Geodetic constraints on the September 2022 Guanshan and Chihshang earthquakes, eastern Taiwan

We characterize the spatiotemporal patterns of ground deformation caused by an earthquake doublet: the September 17, 2022, M_L 6.6 Guanshan and the September 18, 2022, M_L 6.8 Chihshang earthquakes occurred on the Central Range fault, eastern Taiwan. We use geodetic data collected from continuous and campaign-mode GNSS stations, as well as two precise leveling routes to estimate coseismic displacements and invert for fault slip distributions. The M_L 6.6 foreshock caused northwestward horizontal displacements and uplift reaching 200 mm and 170 mm, respectively, in the region between Chihshang and Taitung. Seventeen hours later, the M_L 6.8 mainshock generated coseismic displacements about four times larger than the foreshock, with horizontal displacements exceeding 900 mm and vertical displacements of 800 mm in the area between Guanshan and Ruisui. The maximum horizontal and vertical coseismic displacements of the entire earthquake sequence exceed one meter. The epoch-by-epoch high-rate GNSS data reveal significant seismic shaking, with maximum displacement exceeding 600 mm and 1100 mm during the foreshock and mainshock ruptures, respectively, correlating with severe infrastructure damage near surface ruptures. The dense spatial coverage of networks allows us to map the largest surface deformation along the Yuli fault, a branch of the steeply west-dipping Central Range fault, as well as the associated pop-ups along the east-dipping Longitudinal Valley fault. This observation suggests a likely coseismic and/or postseismic slip along the Longitudinal Valley fault. Our slip model indicates a maximum slip of approximately 3 m at a depth of 4.5 km to the west of Yuli, primarily on the Central Range fault. The coseismic slip extends over 50 km along the fault with two asperities near the hypocenter and Yuli. In addition, the Longitudinal Valley fault is characterized by shallow slip, with a maximum of 0.85 m at depths of 0–3 km.