are very well drafted and nicely explanatory; the tables are appropriate, I have no edits to recommend there! Below are specific comments to help with revision:
Line-by-line comments:
3 - should the second instance of “displacement” in this sentence rather say “offset” as it is referring to the relative total displacement across the fault rather than the net displacement of either side, which is what the displacement fields show. Also can it be described with kinematics (i.e., left-lateral) as the postseismic offset is?
3 - rather than “in Cataingan”, the location of this asperity should be described relative to the fault rupture or epicenter; rather than “lone” one might instead say “single” as this result does not necessarily require that this represents the only asperity present on the fault.
4 - “left lateral” is less jargon-y than “sinistral” and may make for a clearer alternative phrase; repeating comment regarding using “offset” instead of “displacement” when describing the relative displacement across the fault
6 - does tapering slip require a slip barrier? Any rupture will inherently have slip taper toward the crack tips; consider rephrasing this to remove “barrier” or insert text that explains what about the tapering slip suggests a barrier rather than just a rupture tip
7 - the rupture length has not yet been mentioned—have you derived it from the remote sensing data? If so state it in an additional sentence in the abstract here to justify this statement.
14 - what magnitude was the strong aftershock? The specific magnitude is relevant to understand whether it would be likely to produce further ground deformation or was below that threshold.
17 - hypocenter, since the depth is listed
21 - add a word to modify “butterfly” for clarity, such as “butterfly-shaped”
22 - double-check the grammar of this sentence, perhaps a simple correction would be to move “descending track” to just before “line-of-sight” and add another “LOS” after ascending track or before “ground deformation”
25 - spectral analysis “of seismic waveforms”* should be specified for clarity, as remote sensing and displacement fields can also involve spectral analysis
27 - insert “compared to the slip predicted from global scaling relationships based on moment magnitude”
45 - consider using “left-lateral” instead of “sinistral”
47 - ASEAN should perhaps be spelled out on the first instance
49 - why convex? difficult way to describe a 2D fault trace, as it must be relative to one side… would be concave on the other side…. Perhaps just “curved” would be better? or curving
55 - segment “… of the PF…”
63 - left lateral
64 - “extensional” instead of “extension”
66 - this sentence should be rewritten for clarity; firstly, the relationship of a fault and the stress regime is perhaps more two-way causal. The fault is present, so what is the stress regime altered *from*? It’s unclear exactly how you mean the stress state is influenced by the fault, perhaps “the SSF influences the local stress regime due to multiple bends and varying orientation”. “in between the junction of the SSF and PF” doesn’t make sense as a local for either of the possible subjects it refers to, which is also unclear—the SSF itself or the seismic profiles?
84 - Survey, not Service; consider citation specifically of ANSS ComCat (https://earthquake.usgs.gov/data/comcat/)
85 - epicenters, rather than centers
90-91 - presumably these offsets were measured over a period that included the postseismic slip and so co- vs post-seismic slip magnitudes cannot be deconvolved; if accurate, this should be stated here to explain the subsequent conclusion that the excess slip/moment may have been the post-seismic component
95 - hairline fractures… along the fault? Specify whether these were clearly tectonic or possibly had some other origin
98 - rather than presumed to *cause* the subsequent earthquake, it would be more accurate in a current understanding to say “presumed to have reduced the time elapsed before the 2020 earthquake”
106 - would be good convention to specify the spatial resolution of the PlanetLabs images
145-147 - this sentence could be rewritten for a bit more clarity. Something like, “the rasters chosen to extract profiles and calculate across-fault offsets were selected based on…”
148 - specify when using “offset”: “pixel-offset” or “ground-offset”. Again I suggest using “displacement” to refer to absolute position change, and “offset” to refer to relative change across the fault (generally 2x displacement magnitudes). Thus the rasters you have are displacement rasters. They might also reasonably be called “pixel offset” rasters but note that offset of pixels from their pre-event positions is different from our domain-specific use of “offset” to refer to separation of features across the fault, which is generally double the value of displacement on either side and is a closer approximation of a different conceptual quantity: the fault slip.
