the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Dec 2023
Seismic Deformation of Himalayan Glaciers Using Synthetic Aperture Radar Interferometry
Abstract. The Himalayan belt, formed due to Cenozoic convergence between the Eurasia and Indian craton, acts as a storehouse of large amounts of strain that results in large earthquakes that occur from the western to the eastern Himalayas. Glaciers also occur over a major portion of the high-altitude Himalayan region. In this study, we attempt to understand the impact of earthquakes on and around Himalayan glaciers in terms of vertical deformation and coherence change. Eight earthquake events of various magnitudes and hypocenter depths occurring in the vicinity of Himalayan glacial bodies have been analyzed using C-band Sentinel1-A/B synthetic aperture radar (SAR). Differential interferometric SAR (DInSAR) method is applied to the pre-and co-seismic single look complex (SLC) SAR imagery to capture glacial surface deformation potentially related to earthquake occurrence. The influence radius of each earthquake (Mw>4.5) is defined using shake maps. For the lower magnitude earthquakes (Mw<4.5), influence radii are computed using the linear relationship between influence radii and magnitudes of past earthquakes. The mean glacial displacement varies from -38.9 mm to -5.4 mm for the 2020 Tibet earthquake (Mw 5.7) and the 2021 Nepal earthquake (Mw 4.1). However, small glacial and ground patches processed separately for vertical displacements reveal that the glacial mass shows much greater seismic displacement than the ground surface. This indicates potential site-specific seismicity amplification properties of glacial bodies that need additional studies. Reduction in co-seismic coherence around the glaciers is observed in some cases, indicative of possible changes in the glacial moraine deposits and/or vegetation cover. The effect of two different seismic events (the 2020 and 2021 Nepal earthquakes) with different hypocenter depths but the same magnitude at almost equal distances from the glaciers is assessed; a shallow earthquake is observed to result in a larger impact on glacial bodies in terms of vertical displacement. Earthquakes may exacerbate or induce glacial hazards such as glacial surging, ice avalanches, landslides and failure of moraine-/ice-dammed glacial lakes. This research can assist in identifying areas at risk and provide valuable insights for planning and implementing measures for disaster risk reduction in the near future.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2253', Anonymous Referee #1, 29 Jan 2024
General comments
The study presented demonstrates a commendable use of English. There is major room for improvement in the presentation of the figures. Overall, the chosen subject matter is captivating.
Nevertheless, problems of basic methodological elements weaken the study considerably.
For this reason at this point I would propose a major revision.
Specific comments
The authors employ the InSAR technique. The theoretical part of the SAR methods adopted are clearly explained and in detail. However, at first the authors focus on the Tibet 2020 earthquake (Mw5.7), which caused significant deformation detectable by InSAR, as evident from the fringes displayed in the wrapped interferogram in Fig. 3D. However, starting from the second event studied (Leh earthquake, Mw5.3), the magnitudes are too low to be reliably detected by InSAR. Additionally, subsequent minor events with magnitudes ranging from 4.1 to 4.5 and varying depths of 35, 49, or even 74 kilometers, further challenge the feasibility of using InSAR for surface deformation monitoring. Consequently, it becomes apparent why the wrapped interferograms for these events are not presented -except for the Mw5.7 Tibet 2020 earthquake. Thus, the approach utilized to measure surface deformation for the second event (Leh earthquake, Mw5.3, depth: 10 kilometers) could be questionable, and for all subsequent events, it is deemed unsuitable.
The authors make assumptions regarding the influence radius of the smaller events, their methodology could potentially be considered to lack sufficient data to ensure reliable results. However, what is more crucial is the absence of estimations for the expected deformation of each event. I consider, that it is imperative for the authors to develop forward surface displacement models, potentially based on formulations such as those of Okada. This missing element is of significant importance. Such models could be constructed using the focal mechanisms of the events or -for more detailed results- on slip distributions, if available. Understanding the anticipated surface deformation based on the event source is paramount, allowing for a comparison with observed patterns to ascertain whether the glaciers are following expected trends or displaying deviation.
Technical corrections
1. Add the wrapped and unwrapped interferograms for all the events.
2. Add the available shakemaps in the figures, or in the supplementary material.
3. Add a global map where you can show the area of interest. Also since you refer to them, add the Indian and Eurasian plate boundaries, Tibetan plateau Himalayan frontal thrust etc. All that is mentioned in the text, would be better to be shown in the figures too.
4. According to gacos.net, when using GACOS, all the following papers should be cited:
- Yu, C., Li, Z., Penna, N. T., & Crippa, P. (2018). Generic atmospheric correction model for Interferometric Synthetic Aperture Radar observations. Journal of Geophysical Research: Solid Earth, 123(10), 9202-9222.
- Yu, C., Li, Z., & Penna, N. T. (2018). Interferometric synthetic aperture radar atmospheric correction using a GPS-based iterative tropospheric decomposition model. Remote Sensing of Environment, 204, 109-121.
- Yu, C., Penna, N. T., & Li, Z. (2017). Generation of real‐time mode high‐resolution water vapor fields from GPS observations. Journal of Geophysical Research: Atmospheres, 122(3), 2008-2025.
