Analysis of seismic strain release related to the tidal stress preceding the 2008 Wenchuan earthquake

Abstract

Abstract. Tidal stresses could load or unload focal media and trigger small to moderate earthquakes in and around the focal region before a large or great earthquake. Based on the preliminary reference Earth model, we calculated the time series of tidal Coulomb failure stress (TCFS) acting on the focal fault plane of the Wenchuan earthquake. For earthquakes (2.5≤ML≤4.0) that occurred in and around the focal region from January 1990 to April 2008, we calculated the time rate of TCFS, ΔTCFS, at the occurrence time of each earthquake. These earthquakes were divided into two categories on the basis of the signs 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. First, we obtained cumulative seismic strain release (CSSR) curves of NEQs and PEQs and found that the two curves nearly overlapped prior to September 2004 and then began to separate increasingly with time. We used a parameter Rp, the proportion of seismic strain release of PEQs, to investigate the effect of TCFS on earthquake occurrence and found that Rp was significantly higher than 0.5 about 6 months before the Wenchuan event at a 99 % confidence level, indicating a significant correlation between earthquake occurrence and increasing TCFS. Furthermore, we calculated the slope k (time rate) of the CSSR curve vs. time for PEQs and NEQs separately. It was observed in the pre-event period that the seismic strain release accelerated when TCFS increased but decelerated when TCFS decreased. The difference in the time rate of seismic strain release between PEQs and NEQs was quantified using Rk, the ratio of k for PEQs to that for NEQs. We discovered stable Rk values (around 1.0) until they began to rise rapidly with time in early 2005, reaching their highest value of 2.7 just before the Wenchuan event. Rk could reveal the promoting and inhibiting effects of tidal stress on seismic strain release. When kp increases alone or kn decreases alone, Rk will increase. Thus, an increase in Rk corresponds to a promoting effect during times of increasing TCFS and an inhibiting one during that of decreasing TCFS. Both effects were observed in the focal region prior to the Wenchuan mainshock. The b value in the Gutenberg–Richter relationship decreases as the tectonic stress in the crust increases. We also calculated the temporal evolution of the b value in the study region. It was observed that after 2.5 years of increasing tectonic stress, the focal region became unstable, and the tidal stress began to take effect. The effects of the tidal stress were gradually enhanced as the tectonic stress increased further. The increase in the tidal Coulomb failure stress may have aided the occurrence of earthquakes, whereas the decrease had the opposite effect. This observation may shed light on the seismogenic processes that led to the Wenchuan earthquake and its precursors.