판의 섭입 및 충돌로 인한 대만지역의 3차원 속도구조와 지진인자 재결정

Abstract

3-D Vp and Vs models for the crust and upper mantle beneath the Taiwan area have been determined using selected high-resolution earthquake data from an island-wide seismic network and two local seismic arrays. All earthquakes in the Taiwan CWB catalogue are then relocated using the resultant 3-D Vp and Vs models. The depth of the Moho varies significantly, especially along the E-W direction. In the western Coastal Plain and Western Foothills the depth of the Moho is around 35 km, which deepens gradually eastward, reaches a maximum depth of ~55 km beneath the eastern Central Mountain Range, shallows up rapidly beneath the Longitudinal Valley and Coastal Range, and merges with the thin Philippine Sea Plate offshore of eastern Taiwan. In central Taiwan, the Central Mountain Range (CMR) is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km. Therefore, the uplifted and thickened CMR serves as a backstop for the converging Eurasian and Philippine Sea plates. The crust beneath the CMR is characterized by a brittle, high-velocity and seismically active upper crust (<15 km) and a ductile, low-velocity and aseismic mid-to-lower crust (below 15 km), most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east, from the surrounding hotter upper mantle beneath the thickened continental crust, and from shear heating during active collision. The collision zone in eastern Taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low Vp and high Vp/Vs ratio near the Taitung region in SE Taiwan. It transforms into an active westward steeply dipping seismic zone along a transition zone between the high Vp and Vs oceanic crust and the low Vp and Vs continental crust near Hualien region in central eastern Taiwan. The geometry of the subduction zone in northeastern Taiwan can be clearly imaged