Seismogenic fault geometry of the Taiwan suture zone from arc-continent collision to the collision-subduction transition

We investigate the three-dimensional seismogenic architecture and along-strike evolution of the Longitudinal Valley (LV) suture zone in eastern Taiwan, where arc–continent collision between the Philippine Sea Plate (PSP) and Eurasian Plate progressively evolves into northward subduction of the PSP. Using seismic waveforms recorded by CWASN, BATS, and TSMIP between 1991 and 2024, we apply a machine-learning phase picker to obtain internally consistent P- and S-wave arrivals and relocate all events with a nonlinear probabilistic location algorithm and a 3-D velocity model of Taiwan. This procedure yields a more complete and structurally coherent earthquake catalog, allowing us to image the geometry of major fault systems and their relation to tomographic velocity and Vp/Vs anomalies. The relocated seismicity and tomography show that the west-dipping Central Range Fault (CRF) and east-dipping Longitudinal Valley Fault (LVF) and interaction change systematically from south to north. In the southern collision segment, the CRF is steep and bounds a strong, uplifted Central Range block, whereas the LVF is a moderately to steeply east-dipping fault localized beneath the Coastal Range. Northward, the CRF dip shallows and truncates the shallow LVF, producing a structurally complex interaction zone. In the northern transition to subduction, LVF-related seismicity deepens into a near-vertical, high-Vp/Vs structure accompanied by a west-dipping back-thrust, together defining the western boundary of the descending Philippine Sea Plate. We further document a high-angle left-lateral strikeslip fault within the suture zone that is active mainly in the southern LV but remains confined to the Central Range. Finally, a high-velocity, low-Vp/Vs block beneath the Southern Longitudinal Trough is interpreted as remnant forearc crust. Together, these results provide a unified seismological framework for the structural evolution of Taiwan’s suture zone from collision to incipient subduction.