Stress-Strain Characteristics Before the 2021 M s 6.4 Yangbi Earthquake in Yunnan Province, China, and Implications for the Seismogenic Mechanism

On May 21, 2021, an M s 6.4 earthquake occurred in Yangbi County, Yunnan Province, China. The epicenter was located on a secondary blind fault on the southwestern boundary of the Sichuan-Yunnan Block, and this area has a complex tectonic setting and seismogenic background. To improve the understanding of the seismogenic mechanism of this earthquake, we conduct an in-depth analysis on the regional stress and strain characteristics based on geodetic data from the Global Navigation Satellite System (GNSS) and seismological data (focal mechanism solutions) and analyze the stress changes at the epicenter of this earthquake from the perspective of stress triggering. The results show that the epicenter area of the Yangbi earthquake was characterized by significant shear and extension deformation, with a dextral shear rate of 2.5 × 10 −8 /a and an extension rate of 3.5 × 10 −8 /a. The present tectonic stress of the study area is dominated by NNW-directed principal compressive stress and NEE-directed principal extension stress, and the Yangbi main shock occurred on the optimal release nodal plane of the tectonic stress field where the relative shear stress reached 0.984. Furthermore, the cumulative Coulomb stress changes caused by four strong historical earthquakes exceeded the stress trigger threshold of 0.1 bar. All of these results suggest that the Yangbi M s 6.4 earthquake was the result of the maximum effective shear stress concentration on the blind fault on the southwest side of the Weixi-Qiaohou fault, which was mainly driven by the regional tectonic stress and the surrounding historical strong earthquakes.