The multi-fault rupture and strain partitioning revealed by the coseismic surface deformation of the 2022 MS6.9 Menyuan earthquake

This study integrated centimeter-level unmanned aerial vehicle (UAV) data, meter- to sub-meter-resolution Gaofen (GF) satellite imagery, and field investigations to map the coseismic surface ruptures and obtain a high-resolution three-dimensional surface deformation field for the 2022 M S 6.9 Menyuan earthquake. Seismological evidence, particularly from high-precision aftershock relocation, was incorporated to constrain the deep fault geometry. The results reveal that the earthquake produced three surface rupture zones: a continuous NWW-trending primary rupture zone (approximately 23 km) along the western segment of the Lenglongling fault; a discontinuous NWW-trending secondary rupture zone (approximately 8.5 km) on its northern side; and a discontinuous NEE-trending secondary rupture zone (approximately 5.6 km) along the eastern segment of the Tuolaishan fault. The maximum surface sinistral offset (3.4 ± 0.3 m) occurred on the primary rupture zone approximately 3 km east of the epicenter, coinciding with the peak seismic moment release at depth. Notably, the northern secondary fault (S3) exhibited dominant thrust motion, different from the strike-slip-dominated primary rupture, highlighting kinematic partitioning among faults. Surface uplift and horizontal shortening were identified near the epicenter within a restraining bend (approximately 800 m long, 200 m wide). We propose that the apparent negative off-fault deformation (OFD) values in the strike-slip direction are compensated by near-field horizontal shortening and vertical uplift. This mechanism, particularly evident in geometrically complex regions such as fault bends and branches, underscores the importance of incorporating multi-fault interactions and geometric complexities into OFD assessments for a comprehensive understanding of coseismic strain partitioning.