Seismotectonics of northeastern Taiwan: Kinematics of the transition from waning collision to subduction and postcollisional extension

Abstract

The northeastern region of Taiwan is one of the few places on Earth that experienced a transition from collision to subduction and back‐arc opening in a young orogenic belt. This provides us a rare opportunity to study the structural characteristics in such a transition. We attempt to analyze the seismicity and stress patterns in this transitional region using data obtained from a dense and high‐quality seismic network. Our results show that from south to north, fault types change from thrust to strike‐slip, then to normal faulting. The transitional domain between Ilan and Hualien is dominated by left‐lateral strike‐slip faulting with a NNW‐SSE minimum principal stress axis, consistent with the opening of the Okinawa Trough. To the north, the Ilan Plain is formed by structural activities of a series of normal fault systems. To the south, the spatial distribution of an earthquake cluster north of Taiwan's Coastal Range appears to parallel the subducting interface between the Philippine Sea and Eurasian plates. This implies the northern extension of the Coastal Range has subducted. Consequently, the opening of the Okinawa Trough becomes an important tectonic driving force in northeastern Taiwan. The westernmost Ryukyu arc is pushed by the opening Okinawa Trough, and moves southward. The northern part of Taiwan's Central Range moves with the arc to bend southeastward, and the Ilan Plain forms in the space north of the range. The bending of the Central Range belt results in bedding plane slip with left‐lateral strike‐slip earthquakes in the transitional domain to accommodate the deformation.