关键词: 青藏高原南部, 定日地震, 同震形变, 震后形变, 平面状正断层, 重力势能

Abstract:

On 20 March 2020, an M_W5.7 shallow normal-faulting earthquake struck the Shenzha-dingjie fault zone, southern Xizang, providing a significant opportunity to understand the regional seismogenic structure in the little-studied area using the modern satellite geodetic technology. The ascending and descending SAR images from the Sentinel-1A satellite are utilized to derive both the coseismic LOS deformation and the first half-year postseismic LOS deformation associated with this earthquake. The ranges of the coseismic LOS displacements in the ascending and descending interferograms are -13.6 to 2.8cm and -14.6 to 2.2cm, respectively, while the range of the postseismic LOS displacements in the ascending interferograms is -4.8 to 2.6cm. Our coseismic modeling result shows that the earthquake nucleated on a previously unidentified normal fault buried at depths of 1.6 to 5.7m, and that the maximum coseismic slip of ~0.85m is located at a depth of ~4.7km. The coseismic slip distribution model generated a seismic moment of ~3.65×10¹⁷N·m, equivalent to a moment magnitude(M_W)of 5.7. The result of the postseismic deformation throughout nearly half a year following the mainshock shows that the deformation magnitude gradually increases over time and that the deformation increased significantly in the first three months, while the increase trend was relatively slow in the later period. The ranges of the cumulative postseismic surface deformation are -4.8 to 2.6cm. In-depth postseismic analysis reveals that postseismic afterslip and coseismic slip are located at the same fault plane and that the postseismic slip limited to a depth of 0.8 to 4.8km, lies in the up-dip area of the coseismic slip. The seismic moment released by postseismic slip distribution is ~7.8×10¹⁶N·m, corresponding to an M_W5.2 earthquake. The peaking postseismic slip of ~0.23m is located at a depth of ~2km. The deep coseismic slip and shallow postseismic afterslip of this earthquake image an almost complete single planar seismogenic fault structure. By a comprehensive analysis of the inversions, regional topography, landforms and active fault kinematics, we conclude that contrasts in the gravitational potential energy of the regional structure may be the main cause of this earthquake. In addition, coseismic Coulomb stress modelling shows the fairly strong stress loading at both the southern and northern branch fault segments of the Shenzha-dingjie fault zone, with the stress values reaching 0.04MPa and 0.18MPa, respectively. Given that these two branch faults exhibit relatively large positive Coulomb failure stress changes without seismicity during and after this earthquake, the risk associated with these two faults should be given attention. This study demonstrates that the combined data of coseismic and postseismic surface deformation can precisely detect the underground seismogenic fault structure of the hidden normal-faulting earthquake.

Key words: Southern Qinghai-Xizang Plateau, Dingri earthquake, coseismic deformation, postseismic deformation, planar normal fault, gravitational potential energy