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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. Approximately 18 h after the mainshock, we began to document the surface rupture near Yuli Town. Our result suggests the surface rupture 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 1951 surface ruptures inside Yuli Town, with a similar amount of left-lateral cross-fault 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 1.4 m just south of Yuli, with the fault scarp resembling 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.

To provide information towards understanding Taiwan’s earthquake hazard and risk, the multi-disciplinary Taiwan Earthquake Model (TEM) project, supported by the Ministry of Sciences and Technology, had prepared and published a first version of seismic hazard assessment maps of Taiwan in 2016, together with an on-land seismogenic structure database of the island. In the years following the publication of this first version, we have constructed an updated version of this database. Seven structures were identified and added to the database. Based on additional information, we have also updated the structural parameters of four existing structures, as well as systematically updated the parameters of all structures to include more complete parameter uncertainties. This update of the database represents the most up-to-date information of seismogenic structures in Taiwan, and would provide better constraints for future seismic hazard assessment and mitigation studies.