Analysis of Crustal Deformation of the 2023 Ms 6.2 Jishishan Earthquake in Gansu Province

Objectives: An earthquake with Ms 6.2 occurred in Jishishan county, Gansu province on December 18th, 2023. Due to the lack of prior observations, the crustal deformation of the study area is still not clear, hindering the understanding of the mechanism of the earthquake. Methods: To solve this issue, we collect co- and pre-seismic Sentinel-1 SLC images covering the study area, and use D-InSAR (Differential Interferometric Synthetic Aperture Radar) and SBAS (Small Baseline Subset) technology to obtain the co-seismic and inter-seismic deformation, respectively. Furthermore, both inter-seismic InSAR and GNSS observations are integrated to obtain the high-accuracy and high-resolution three-dimensional deformation field. Subsequently, we invert the slip distribution of the seismogenic fault using the Steepest Descent Method (SDM) program with the constraint of coseismic deformation. The fault model is constructed based on 3D electrical structure, aftershocks, and InSAR deformation field. In addition, we invert the dextral strike slip rate, dip slip, and locking depth of the seismogenic fault with the constraint of three-dimensional deformation. Results: Both ascending and descending coseismic InSAR deformation exhibit symmetrical elliptical uplift with a maximum uplift of 8.9 cm along the radar line of sight direction, indicating a thrust earthquake, and there is no obvious surface rupture. The seismogenic fault is mainly characterized by reverse movement and did not rupture the surface, and the peak slip is 0.24 m at the depth of 13.4 km. The geodetic moment is~1.93×10¹⁸ N·m, corresponding to an Mw 6.1 event. The dextral strike slip and dip slip rate of the seismogenic fault are~1.9 mm/a and~2.4 mm/a, respectively. The locking depth of the seismogenic fault is about 17 km, close the the depth of this earthquake. Conclusions: The seismogenic fault may be a SWW-dipping hidden fault near the southern segment of the Northern Lajishan Fault, which thrusts towards the basin direction and parallel to the Lajishan fault. The main seismogenic fault may be southern segment of the the SWW-dipping Northern Lajishan Fault. This main thrust fault and the NEE dipping back-thrust fault jointly uplift when the earthquake occurs. The Static Coulomb stress calculation result indicates that the future seismic risk of the southern section of the Northern Lajishan Fault, the southern end of the Southern Lajishan Fault, the central and southern section of the Daotanghe-Linxia fault, and the western end of the Northern West Qinling Fault cannot be ignored. This earthquake is the result of a regional stress regulation under the the northeast expansion of the Tibet Plateau.