Neotectonic Framework of Eastern Taiwan: Insights from Coseismic Rupture Patterns of the 2022 Chihshang Earthquake and Paleoseismic Investigations

The Mw 6.9 Chihshang earthquake on 18 September 2022 produced ∼65 km of surface ruptures in eastern Taiwan, marking the most extensive surface faulting since the 1999 Mw 7.6 Chi-Chi earthquake. Field and geophysical data reveal that the event ruptured the west-dipping Central Range fault system (CeRFS), rather than the traditionally dominant Longitudinal Valley fault system (LVFS). The rupture exhibited thrust motion with a significant left-lateral component, showing maximum vertical and horizontal offsets of ∼1.2 and 2.0 m, respectively. In the Changliang area, the 2022 Chihshang earthquake rupture reactivated a pre-existing scarp (∼1.7 m high), indicating cumulative displacements from past earthquakes. Paleoseismic trenching at Songpu identified five surface-rupturing events since ∼A.D. 1525, including the 1951 Longitudinal Valley earthquake sequence and 2022 Chihshang earthquake. Radiocarbon dating constrains three earlier events at A.D. 1658–1796, A.D. 1591–1689, and A.D. 1525–1611, yielding long-term vertical slip rates of 6.7–7.3 mm/yr and up to ∼11.5 mm/yr from stratigraphic reconstruction. The contrasting rupture geometries of the 1951 Longitudinal Valley earthquake sequence and 2022 Chihshang earthquake suggest mechanical interaction and stress transfer between the west-dipping CeRFS and the east-dipping LVFS. These findings redefine the fault architecture of the Longitudinal Valley and highlight the CeRFS as a critical but previously underappreciated source of seismic hazard in eastern Taiwan.