Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing are-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10-15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.