Seismicity and tectonics of a continent‐island arc collision zone at the Island of Taiwan

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Abstract
A collision between the Eurasian plate continental margin and a volcanic island arc on the Philippine Sea plate about 4 m.y. ago uplifted the island of Taiwan. Many lines of geological and geophysical evidence suggest that the collision is still active today and is currently propagating southward. Due to plate interaction, geological and tectonic settings have become more complicated since the onset of the Collision. The collision suture is along the Longitudinal Valley (LV) in eastern Taiwan. Seismic studies including earthquake monitoring and seismic refraction lines conducted in 1985 in eastern Taiwan and its offshore area using land seismic stations and ocean bottom seismographs have generated results that better constrain the present‐day plate interaction. Both the crustal velocity structure and earthquake hypocenter distribution suggest that the subduction of the Philippine Sea plate along the Ryukyu trench extends to the east coast of Taiwan near about 23.5°N. Very high seismicity with dominant right‐lateral strike‐slip faulting is found in the back arc region behind the Ryukyu trench. This seismic activity is interpreted as a result of plate convergence and back arc wrenching due to the relative motion between the “Okinawa Platelet” and the Eurasian plate. The region close to the LV in eastern Taiwan showed only shallow and a relatively low level of seismicity during this experiment with higher seismicity to the east of the LV. The lack of intermediate‐depth events suggests that subduction is no longer active along the collision zone and the difference in seismicity between the regions east and west of the LV reflects the difference in strength of the adjacent continental and island arc crusts. To the south of the LV, seismicity continued southward with intermediate‐depth events distributed mainly to the east of Taiwan along a nearly vertical Benioff zone. This supports the argument that convergence and collision are still active south of Taiwan.