155 - “profiling” hasn’t been methodologically defined yet here and may thus be an ambiguous term to use without that defintion. You may wish to define it in a prior sentence, or explicitly say, “taking fault-perpendicular profiles (transects) of displacement data in order to measure the net fault-parallel offset”
191 - presumably this means decorrelation in the coseismic interferogram due to higher magnitudes of deformation; you may wish to state that explicitly for clarity.
193 - across the ground rupture instead of “of” the ground rupture
224 - may be worth noting relative horizontal location uncertainties; I see USGS is 7km which may explain the discrepancy
257 - left lateral
279 - describe the distribution along strike more accurately. “followed by an increase” is a temporal term… quantify slip values with numbers and specify relative location along strike using geographical terms like northwest or southeast.
280 - not followed by, that’s a time term; “south of the 60 cm high, displacement declines southward toward the shoreline”
288 - left-lateral
341 - are “subparallel” and “at a low angle” meant to contrast? They sound synonymous; perhaps omit “and at a low angle” or quantify the range of observed strikes in addition to the mean
342 - “the northern terminus was observed” - does this mean the end of the crack tip? Because a measurement of 0.5 cm may be the last resolvable measurement of offset along the surface rupture but care should be taken to explain whether it simply provides a minimum northward constraint on where surface rupture is observed or whether the actual end of the crack is observed and no others can be discovered north of it
345 - remove “down”
350, 355 - “the rupture is continuous southward” is a more neutral description of its characteristics; “the rupture continued” would be how to describe its coseismic propagation, which presumably was northward from the hypocenter
351 - can omit “for”
379 - nothing you have measured is definitively coseismic, correct? In fact it is presumed all of these methods capture co- and post-seismic displacement without being able to fully discern them
383-384 - this sentence is meant to speifically apply to the postseismic interferograms, correct? Specify.
384 - rather than qualitatively say “low”, quantify the uncertainty
391 - “higher”* uncertainties; again, quantify
412 - inverse, rather than indirect
431-432 - I do not believe that after all this your null hypothesis would be that the remote sensing results are “merely arbitrary”. You would perhaps rather say that the agreement with field measurements “indicate that remote sensing results accurately capture ground displacements and provide greater along-strike coverage”
451-454 - this is a slightly unclear sentence, as phrased. Rephrase to make clearer inferences and conclusions based specifically on well articulated observations. Consider changing to the active voice to help explain what observations lead to which inferences. “mostly vertical rupture propagation” doesn’t make sense for a long strike-slip rupture; what does “a southwesterly component” mean or refer to? Is this sentence describing fault attitude? Subvertical with a slight southwest dip? The subject as written is “rupture propagation” which doesn’t make sense in this context.
458 - how does the decreasing slip trend “mark” or indicate the presence of a specific barrier? Declining slip is expected for any secular crack; in fact an anomalous barrier may be expected to make slip decline more abruptly rather than smoothly? This conclusion bears further exploration and/or explanation in your text. Such a gradual, low magnitude and low gradient decline in slip actually suggests an anomalously low stress/strain concentration toward the northern end of the rupture…
467 - The relationship described here seems inverted: those various processes are accommodated by postseismic deformation; postseismic deformation is caused by one or all of those processes.
468 - I do not believe you have presented a coseismic slip distribution. At best you capture the earthquake and the first two days of postseismic (presumably the fastest!), or, more realistically since coseismic profiles were only derived from optical images, the first month (!) of afterslip. You say as much in the subsequent sentences. This sentence warrants a more in depth and logical explanation or walk-through. First you must define how you assess the component of the measured slip which is coseismic versus postseismic; then explain how they relate (proportionally and along strike), and then explain how that means or suggests that postseismic slip is stress driven (mustn’t it be? Is this a novel conclusion?) Your subsequent paragraphs may begin to do this, but that means this sentence is preemptive.
