the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2024
Coda-derived source properties estimated using local earthquakes in the Sea of Marmara, Türkiye
Abstract. Accurate estimates of the moment magnitude of earthquakes that physically measures the earthquake source energy are crucial for improving our understanding of seismic hazards in regions prone to tectonic activity. To address this need, a method involving coda wave modelling was employed to estimate the moment magnitudes of earthquakes in the Sea of Marmara. This approach enabled to model the source displacement spectrum of 303 local earthquakes recorded at 49 seismic stations between 2018 and 2020 in this region. The coda wave traces of individual events were inverted across twelve frequency ranges between 0.3 and 16 Hz. The resultant coda-derived moment magnitudes were found to be in good accordance with the standard local magnitude estimates. However, the notable move-out between local magnitude and coda-derived moment magnitude estimates for smaller earthquakes less than a magnitude of 3.5 likely occurs due to potential biases arising from incorrect assumptions for anelastic attenuation and the finite sampling intervals of seismic recordings. Scaling relations between the total radiated energy and seismic moment imply a nonself-similar behaviour for the earthquakes in the Sea of Marmara. Our findings suggest that larger earthquakes in the Sea of Marmara exhibit distinct rupture dynamics compared to smaller ones, resulting in a more efficient release of seismic energy. In conclusion, here we introduce an empirical relationship devised from the scatter between local magnitude and coda-derived moment magnitude estimates.
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RC1: 'Comment on egusphere-2024-721', Gizem Izgi, 30 May 2024
This study makes a significant contribution to the field of seismology, particularly through its innovative use of coda wave modeling to estimate moment magnitudes of earthquakes in the Sea of Marmara. The comprehensive dataset, consisting of 303 local earthquakes recorded at 49 seismic stations between 2018 and 2020, enhances the robustness of the findings. The inversion of coda wave traces across twelve frequency ranges is a noteworthy methodological strength, providing detailed insights into the source displacement spectrum. The alignment between coda-derived moment magnitudes and standard local magnitude estimates is promising, despite some discrepancies for smaller earthquakes. The introduction of an empirical relationship between local magnitude and coda-derived moment magnitude is particularly valuable for future applications.
Some minor comments:
Abstract
- Line 19: What the authors mean by "devising an emprical relationship". Do they suggest a new methodology or do they just mean observed by the comparison.Introduction
- Line 21: The authors may consider to cut the sentence into two or avoid using the phrase of "such moment magnitude estimates".
- Line 36: Could the authors discuss this sentence in more detail. "It is widely recognized that local or regional coda waves mainly consist of scattered waves." The shape of the coda wave is mainly driven by other attenuation types. Thus this statement can be discussed further or do they refer to attenuation in general instead of scattering?Data
- The authors may explain why the central frequencies chosen as these.
- It would be nice to see a waveform example with chosen parameters shown. When the datasets of similar studies considered, the dataset seems extremely lucky.
- The authors could explain why they chose minimum of 4 stations for each event.
- How many earthquakes are presented in Figure 1. I believe the authors are showing 375 events. Visually they seem a lot less probably because of the circle size. At least mentioning the earthquake count in the figure caption would help.Method
- Why the Figure 2 shows 5.5 to 10.5 Hz whereas in data section the authors presented different central frequencies.
- Also at Figure 2, I do not see any colors as the authors mentioned in the figure caption. I am not sure if its caused by the printing but I can see the colors on the other figures but not this one. Although one might figure out where blue cross is, it would be nicer to show it.Result and Interpretations
- Line 266: In which way the result fits are comparable to the other mentioned studies.
- Line 274: Authors state that Figure 1 shows 303 selected events. Is this information correct? Please check also my comment related to Figure 1 under Data section.
- Line 298: "two corner frequencies More recently," dot is missing between two sentences.
- Line 319: "A comparison between ML-based catalogue magnitudes (KOERI earthquake catalogues) and our 𝑀𝑤−𝑐𝑜𝑑𝑎 indicates, an overall good accordance between them, except only for a few outliers caused by small-magnitude earthquakes." Can the authors provide a percantage of "good accordance" and how many outliers there are and would it be possible to somehow mark them on figures 5 and 6.
- Line 368: There seems like to be a font issue. Also the sentence is hard to understand.
- The discussion highlights the good agreement between coda-derived moment magnitudes (Mw-coda) and local magnitudes (ML) for earthquakes with Mw-coda > 3.5, while noting potential biases for smaller events due to anelastic attenuation and recording limitations. Could you elaborate on how these biases specifically impact the empirical relation between Mw-coda and ML, and what methodologies could be adopted in future studies to better account for or mitigate these biases, particularly for smaller magnitude earthquakes?Conclusion
- Line 419: "A linear regression analysis further provided an empirical relation developed between 𝑀𝑤−𝑐𝑜𝑑𝑎 and 𝑀𝐿 , which can be a useful tool in the future to quickly convert catalog magnitudes into moment magnitudes for local earthquakes in the study area." Is the process really quick for conversion compared to other methods? If so that would be nice to see in discussion part. Also I presume this analysis would be useful in any study area for local earthquakes so that the authors maybe rephrase the sentence not emphasizing on 'the' study area.Overall, this study is methodologically rigorous and offers meaningful advancements in understanding earthquake dynamics and seismic hazard assessment.
