Impact of stress unloading induced by the 2021 M_S6.4 Yangbi，Yunnan earthquake on the surrounding areas from the joint analysis of InSAR observations and small earthquake distribution

In order to analyze the impact of stress changes on surrounding faults after the Yangbi M_S6.4 earthquake in Yunnan Province on May 21, 2021, this paper used InSAR technology to perform the inversion for the coseismic deformation field of the Yangbi earthquake. Additionally, small earthquake distribution data was incorporated to establish a fault rupture sliding model. By calculating the coseismic Coulomb stress on the fault plane, the evaluation of the impact of earthquakes on surrounding faults was conducted. The analyses effectively examined the spatio-temporal resolution of earthquake rupture. The results indicate the following: ① In the high-resolution coseismic deformation field obtained from the InSAR data of the ascending orbit, the maximum line of sight deformation is approximately 5.00 cm, while for the descending orbit it is around 7.80 cm. ② The epicenter of the main shock of the Yangbi earthquake sequence, determined through precise positioning of aftershocks, is located at （99.89°E, 25.67°N） with a focal depth of 13.29 km. Apart from the main shock, the focal depths of the aftershocks are primarily concentrated in the range of 5−15 km. By analyzing the location of small earthquakes, the strike of the seismogenic fault is is determined to be NW-SE （316.69°） with a dip angle 88.56° and a slip angle 177.97°. ③ Based on the joint inversion of InSAR coseismic deformation field result and small earthquake fitting fault parameters, it was determined that the fault slip during this earthquake was primarily dextral strike-slip. The maximum slip amount observed during the ascending orbit was 0.80 m, corresponding to a depth of 8.85 km. Additionally, the average slip amount was measured to be 0.22 m, and the moment magnitude of the earthquake is M_W6.41. The maximum slip of the fault during the descending orbit is 0.30 m, corresponding to a depth of 6.88 km. The average slip amount during this orbit is measured to be 0.05 m, and the moment magnitude of the earthquake is M_W6.01. Furthermore, stress calculations on fault planes at the depths of approximately 7.50 km and 15 km revealed a significant decrease in stress difference between the fault itself and its surroundings after the earthquake. This suggests a notable stress unloading effect on the surrounding faults. Based on the distribution area of aftershocks, simulation of fault traces, and the direction of coseismic Coulomb stress reduction, it can be determined that the seismogenic fault of the Yangbi earthquake is a concealed secondary fault of the Weixi-Qiaohou-Weishan fault. The release of regional stress through the unloading of stress from the surrounding faults during the Yangbi earthquake has effectively reduced the seismic risk associated with these faults.