Analysis of seismic strain releases related to tidal stress before the 2008 Wenchuan earthquake
- 1Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
- 2Chongqing Earthquake Administration, Chongqing 401147, China
- 1Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
- 2Chongqing Earthquake Administration, Chongqing 401147, China
Abstract. Tidal stresses could load or unload the focal media and trigger small to moderate earthquakes around the epicentral area before a large or great earthquake. Based on the Preliminary Reference Earth Model, we calculated the time series of the tidal Coulomb failure stress (TCFS) acting on the focal fault plane of the Wenchuan earthquake. For the earthquakes (2.5 ≤ ML ≤ 4.0) that occurred around the epicentral area from January 1990 to April 2008, we calculated the rate of TCFS, ΔTCFS, at the occurrence time of each earthquake. These earthquakes are divided into two categories on the basis of the sign of ΔTCFS: One is positive earthquakes (PEQs) occurring at times of ΔTCFS > 0 and the other negative earthquakes (NEQs) occurring at times of ΔTCFS < 0.
Firstly, we obtained the cumulative seismic strain release (CSSR) curve for NEQs and PEQs respectively, and found that two curves almost overlapped before September 2004 and then began to diverge increasingly with time. We employ a parameter Rp, the propotion of the seismic strain release of PEQs, to reveal the effect of TCFS on the occurrence of eartquakes, and found that Rp was significantly higher than 0.5 about six months before the Wenchuan event at the 99 % confidence level, indicating a significant correlation betwen the occurrence of earthquakes and the increasing TCFS.
Furthermore, we worked out the slope k (time rate) of the CSSR curve vs. time for PEQS and NEQs respectively. It shows that several years before the Wenchuan event the seismic strain release accelerated when TCFS increased, while it decelerated when TCFS decreased. Rk, the ratio of k for PEQS to that for NEQs, was used to depict quantificationally the difference of the time rate of seismic strain release between PEQS and NEQS. We found that Rk remained stable, around 1.0, until it started to increase rapidly with time from the beginning of 2005, reached its highest value of 2.7 just before the time of the occurrence of the the Wenchuan event. Rk could reveal the promoting and inhibiting effects of the tidal stresses on the release of seismic strain. The increase of Rk corresponds to the promoting effect when TCFS increases or the inhibiting one when it decreases. Both effects took place in the focal region before the Wenchuan mainshock.
When the tectonic stress in the crust increases, the b-value in the Gutenberg–Richter relation will decrease. We also calculated the temporal variation of the b-value in the study region. By comparing Rk with the b-value, we found that after the tectonic stress had increased for about two and a half years, the focal region started to become unstable and the tidal stress began to take effect. With the further increase in the tectonic stress, the effects of the tidal stresse were enhanced gradually. The increase of the tidal Coulomb failure stress might have promoted the occurrence of earthquakes, whereas its decrease had an opposite effect. This observation may provide an insight into the processes leading to the Wenchuan earthquake and its and precursors.
Xuezhong Chen et al.
Status: final response (author comments only)
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CC1: 'Comment on nhess-2021-405', Natarajan Venkatanathan, 07 Feb 2022
Dear Editor,
Herewith I have enclosed my comments on paper titled "Analysis of seismic strain releases related to tidal stress before the 2008 Wenchuan earthquake". Overall the authors have made observation that there is a seismic strain release, but the authors have to explain further why such behaviourial changes oberved between PEQs and NEQs.
Yours faithfully,
Venkatanathan
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CC2: 'Reply on CC1', Xuezhong Chen, 07 Feb 2022
We thank Venkatanathan for the comments. Some researches reported the tidal triggering of earthquakes prior to moderate to large earthquakes (Tanaka et al., 2002b; Tanaka, 2010, 2012; Li et al., 2018). They investigated the tidal triggering of earthquakes in terms of event count. We investigated the tidal triggering of earthquakes before the Wenchuan earthquake in terms of seismic strain release,but not event count. Therefore the behaviourial changes oberved between PEQs and NEQs result from the tidal triggering of earthquakes prior to the Wenchuan earthquake indeed. The tidal triggering of earthquakes focus on the promoting effect of the tidal stresses, but our results reveal not only the promoting effect, but also the inhibiting effect of the tidal stresses. The increasing tidal stress will promote the occurrence of earthquakes, while the decreasing tidal stress will inhibit the occurrence of earthquakes when a large earthquake is impending.
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CC3: 'Reply on CC2', Natarajan Venkatanathan, 07 Feb 2022
Dear authors, I agree you have worked on seismic strain release. You have observed behaviourial changes of PEQs and NEQs , but there is no explanation that why NEQs decreases prior to the main event compared to the PEQs. You need to explain, when NEQs decreases due to decreasing tidal stress, then why PEQs increases.