5. Add the focal mechanism of all the events in the corresponding figures.
6. In section 4.2 around line 245 you mention: “…regions close to the epicenter location show positive vertical displacement for the 2020 Leh earthquake, with negative vertical displacements moving away from the epicenters.” This is an example where you should present what you measured and which is the theoretically expected deformation pattern.
7. Figure 5A. Why is there such a sharp discontinuity?
6. Provide a citation for the SRTM DEM.
FINAL COMMENTSI find the arguments of the authors intriguing: the presence of metamorphosed glacial ice over the ground surface leads to seismic amplification and the effects of earthquake events on glaciers can be notably more significant than their impact on the surrounding terrain. But as previously mentioned, the analysis falls short in terms of robustness. The methods adopted in this manuscript are not suitable for most of the part of the study that the authors present.
However, considering that the subject is interesting, I would invite the authors to present an improved version of it, after a major revision.
Citation: https://doi.org/10.5194/egusphere-2023-2253-RC1 -
RC2: 'Comment on egusphere-2023-2253', Anonymous Referee #2, 03 Apr 2024
In this study the authors attempt to correlate deformation measured using the InSAR technique from minor earthquakes with magnitudes ranging between Mw 4.1 and 5.7 with deformation at nearby glaciers. While the idea in principle is interesting and worthy of investigation, I am not persuaded by the arguments presented in this study of the correlation between earthquakes and glacial deformation.
Major:
- Fundamentally I am unconvinced by your arguments that these small (Mw<5.7) earthquakes have resulted in deformation at glaciers. You’ve shown interferograms of several earthquakes and unconvincing maps of deformation at glaciers. However, the causality is lacking. One convincing way to do this would be to generate a deformation time series at the glacier and how that there is an acceleration following the earthquake.
- It’s not clear to me why you have only selected earthquakes between magnitudes 4.1 and 5.7. Why have you missed out the potential impacts from larger events, for example the 2016 Nepal earthquake?
- You are missing a step in your assumption that the InSAR data provides vertical deformation (e.g. Figure 2, Line 161). It doesn’t, it gives you deformation in the Line-of-Sight direction. You can assume that all your deformation is vertical, which is not great but fine. But you need to clearly state this assumption and discuss the impacts of it in your discussion.
- You don’t say anything about how you do the spatial reference selection for the InSAR images. This is particularly important when you are looking at differences between pre- and coseismic image pairs. This also becomes extremely problematic when you unwrap areas separately as you say you do in Lines 258-261. Did you pick the same point for the both the pre- and co-earthquake pairs? If not, how do you know the differences you see are not due to reference selection?
- To me looks like there are major unwrapping errors in Figures 5/11. The jump between and red (uplift) and subsidence (blue) is far too sharp. Please can you confirm and show that this is not the case. I also can’t see the earthquake in the interferograms. For all earthquake figures can you plot the wrapped image like you did for Fig 4, please also plot the epicentre location and the USGS contours you use to delineate the influence area.
- I’m not entirely sure I agree with your influence radius argument. Mostly because I’m not clear what you mean by the influence radius. The shakemap contours are the shaking intensity contours, not the deformation radius. It isn’t clear how you’ve done your calculations in section 4.4. I’m not entirely sure what the 4th column in Tables 3 and 4 represents. How reasonable is it to assume a linear relationship between IR and Mw and hypocentral depth, when we know that in terms of energy release Mw does not scale linearly? Please provide extra clarity on this point.
Minor:
The introduction is largely irrelevant. It can be improved by removing all the sections giving a background to the techniques and instead focusing on the main aims/purpose of your study. I would say the same for your Data section, you don’t need to go into so much unnecessary detail about the system/data specifics of the Sentinel-1/2 satellites.
Line 17: Shakemaps from where? Did you calculate these yourself?
Line 18-19: I think this statement needs more information/detail.
Line 43: …assessment is a crucial…
Line 54: … with the help of Synthetic Aperture Radar (SAR)…
Line 104: …resolution of approximately 5m×20m…
Sections 3.3 and 3.4: Much of the text in these sections is unnecessary. Most of the processing steps and background are largely known and now routine and can be cited away. Please just state what you did and focus on anything you’ve done differently to the established norm. For example all the text explaining the background of coherence is not needed. Also, please state which software you used to do this processing, e.g. SNAP, ISCE, GAMMA etc.?
Line 130 : Please reference in the text the section in the paper where you do this (section 4.4).
Line 166: What resolution? Also, how does the ZTD delay maps help correct unwrapping errors?
Line 173: I’m not sure this makes sense. Interferometry is not the complex coherence…
Line 224: Please state which isoseismal contour threshold you used. Can you also plot these on Figure 3?
Line 226: Are you sure your subsidence is 47.3mm? The blue colours inf Figure 3 correspond the subsidence ~150mm!
Figure 4: In panel A, please label the boxes that represent panels B and C.: You say that you can see a drop in coherence. I can’t see this. Could you show a zoom in of the coherence change? Please plot on the map the earthquake location. Can you do this for all panelled figures (e.g. Fig 12) as it is confusing to see which panel corresponds to which geographic location.
Line 254: I’m not sure I understand what you’re saying here. Fig 6B is the unwrapped vertical deformation. If it is difficult, how did you get this figure?
Citation: https://doi.org/10.5194/egusphere-2023-2253-RC2
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