Subsurface structure, seismicity patterns, and their implications to tectonic evolution in Taiwan

Three-dimensional velocity structure and relocated earthquake hypocenters have been studied to understand the active tectonic process in and around the island of Taiwan using data collected by an island-wide seismic network and two temporary seismic arrays. Important observations from the resultant 3-D velocity model and relocated earthquake hypocenters include an apparent crustal thickening of the Eurasian plate from west to east, a large-scale low velocity and aseismic region at mid to lower crustal depths beneath the eastern Central Mountain Range, at least five anomalous low velocity regions at shallow depth that can be associated with sediment-filled basins, an anomalous thin zone of east dipping deep seismicity from 20 km to 50 km beneath western central Taiwan, complex velocity structures and seismicity representing current active collision status beneath the Longitudinal Valley and Coastal Range, west dipping seismicity beneath the northern Coastal Range which can be explained by recent GPS observations, east-dipping seismicity at the north extension of Taitung trough in the southern Coastal Range, and an anomalous region of low V_P and high V_P/ V_S ratio beneath the Tatun-Chilung volcano group that may reveal the existence of a magmatic reservoir.; Mountain building process in Taiwan can be subdivided into three characteristic regimes based on observations of seismicity and the resultant 3-D velocity structure. Tectonic processes in northern and southern Taiwan are governed mainly by subduction of the Philippine Sea plate beneath the Eurasian plate and subduction of the South China Sea plate beneath the Philippine Sea plate, respectively. In central Taiwan, the active collision of two plates contributes to the mountain building process and seismicity. Two steeply west-dipping boundary faults defined by earthquake hypocenters separate central Taiwan into three different tectonic environments including (1) active collision in eastern central Taiwan, (2) brittle deformation at shallow depth and ductile deformation at depths deeper than 15 km beneath the eastern Central Mountain Range, and (3) thin skinned tectonics at shallow depth and crustal thickening below the shallow seismogenic zone beneath western central Taiwan resulting from active plate convergence.