Earthquake Focal Mechanisms in Yunnan and their Inference on the Regional Stress Field

Abstract

Using the broadband records from a regional permanent network, we determined the focal mechanisms of 174 small and moderate earthquakes (3.4≤ M w≤6.0) from January 2008 to March 2011 in Yunnan, China, by fitting the three‐component waveforms of P and S waves. Results show that the earthquakes predominantly have strike‐slip mechanisms, with occasional normal and very few thrust‐faulting events. Most earthquakes occur in the upper crust above 16 km depth. We then derive the regional distribution of the stress field through a damped linear inversion using the focal mechanisms obtained in this study and augmented by the solutions in the Global Centroid Moment Tensor (GCMT) catalog and in Xu et al. (2010). Inversion results suggest that the entire region is predominantly under a strike‐slip faulting regime. Orientations of both maximum and minimum principle axes exhibit a clockwise rotational pattern. The axes of maximum compression are oriented roughly north–south, with a gradual change across the region from trending northwest–southeast in the South China block to trending northeast–southwest in the Indo‐China block. This pattern is consistent with the surface horizontal velocity derived from geodetic observations. Localized normal faulting stress regions are observed in the Tengchong volcanic zone and in the Qiao‐Jia segment of the Xianshuihe–Xiaojiang fault zone. Online Material: Tables of earthquake catalogs and figures describing stress inversion tests.