501 - “slip distributions” rather than “profiles”
557 - “the peak coseismic offset is equivalent to a moment magnitude…” doesn’t make sense or is stated incorrectly. Based on scaling relationships the peak slip is consistent with that magnitude. Also rememnber that you have no strictly coseismic displacement measurements
Citation: https://doi.org/10.5194/egusphere-2023-978-RC3 -
AC2: 'Reply on RC3', Khelly Shan Sta. Rita, 29 Sep 2023
We would like to express our sincere gratitude to the reviewer for his favorable and thoughtful comments and suggestions. We appreciate the reviewer’s support for promulgating the techniques used in the study throughout the Philippines. We have taken the reviewer's comments and suggestions into careful consideration, and we are committed to addressing them in a thorough manner.
- The distinction between co-seismic and post-seismic deformation is a key aspect of this project, yet no truly purely coseismic imaging was possible. At best the remote sensing data include two days of afterslip (the period when that rate is presumably highest) and in general, the measurements are made on data that include months of afterslip. Care should be taken to very explicitly distinguish these results; multiple time windows might provide an opportunity to define the afterslip rate decay and thus estimate how much may have accumulated in the first two days, thus providing a better estimate of coseismic slip. Otherwise what you have is just total slip and a minimum estimate of postseismic slip, and it should be presented as such.
Response: Following the paper of Elliot et al (2020), we chose to address this comment by adding the words “may be inferred to represent the coseismic slip” whenever necessary in our manuscript. This addresses the lack of a “truly purely coseismic slip imaging”. For the second point, we will be emphasizing that the post-seismic measurements are minimum estimates, in accordance with this comment and to a similar point raised by reviewer #2.
- In a couple places, the authors refer to a presumed slip barrier, but it is unclear what exactly this interpretation involves or implies or is based on, and given the legitimacy of scientific debate over rupture barriers/boundaries, etc., these terms should only be introduced confidently with clear explanation. In this case, it is unclear to me how a gradually tapering slip distribution implicates a “slip barrier” as opposed to simply being a manifestation of a slip profile across a finite crack. The authors have not explored the possibility that it results from an increase in the depth at which slip is concentrated, with less reaching the surface the farther north it goes. These ideas should be discussed if you wish to invoke a “slip barrier” model/conclusion; otherwise, as it is only mentioned briefly twice and is not what the paper hinges on, it may be better simply omitted.
Response: Thank you and we agree that further research is needed to define this in the study area. In line with this, we will omit this point to avoid scientific inaccuracies.
For the specific comments, the following are our responses:
3 - should the second instance of “displacement” in this sentence rather say “offset” as it is referring to the relative total displacement across the fault rather than the net displacement of either side, which is what the displacement fields show. Also can it be described with kinematics (i.e., left-lateral) as the postseismic offset is?
Response: Thank you for the comment and noticing. We will replace the particular word with “offset”. We believe that adding the movement sense of the fault will add an important context in the abstract, and we will be adding it as well.3 - rather than “in Cataingan”, the location of this asperity should be described relative to the fault rupture or epicenter; rather than “lone” one might instead say “single” as this result does not necessarily require that this represents the only asperity present on the fault.
Response: Much appreciated. This will be modified.4 - “left lateral” is less jargon-y than “sinistral” and may make for a clearer alternative phrase; repeating comment regarding using “offset” instead of “displacement” when describing the relative displacement across the fault
Response: Thank you. We will update each mention of the word accordingly.6 - Does tapering slip require a slip barrier? Any rupture will inherently have slip taper toward the crack tips; consider rephrasing this to remove “barrier” or insert text that explains what about the tapering slip suggests a barrier rather than just a rupture tip
Response: Thank you for the clarifications. We will avoid invoking the slip barrier concept.7 - the rupture length has not yet been mentioned—have you derived it from the remote sensing data? If so, state it in an additional sentence in the abstract here to justify this statement.
