Surface ruptures of the 2022 Guanshan-Chihshang earthquakes in central Longitudinal Valley area, eastern Taiwan

Abstract

Abstract The Mw 6.4 and 6.8 Guanshan-Chihshang earthquakes occurred on 17 and 18 September 2022 resulted in prominent surface ruptures within the Longitudinal Valley in eastern Taiwan, particularly along the Yuli fault in the middle of the valley. Approximately 18 hours after the mainshock, we began to document the surface rupture in the vicinity of Yuli Town, where the rupture transected through the center of the residential area. Our result suggests the surface rupture of the mainshock formed a confined single left-lateral trace in the town of Yuli, characterized by a series of en échelon right-stepping left-lateral faulting geometry. The rupture of 2022 roughly matches the locations of surface ruptures of 1951 inside the Yuli Town, with similar amount of cross-fault left-lateral displacement. North and South of the Yuli residential area, we identified several sections of the surface rupture distributed in the water-saturated paddy fields. The maximum left-lateral displacement recorded across the rupture can reach to 1.4 meters just south of Yuli, with the fault scarp resembles a high-angle west-dipping fault geometry. In addition to the co-seismic surface ruptures, our repeating cross-fault measurements show significant post-seismic shallow after-slip along the Yuli fault. The amount of post-seismic deformation within 3 months after the mainshock is close to, or even higher than the co-seismic cross-fault displacement, consistent with local witness accounts and post-event field photos which showed continuous damage and displacement of building floors and roads after the earthquake. Such shallow post-seismic slips were also observed along the main fault trace in the 2014 South Napa earthquake, and likely represent the shallow elastoplastic behavior of the sub-vertical fault in the young alluvial sediments.