Regional stress field in the SE margin of the Tibetan Plateau revealed by the focal mechanisms of small and moderate earthquakes

Abstract

In this study, we investigate the stress field in the southeastern margin of the Tibetan Plateau. We first determine the focal mechanism solutions of 1537 small and moderate (3.2 ≤ M_W ≤ 6.7) regional earthquakes from January 2009 to June 2021, and then use the focal mechanisms to invert for the spatial variation of crustal stress field by a damped linear inversion method. Our result suggests that in the southeastern margin of the Tibetan Plateau the seismogenic zone is in the upper crust above 15-km depth, and the stress field is predominantly strike-slip in the Sichuan-Yunnan Rhombic Block (SYRB). The maximum compressional stress axis is oriented in a fan-shaped pattern, rotating clockwise from nearly east-west in the Songpan-Ganzi Terrain in the north to northwest-southeast in the SYRB to nearly north-south across the Red River Fault in the Indo-China Block (ICB), consistent with the GPS-derived surface strain rate. The stress field around the border of the Tibetan Plateau with high elevation relief appears to be largely caused by gravitational effect with the maximum extensional axis perpendicular to the topography gradient. The stress field in the vicinity of the Longmenshan Fault Zone and in the Yangtze Craton is mainly thrust as a result of the eastward expansion of the Tibetan Plateau and the resistance of the Sichuan Basin. Near the epicenter of the 2008 Wenchuan earthquake and in the northeastern end of the Longmenshan Fault Zone, the thrust stress field shows spatial variations as a result of the perturbation by complex geometry and the post-seismic healing process. Our result provides multi-resolution images of the stress field for better understanding about the mechanisms of seismic activity and crustal deformation in the southeastern margin of Tibetan Plateau.