Seismotectonics of northeastern Taiwan: Structural characteristics of a transitional area from waning collision to subduction and post-collisional extension

Northeastern Taiwan (121.0-123.0°-23.5-25.5°) is a tectonically complex area composed of subduction, backarc opening, and continental-arc collision. In previous studies, this area was usually divided into two parts, Ilan Plain and Longitude valley, to discuss, but the detail of the transition area between them is still unclear. Therefore, a total of 12030 seismic events recorded by Central Weather Bureau Seismic Network (CWBSN) from January 1994 to July 2005 were used in this study. We relocated these events using three-dimensional velocity model (Wu, 2007) and combined the data from Taiwan Strong Motion Instrumentation Program (TSMIP) to help better determine first motion focal mechanism. In the results, the spatial distribution of the focal mechanisms sequentially switches from thrust, strike-slip, and normal fault northward, and the transition area is dominated by left lateral strike-slip fault systematically. In transition area, the direction of the maximum stress axis, different from the direction of plate collision, is EW and the minimum is consistent with Okinawa trough opening. Then, based on the seismic profiles correlated with tomography (Wu, 2007), a suspected detachment beneath Ilan Plain related to a set of normal faults and a concentrated seismicity zone parallel to the subduction of Phillipine Sea Plate are observed. As a consequence, Coastal Range probably has already subducted along with Phillipine Sea Plate at current time. The southward opening of Okinawa trough was a main driving force in northeastern Taiwan and bending the Taiwan mountain belt, so a left-lateral fault system appears at transition area because of the bedding-slip motion.