Response: Thank you. We will include the parameter source in this line.14 - what magnitude was the strong aftershock? The specific magnitude is relevant to understand whether it would be likely to produce further ground deformation or was below that threshold.
Response: We will update and specify the magnitude.17 - hypocenter, since the depth is listed
Response: Thank you. We will correct it.21 - add a word to modify “butterfly” for clarity, such as “butterfly-shaped”
Response: This will be included.22 - double-check the grammar of this sentence, perhaps a simple correction would be to move “descending track” to just before “line-of-sight” and add another “LOS” after ascending track or before “ground deformation”
Response: Thank you for the suggestion. We will correct it.25 - spectral analysis “of seismic waveforms”* should be specified for clarity, as remote sensing and displacement fields can also involve spectral analysis
Response: Thank you for the clarification. We will add the phrase.27 - insert “compared to the slip predicted from global scaling relationships based on moment magnitude”
Response: Much appreciated. We will add this for clarity.45 - consider using “left-lateral” instead of “sinistral”
Response: All instances of “sinistral” have been modified.47 - ASEAN should perhaps be spelled out on the first instance
Response: This is noted. We will spell this out.49 - why convex? difficult way to describe a 2D fault trace, as it must be relative to one side… would be concave on the other side…. Perhaps just “curved” would be better? or curving
Response: Thank you for this. This notation is to indicate the direction of the curve since a simple “curve” inherently lacks direction. Also, the term is adapted from the cited author.55 - segment “… of the PF…”
Response: Much appreciated. We will include this for clarification.63 - left lateral
Response: All instances of “sinistral” have been modified for simplicity.64 - “extensional” instead of “extension”
Response: Thank you for this. It will be corrected.66 - this sentence should be rewritten for clarity; firstly, the relationship of a fault and the stress regime is perhaps more two-way causal. The fault is present, so what is the stress regime altered *from*? It’s unclear exactly how you mean the stress state is influenced by the fault, perhaps “the SSF influences the local stress regime due to multiple bends and varying orientation”. “in between the junction of the SSF and PF” doesn’t make sense as a local for either of the possible subjects it refers to, which is also unclear—the SSF itself or the seismic profiles?
Response: Much appreciated. We propose modifying it to “The SSF influences the local stress regime expressed by alternating extensive and compressive structures, as it interacts with the PF.”84 - Survey, not Service; consider citation specifically of ANSS ComCat (https://earthquake.usgs.gov/data/comcat/)
Response: Thank you for pointing it out. We will correct it and add the citation.85 - epicenters, rather than centers
Response: Thank you. This will be corrected.90-91 - presumably these offsets were measured over a period that included the postseismic slip and so co- vs post-seismic slip magnitudes cannot be deconvolved; if accurate, this should be stated here to explain the subsequent conclusion that the excess slip/moment may have been the post-seismic component
Response: The mentioned measurements and statements on excess slip/moment were from the cited authors. Per their papers, the field investigation began on 18 Feb. 2003, a mere 3 days after the 15 Feb. 2023 event.95 - hairline fractures… along the fault? Specify whether these were clearly tectonic or possibly had some other origin.
Response: The original authors suggested that these may indicate the presence of post-seismic slip. As for their location of occurrence, these fractures were located on an asphalt pavement along the southern terminus of the 2003 rupture. The possible origin of these were discussed in the succeeding sentences 96-97.98 - rather than presumed to *cause* the subsequent earthquake, it would be more accurate in a current understanding to say “presumed to have reduced the time elapsed before the 2020 earthquake”
Response: Thank you for this suggestion. We will modify this accordingly.106 - would be good convention to specify the spatial resolution of the PlanetLabs images.
