Crustal structure and seismicity characteristics based on dense array monitoring in northwestern Yunnan, China

Abstract

Owing to the complex geological environment and frequent seismic activity in northwestern Yunnan, China, the northwest Yunnan dense array was installed. The dense array monitored the Mw6.1 Yangbi earthquake that occurred on May 21, 2021, on the western boundary of the Sichuan-Yunnan block. The earthquake exhibited a typical foreshock-mainshock-aftershock sequence and presented a clear process of earthquake migration. From April 1, 2018 to December 31, 2021, 11,623 earthquakes were recorded by 72 stations in the dense array. Based on their arrival time data, the Vp/Vs model consistency-constrained double-difference seismic tomography method was employed. We obtained high-precision crustal 3-D Vp, Vs, Vp/Vs models, and relocation results for northwestern Yunnan. A wide distribution of low-velocity anomalies was observed in the middle and upper crust of northwestern Yunnan. Based on GPS measurements, the contour of the low-Vp zone is aligned with the clockwise movement of crustal material caused by the compression of the Tibetan Plateau. A seismic cluster with a banded distribution pattern was identified in an intersection area of multiple faults. And its formation might be related to the infiltration of geothermal fluid in a fluid channel composed of unknown fractures. The geothermal fluid originates from the intersection of the fault-enriched hot springs. The flow and seepage from the fluid channels spread to the Yangbi earthquake source region, reactivating the previously existing unmapped faults, ultimately leading to the occurrence of Yangbi foreshocks. Because of the stress transmission, the Coulomb stress of the foreshock triggered the high-strength part of the main fault, which triggered the occurrence of the main earthquake.