Earth tidal stress as an earthquake trigger prior to the Wenchuan earthquake, Sichuan, China

Abstract

Cyclic stress variations in the crust of the earth are caused by the earth tide. Although these stress variations are small in comparison to tectonic stress, tidal stress rates are often larger than those of tectonic stress accumulation. Thus, tidal stress could be considered as one of the possible triggers for an earthquake. Many scientific studies have been conducted to examine the correlation between the earth tide and earthquake occurrence. In this study, the shear and normal stresses caused by the earth tide on the focal fault plane of the Wenchuan earthquake, Sichuan, China, are calculated using the focal mechanism solution published by the Global Centroid Moment Tensor catalog and the Preliminary Reference Earth Model. We analyzed how the tidal stresses on the fault plane changed when the Wenchuan earthquake occurred. The Schuster’s test was applied to define correlations between earthquakes occurring around the epicenter area before the Wenchuan earthquake and stresses caused by the daily earth tide. A P -value was calculated and used to quantify the statistical significance of the evidence. Earthquake data used in this study were obtained from the China Earthquake Networks Center, China Earthquake Administration. The temporal variation and spatial distribution of the P -value, indicating the significance of the test, were investigated. The obtained results show that the daily tidal stress was in favor of slip of the focal fault when the Wenchuan earthquake occurred. For earthquakes ( M L≥2.7) that occurred in the study area, the P -value began to drop in June 2007, approaching the lowest value (0.85%) very close to the occurrence time of the M S8.0 Wenchuan main shock. Most of low- P -value ( P -value situated in the near northeast of the epicenter can be found prior to the Wenchuan earthquake from the spatial distribution of P -value in the Longmenshan Fault. It can be concluded from the above-mentioned results that significant correlations between earthquakes occurring around the epicenter area and the daily tide existed prior to the Wenchuan earthquake. These tide-triggered earthquakes were in the south part of the rupture zone of the Wenchuan earthquake, northeast of the epicenter. Therefore, tidally triggered earthquakes occurred around the epicenter area prior to the Wenchuan earthquake.