Response: Thank you for this. We will add this here.145-147 - this sentence could be rewritten for a bit more clarity. Something like, “the rasters chosen to extract profiles and calculate across-fault offsets were selected based on…”
Response: Thank you. We will modify this.148 - specify when using “offset”: “pixel-offset” or “ground-offset”. Again I suggest using “displacement” to refer to absolute position change, and “offset” to refer to relative change across the fault (generally 2x displacement magnitudes). Thus the rasters you have are displacement rasters. They might also reasonably be called “pixel offset” rasters but note that offset of pixels from their pre-event positions is different from our domain-specific use of “offset” to refer to separation of features across the fault, which is generally double the value of displacement on either side and is a closer approximation of a different conceptual quantity: the fault slip.
Response: This is well noted. Thank you for the clarification. We will update the terminologies.155 - “profiling” hasn’t been methodologically defined yet here and may thus be an ambiguous term to use without that definition. You may wish to define it in a prior sentence, or explicitly say, “taking fault-perpendicular profiles (transects) of displacement data in order to measure the net fault-parallel offset”.
Response: Thank you. We will add an introductory sentence “Measurements were acquired by taking profiles across the fault using displacement data to determine the net fault-parallel offset.” to discuss profiling.191 - presumably this means decorrelation in the coseismic interferogram due to higher magnitudes of deformation; you may wish to state that explicitly for clarity.
Response: Thank you for this one. We will add this to clarify.193 - across the ground rupture instead of “of” the ground rupture
Response: Thank you for pointing it out. We will correct it.224 - may be worth noting relative horizontal location uncertainties; I see USGS is 7km which may explain the discrepancy
Response: Thank you. We have gone over the reference files again and the uncertainty ranges are unavailable. However, the discrepancies aren’t surprising knowing that PHIVOLCS would have a denser network of instruments for the study area.257 - left lateral
Response: Thank you. All instances of “sinistral” will be replaced.279 - describe the distribution along strike more accurately. “followed by an increase” is a temporal term… quantify slip values with numbers and specify relative location along strike using geographical terms like northwest or southeast.
Response: Thank you for this. We will modify this in the revisions.280 - not followed by, that’s a time term; “south of the 60 cm high, displacement declines southward toward the shoreline”
Response: Thank you for pointing it out. We will correct this.288 - left-lateral
Response: Thank you. All instances of “sinistral” will be replaced.341 - are “subparallel” and “at a low angle” meant to contrast? They sound synonymous; perhaps omit “and at a low angle” or quantify the range of observed strikes in addition to the mean
Response: Thank you for this. We agree that it can become redundant.342 - “the northern terminus was observed” - does this mean the end of the crack tip? Because a measurement of 0.5 cm may be the last resolvable measurement of offset along the surface rupture but care should be taken to explain whether it simply provides a minimum northward constraint on where surface rupture is observed or whether the actual end of the crack is observed and no others can be discovered north of it
Response: This is noted. We will modify it with “onshore surface rupture” and add “minimum northward constraint”.345 - remove “down”
Response: Thank you. We will take it out.350, 355 - “the rupture is continuous southward” is a more neutral description of its characteristics; “the rupture continued” would be how to describe its coseismic propagation, which presumably was northward from the hypocenter
Response: Thank you. We will improve this.351 - can omit “for”
Response: Revised.379 - nothing you have measured is definitively coseismic, correct? In fact it is presumed all of these methods capture co- and post-seismic displacement without being able to fully discern them
Response: Similar to the approach of Elliot et al (2020) in the Tajikistan paper, we will add “may be inferred to represent the coseismic slip” whenever necessary in our manuscript.383-384 - this sentence is meant to specifically apply to the postseismic interferograms, correct? Specify.