Citation: https://doi.org/10.5194/egusphere-2024-721-RC1 -
RC2: 'Comment on egusphere-2024-721', Anonymous Referee #2, 17 Jun 2024
In this paper, Özkan et al., estimate the source parameters of 303 ML [2.5 – 5.7] earthquakes in the Sea of Marmara region, NW Türkiye, using a coda-wave modelling approach. The authors derived a scaling relationship between ML and MW coda. One main result is the reporting of non-self-similar earthquake behavior based on in the increasing ratio of scaled energy (E0/M0) with seismic moment. The topic is relevant to the community, and, especially to the Marmara region, there are not many studies on this topic, making this contribution particularly valuable. The manuscript is well written and (to some extent) logically organized. Some recommendations are provided below:
- I feel certain imbalance in the manuscript between the introduction to the region and the methodology, which is extensive and detailed, and the results, interpretation and conclusions, which I feel are rather scarce and limited. For example, in Section 2, the description focuses on the potential for large earthquakes from different fault segments. However, this important issue is never brought up in the results, or, more importantly, on the interpretation of the results. These two parts of the paper seem somewhat disconnected. The estimation of the Coda parameters was quite extensive and difficult. What new information do these results provide, in the context of the Marmara region and the ongoing deformation there? I would suggest working on extending this part.
- Unfortunately, I have strong concerns about the reported breakdown in the self-similarity of the earthquakes for this region. As the authors note, many artefacts can lead to such a plot (Figure 7). The fit looks so sharp that it is hard to believe that this is a real feature. Simply reporting this as a “result that deserves further analysis in the future” may be misleading to the community, who may believe that these are rigorous results. Instead, as this is a major result of their analysis, I encourage the authors to evaluate more deeply the various artifacts that can promote such an artificial trend. Some of them include (1) limited bandwidth, (2) Biases in the station See Cocco et al., (2016) for a broader overview on factors that could potentially contribute to biases in this relation.
- Introduction: there is a certain amount of repetition, which can also be seen in the repetition of sequences of references. For example, see lines 110 and 140.
- Section 2: The selected data set extends beyond the Main Marmara Fault and also covers the Armutlu Peninsula. I suggest to also add a description of the seismological and tectonic processes of this region, as it seems to be an important part of your data set (see e.g. the event shown in Figure 3). Some literature from the region includes Martínez-Garzón et al., (2019; 2021); Bocchini et al., (2022).
- Section 3: Why there is a need to restitute if these are broadband stations? Also, is it necessary to give only central frequencies instead of detailing both frequency cutoffs? It would help to exclude bandwidth limitation problems.
Other comments:
- 164: “The noise level before P-wave onset was removed” à How?
L 165: Why these choices on window length, and do small variations play a role ?
L 168: SNR from where?
L 172: “Earthquakes” should be “Stations” ?
L 185: parameter (no s in the end).
L 201: THE three main steps… […] is given à are given
L 203: parameter (no s in the end).
Figure 2 caption: Description of the y axis of the small insets and the actual y label of the insets do not match. Which one is correct?
L 274: Sentence weird. Please revise.
Figure 4, y label: please add units.
L 370: smaller oneS.
L 373: relatively “fort”?
L 396: efficient is doubled.
References:
Bocchini, G. M., Martínez-Garzón, P., Verdecchia, A., Harrington, R. M., Bohnhoff, M., Turkmen, T., & Nurlu, M. (2022). Direct Evidence of a Slow-Slip Transient Modulating the Spatiotemporal and Frequency-Magnitude Earthquake Distribution: Insights From the Armutlu Peninsula, Northwestern Turkey.Geophysical Research Letters, 49(18), e2022GL099077. https://doi.org/10.1029/2022GL099077
Cocco, M., Tinti, E., & Cirella, A. (2016). On the scale dependence of earthquake stress drop. Journal of Seismology, 20(4), 1151–1170. https://doi.org/10.1007/s10950-016-9594-4
Martínez-Garzón, P., Bohnhoff, M., Mencin, D., Kwiatek, G., Dresen, G., Hodgkinson, K., et al. (2019). Slow strain release along the eastern Marmara region offshore Istanbul in conjunction with enhanced local seismic moment release. Earth and Planetary Science Letters, 510, 209–218. https://doi.org/10.1016/j.epsl.2019.01.001
Martínez‐Garzón, P., Durand, V., Bentz, S., Kwiatek, G., Dresen, G., Turkmen, T., et al. (2021). Near‐Fault Monitoring Reveals Combined Seismic and Slow Activation of a Fault Branch within the Istanbul–Marmara Seismic Gap in Northwest Turkey. Seismological Research Letters, 92(6), 3743–3756. https://doi.org/10.1785/0220210047
Citation: https://doi.org/10.5194/egusphere-2024-721-RC2
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