Analysis of seismic strain releases related to tidal stress before the 2008 Wenchuan earthquake

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 ≤ M_L ≤ 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 R_p, the propotion of the seismic strain release of PEQs, to reveal the effect of TCFS on the occurrence of eartquakes, and found that R_p 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. R_k, 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 R_k 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. R_k could reveal the promoting and inhibiting effects of the tidal stresses on the release of seismic strain. The increase of R_k 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 R_k 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.