Response: Thank you for pointing it out. We will specify this.384 - rather than qualitatively say “low”, quantify the uncertainty
Response: This is noted. We will provide either the average uncertainty or the range.391 - “higher”* uncertainties; again, quantify
Response: This is noted. We will provide either the average uncertainty or the range.412 - inverse, rather than indirect
Response: Thank you. This is noted and will be revised.431-432 - I do not believe that after all this your null hypothesis would be that the remote sensing results are “merely arbitrary”. You would perhaps rather say that the agreement with field measurements “indicate that remote sensing results accurately capture ground displacements and provide greater along-strike coverage”
Response: Thank you very much for this one. We really appreciate the suggestion and will incorporate this phrase.451-454 - this is a slightly unclear sentence, as phrased. Rephrase to make clearer inferences and conclusions based specifically on well articulated observations. Consider changing to the active voice to help explain what observations lead to which inferences. “mostly vertical rupture propagation” doesn’t make sense for a long strike-slip rupture; what does “a southwesterly component” mean or refer to? Is this sentence describing fault attitude? Subvertical with a slight southwest dip? The subject as written is “rupture propagation” which doesn’t make sense in this context.
Response: Thank you for this one. We will remove the rupture propagation idea and instead focus on the slip variability aspect to equate to fault complexity.458 - how does the decreasing slip trend “mark” or indicate the presence of a specific barrier? Declining slip is expected for any secular crack; in fact an anomalous barrier may be expected to make slip decline more abruptly rather than smoothly? This conclusion bears further exploration and/or explanation in your text. Such a gradual, low magnitude and low gradient decline in slip actually suggests an anomalously low stress/strain concentration toward the northern end of the rupture…
Response: Thank you and we agree after further deliberation. This will no longer be included.467 - The relationship described here seems inverted: those various processes are accommodated by postseismic deformation; postseismic deformation is caused by one or all of those processes.
Response: Thank you for pointing it out. We will use “by” instead of “as” to correct the conceptual relationship.468 - I do not believe you have presented a coseismic slip distribution. At best you capture the earthquake and the first two days of postseismic (presumably the fastest!), or, more realistically since coseismic profiles were only derived from optical images, the first month (!) of afterslip. You say as much in the subsequent sentences. This sentence warrants a more in depth and logical explanation or walk-through. First you must define how you assess the component of the measured slip which is coseismic versus postseismic; then explain how they relate (proportionally and along strike), and then explain how that means or suggests that postseismic slip is stress driven (mustn’t it be? Is this a novel conclusion?) Your subsequent paragraphs may begin to do this, but that means this sentence is preemptive.
Response: Similar to the responses above, we will add that we infer our measurements to be coeval to the coseismic slip. In addition, its agreement with the field data effectively gives us a combined dataset for this particular period. The most we can add is the improved description of its relationship with the minimum post-seismic dataset, and we are grateful for the suggestions on describing such.501 - “slip distributions” rather than “profiles”
Response: Thank you. We will modify this accordingly.557 - “the peak coseismic offset is equivalent to a moment magnitude…” doesn’t make sense or is stated incorrectly. Based on scaling relationships the peak slip is consistent with that magnitude. Also remember that you have no strictly coseismic displacement measurements
Response: We can insert “the preferred (or inferred) coseismic offset” to emphasize our inference.References:
Elliott, A., Elliott, J., Hollingsworth, J., Kulikova, G., Parsons, B., & Walker, R. (2020, 02). Satellite imaging of the 2015 M7.2 earthquake in the Central Pamir, Tajikistan, elucidates a sequence of shallow strike-slip ruptures of the Sarez-Karakul fault. Geophysical Journal International, 221(3), 1696-1718. Retrieved from https://doi.org/10.1093/gji/ggaa090 doi: 10.1093/gji/ggaa090
Citation: https://doi.org/10.5194/egusphere-2023-978-AC2
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AC2: 'Reply on RC3', Khelly Shan Sta. Rita, 29 Sep 2023
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Khelly Shan Sta. Rita
Sotirios Valkaniotis
Alfredo Mahar Francisco Lagmay
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