Venkatanathan
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CC4: 'Reply on CC2', Natarajan Venkatanathan, 07 Feb 2022
Dear authors, I agree you have worked on seismic strain release. You have observed behaviourial changes of PEQs and NEQs , but there is no explanation that why NEQs decreases prior to the main event compared to the PEQs. You need to explain, when NEQs decreases due to decreasing tidal stress, then why PEQs increases.
Venkatanathan
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CC5: 'Reply on CC4', Xuezhong Chen, 07 Feb 2022
The Earth tide produces cyclic stress variations in the Earth. These stress variations, of the order of 1000~10000 Pa, are far smaller than the tectonic stress. When the stress in the focal region is at lower values, the tidal stress can not influence the occurrence of earthquakes, but when it is close to a critical condition to release a large rupture, the tidal stress could take effect on the occurrence of earthquakes. The tidal stress increase will promote the occurrence of earthquakes, making the seismic strain release accelerate for PEQs (corresponding to the increase of k in Fig.4c), and when the tidal stress decrease will inhibit the occurrence of earthquakes, making the seismic strain release decelerate for NEQs (corresponding to the decrease of k in Fig.4c).
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CC6: 'Reply on CC5', Natarajan Venkatanathan, 07 Feb 2022
Dear Author,
Please include this explanation in the manuscript. For future studies - try to correlate PEQs and NEQs with dip (rake) of the fault.
- CC7: 'Reply on CC6', Xuezhong Chen, 08 Feb 2022
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CC6: 'Reply on CC5', Natarajan Venkatanathan, 07 Feb 2022
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CC5: 'Reply on CC4', Xuezhong Chen, 07 Feb 2022
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CC3: 'Reply on CC2', Natarajan Venkatanathan, 07 Feb 2022
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CC2: 'Reply on CC1', Xuezhong Chen, 07 Feb 2022
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RC1: 'Comment on nhess-2021-405', Anonymous Referee #1, 09 Feb 2022
Dear Editor,
Herewith I have enclosed my comments on paper titled "Analysis of seismic strain releases related to tidal stress before the 2008 Wenchuan earthquake".
With warm regards,
Venkatanathan
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CC8: 'Reply on RC1', Xuezhong Chen, 09 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-405/nhess-2021-405-CC8-supplement.pdf
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AC1: 'Reply on RC2', Xuezhong Chen, 30 Mar 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-405/nhess-2021-405-CC8-supplement.pdf
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AC1: 'Reply on RC2', Xuezhong Chen, 30 Mar 2022
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CC8: 'Reply on RC1', Xuezhong Chen, 09 Feb 2022
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RC2: 'Comment on nhess-2021-405', Andrew Delorey, 25 Mar 2022
Comments for manuscript:
Analysis of seismic strain releases related to tidal stress before the 2008 Wenchuan earthquake
Xuezhong Chen et al.
The analysis shows results for PEQ (positive earthquakes) defined as when (delta)TCFS >0. Did you also do the analysis on PEQ defined as TCFS>0? If so, what were the results? Can you discuss how the choice of PEQ impacts your interpretation and the underlying physics? I think the observation is pretty robust, but there is a lot more analysis you could provide regarding your interpretation of the underlying physics and earthquake processes. As it stands, it is simply an interesting observation.
Can you resolve any changes in behavior with shorter time resolution? You average over 5-years. Is there any change in behavior that you can resolve within the period of time when Rk is increasing?
Why should k(neg) decrease when approaching the Wenchuan earthquake? It seems more intuitive that both k(pos) and k(neg) should increase, even if k(pos) increases faster.
There is another change in behavior around 1999, which can be seen in Figure 4a and 4c. Is there any explanation for that? Is it related to the change in instrumentation?
Figure 1b, what component of strain are you showing?
You write, “as the length of time with (delta)TCFS>0 is almost the same as that with (delta)TCFS<0”. What do you mean by almost? Do you account for the difference in your analysis? You should compare observed versus actual expected, not observed versus “almost” expected. This could impact your results.
There are some minor language problems, that could be fixed by having the manuscript reviewed by a more experienced English writer.
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AC1: 'Reply on RC2', Xuezhong Chen, 30 Mar 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-405/nhess-2021-405-AC1-supplement.pdf
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AC1: 'Reply on RC2', Xuezhong Chen, 30 Mar 2022
Xuezhong Chen et al.
Xuezhong Chen et al.
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