Temporal and Spatial Variation Characteristics of Fluid Solid Tide Parameters before 2021 Yunnan Yangbi M6.4 Earthquake

Taking the Yangbi M6.4 earthquake in Yunnan Province on May 21, 2021 as an example, the water level data of 32 wells in Yunnan Province from January 2017 to July 2021 were collected. The spatio-temporal variation characteristics of M2 wave tidal factor were calculated by baytap-G program, and the relationship between the spatio-temporal distribution characteristics of fluid earth tidal parameters and seismogenesis was discussed. The results show that:(1) Before the Yangbe earthquake, the North-South fluid tide factor of the epicenter showed an upward trend, showing stress accumulation; The East-West fluid tidal factor of the epicenter shows a downward trend, which is manifested in stress release. Combined with the focal mechanism type of the earthquake, it is concluded that the temporal and spatial variation form of tidal factor is mainly affected by the main stress direction of the earthquake, compression in the north-south direction and tension in the east-west direction. (2) According to the linear pore elasticity theory and coseismic volume strain theory, it is concluded that the water level rises in the north-south direction of the epicenter, indicating that the compression stress in the north-south direction is basically controlled; The water level drops in the east-west direction of the epicenter, indicating that the east-west direction is basically controlled by tensile tension. The conclusion is consistent with the focal mechanism solution of the earthquake, which verifies the correctness of the conclusion again. (3) The borehole pressure is distributed in four quadrants around the strike slip fault fracture zone, and the water level tidal factor and water level near the epicenter basically accord with the four quadrant distribution.