Shallow Slip Pattern of the Laohushan-Haiyuan Fault Zone Characterized by Geodetic and Seismicity Observations

Summary Kinematic characteristics (creeping or locked) and high-precision seismic catalogs can constrain the shallow (<20 km) slip pattern of active fault zone. We collect Sentinel-1 Synthetic Aperture Radar (SAR) images and extract a high-resolution deformation velocity field along the active Laohushan-Haiyuan (LHS-HY) fault zone in the northeastern margin of the Qinghai-Tibet Plateau. We invert the shallow fault coupling and slip distribution using two-dimensional (2D) and three-dimensional (3D) models, indicating that the fault zone exhibits an alternating pattern of large strong coupling asperities and creeping zones, and the deep slip rate decreases from 5.2 mm/yr in the west to 3.4 mm/yr in the east, accompanied by a transition from strike-slip to dip-slip components. Then we calculate cumulative seismic energy release, seismic slip rate, and statistical parameters including $b - \textit{value}$, coefficient of variation of seismicity interevent times, and Nearest-Neighbor Distance (NND) with regional seismic catalog. The geodetic and seismic results demonstrate a complex shallow slip pattern in the fault zone. The following characteristics are highlighted. A significant throughgoing locked-creeping transition zone with variable depth range extends from the eastern part of the Laohushan segment (LHS) to the eastern segment of Haiyuan Fault (HYE). A shallow (<6 km) creeping zone with weak coupling and seismicity in the western HYE segment differs from the shallow part of the locked-creeping transition zone between the eastern LHS segment and the western part of the western segment of Haiyuan Fault (HYW). A transition zone with strong coupling and active seismicity in the eastern HYE segment ranges from 4 km to 12 km in depth. The results provide new insights into the shallow slip behavior of the LHS-HY fault zone, and offer valuable references for seismic hazard assessment in the region.