Lithospheric Equilibrium and Anisotropy around the 2021 Yangbi M s 6.4 Earthquake in Yunnan, China

Using the gravity/GNSS data of 318 stations observed in 2020, this paper optimizes the Bouguer and free-air gravity anomalies around the 2021 Yangbi M s 6.4 Earthquake, inverses the litho-spheric density structure of the focal area, and obtains the distribution of isostatic additional force borne by the lithosphere. The results show that the Bouguer gravity anomaly in western Yunnan varies from -120 to -360 mGal. As a whole the anomalies are large in the north and small in the south, and the value in the source area of the 2021 Yangbi M s 6.4 Earthquake is about -260 mGal. Significant lateral differences indicates that the crust around the great earthquake does not belong to a solid and stable tectonic unit. The lithosphere in the source area is basically in equilibrium, indicating that the occurrence of the great event is not relative to the lithospheric equilibrium, but to the differential movement of the crust in the horizontal direction. In addition, we obtain the teleseismic SKS phases of 51 stations. As a whole, the polarization direction of fast wave in western Yunnan is approximately vertical to the maximum gradient change direction of regional Bouguer gravity anomaly that reflects the change of Moho.