New Insights of the Conjugate Seismogenic Structure in the Northernmost Longitudinal Valley Revealed by the 2024 Hualien (Taiwan) Earthquake From Geodetic and Seismic Observations

The Mw 7.4 Hualien earthquake, occurred in the northernmost Longitudinal Valley on 2 April 2024 is the strongest in Taiwan in 25 years. This study investigated fault geometry, slip distribution, and rupture process of the event, using teleseismic, regional strong-motion, and near-field geodetic observations as constraints, along with relocated aftershocks and their focal mechanisms for auxiliary validation and additional constraints. Furthermore, using the preferred rupture model determined in this study and 49 slip models of earthquakes between 1951 and 2022, collected or constructed from previous studies, we investigated the stress triggering of the 2024 Hualien event and reassessed the regional future seismic risk. Based on the joint inversion tests from different data set combinations under three candidate fault geometries, we preferred the model combining SEE-dipping and NWW-dipping faults as the causative structure. The coseismic rupture exhibits unilateral propagation along NNE direction, with significant slip occurring over approximately 30 km during the first 20 s. Combined with tectonic settings, background seismicity and joint finite-fault inversions, we further discussed the seismogenic structure, and inferred that the event may have conjugately ruptured the SEE-dipping deep Longitudinal Valley fault (LVF) and the NWW-dipping offshore backthrust fault. Based on Coulomb stress transfer, we found that preceding events first triggered the SEE-dipping fault, and its initial 6 s rupture subsequently activated the conjugate fault, which aligns with the rupture process we inverted. Additionally, we found that the event further exacerbated the seismic risk of the Ruisui-Shoufeng segment of the LVF.