关键词: 汶川地震, 综合震源机制, 地壳应力场, 应力方向集中

Abstract: Earthquakes are caused by the failure of faults, driven by tectonic stress build-up in the Earth’s crust. Stress field before a large earthquake is directly related to the earthquake preparation process. Firstly, we determine the composite focal mechanisms in Sichuan, China by using P wave polarity data in the period of 2001-the May, 12, M8.0 Wenchuan earthquake. The P and T axes of the composite focal mechanisms are roughly divided by the Longmenshan fault, the Xianshuihe fault, and the Huayingshan fault. The composite focal mechanisms in Songpan-Ganzi block prefer the accumulation process of stress field before the Wenchuan earthquake. The stress fields in the south of the Xianshuihe fault and southeast of the Huayingshan fault show strike-slip pattern and thrust slip pattern respectively. Then we calculate the Focal Mechanism Rotation Angles (FMOAs) of the temporal composite focal mechanisms to the Wenchuan earthquake focal mechanism before the Wenchuan earthquake. We find that the FMOA 1 year before the earthquake show its minimum value and are lower than its standard deviation, which illustrates the convergence of the stress field to promote the Wenchuan earthquake rupture. This result may provide a new idea to archive large earthquake precursor from tectonic stress field.

Key words: Wenchuan earthquake, composite focal mechanism, Crustal stress field, Convergence of stress direction