Seismogenic structure of the major collision boundary in the southeastern Taiwan

Abstract

Located at the heart of the Taiwan orogenic process, the Longitudinal Valley is thesurface representation of the plate collision and characterized by its narrow and long topographic features. Due to the oblique convergence of the Philippine Sea plate with respect to the Eurasian plate, it is widely considered the collision is in its initial stage in the southern Longitudinal Valley and Coastal Range. We explored the lateral variations of the subsurface structure due to the initiation of active collision using 3-D Vp and Vs models and relocated seismicity. High-resolution 3-D Vp and Vs models are determined using data collected by an island wide seismic network and a high-density temporary seismic network. We observe very complex structures with large lateral and vertical variations of seismic property in the study area. Velocity perturbations at shallow depths are largely corresponding to the variations in the surface geology. There is a negative velocity anomaly beneath the Western Foothills in the northwest of the study area where relatively thick sedimentary basins are found at surface. Beneath the Central Mountain Range, higher velocity anomaly becomes dominant at shallow depths where highly metamorphosed rocks are observable in the surface. Velocity anomaly further to the east beneath the Longitudinal Valley and the Coastal Range is characterized by a strong negative anomaly, which may represent the unconsolidated thick sediments observed at the surface. The overall distributions of velocity perturbations at mid-crustaldepths (~16 km) are quite different from those observed in the slice views at the shallower depth. Negative velocity is predominant in the most part at the depth including the Western Foothills and the Central Mountain Range. A narrow and long linear positive velocity anomaly is noticeable beneath the Longitudinal Valley and its southern extension. In the east of the high velocity zone, another linear feature with low velocity perturbation is