Spatio-temporal distribution of tectonic stress field in Longmenshan and its adjacent regions before and after Wenchuan M_S8.0 earthquake

Based on the focal mechanism solutions for Longmenshan and its adjacent regions from July 2000 to June 2009, the spatial distribution of the main direction of tectonic stress fields are retrieved in the regions before and after the Wenchuan M_S8.0 earthquake by using the Gephart & Forsyth method. The results show that the tectonic stress fields change significantly along Xianshuihe fault zone and the region to the north, and that the maximum principal stress direction of the regional tectonic stress shifts from NNW to NW, while the faulting changes from combined normal fault and strike-slip to strike-slip only. Significant changes can also be observed in the tectonic stress field along the Longmenshan fault zone. Although the maximum principal stress direction still approximates to the east-west and north-east, its scope is expanding towards the east-north, and the abnormal belt near the epicenter of Wenchuan earthquake shrinks. On the other hand, the retrieval results of the tectonic stress fields in Longmenshan and its adjacent regions during different periods from July 2000 to June 2009 show that the characteristic parameters of tectonic stress fields, including the R value, Lode parameter μ′, azimuths and elevation angles of three principal stresses, begin to change significantly since the thirteenth window, suggesting that the thirteenth window (December 2006 to January 2007) is a characteristic window with significant changes in the tectonic stress. In the thirteenth window, the seismic energy is accumulated to a critical state, signaling the time node for the occurrence of Wenchuan M_S8.0 earthquake.