Earthquake relocation and regional stress field around the eastern Himalayan syntaxis

The eastern Himalayan syntaxis is located at the front of the collision between the Indian and Eurasian continents. This region is affected by the interaction of the Himalayan, Lhasa, Qiangtang, and Sichuan-Yunnan blocks and the Indian plate and is characterized by strong tectonic deformation with frequent earthquakes primarily distributed linearly. In this study, various seismological methods were used to reveal the seismicity, seismogenic mechanism, and tectonic stress field in this region. First, we used the double-difference location method (HypoDD) to relocate 65663 earthquakes with M ≥1.0 during 2008-2018. Then, the Cut-And-Paste (CAP) method was adopted to invert the focal mechanism solutions of 163 events with M ≥3.5 from 2009 to 2021. Combining the focal mechanisms inverted in this study and 1156 solutions collected from the GlobalCMT catalog and other published studies, we obtained the regional stress field with the damped regional-scale stress tensor inversion method. The results show that the earthquakes in this region are mainly distributed along mapped faults, among which the eastern Himalayan syntaxis, the extensional rift in the middle of the plateau, the Sichuan-Yunnan block, and the Yunnan-Burma block experience significant seismic activity. The earthquakes are distributed in the upper and middle crust (5-25 km), and there is a significant increase in the number and dominant depth distribution of earthquakes within the Sichuan-Yunnan and Yunnan-Myanmar blocks from those in the Lhasa and Qiangtang blocks. Earthquakes of various mechanisms occur frequently at the eastern Himalayan syntaxis; strike-slip earthquakes are mainly distributed along large boundary faults; normal earthquakes primarily occur along the western boundary faults of the Sichuan-Yunnan block, and thrust earthquakes are concentrated at the front of the collision between the Indian and Eurasian plates. The horizontal direction of the principal compressive stress axis rotates nearly clockwise around the eastern Himalayan syntaxis, from the Himalayas, Lhasa, Qiangtang, Sichuan-Yunnan to Yunnan-Burma blocks. Moreover, strong local inhomogeneity in the stress fields are found in the shallow eastern Himalayan syntaxis and northwest Sichuan-Yunnan block.