The island of Taiwan is a by-product of one of the few active collisions between a continent, represented by the Eurasian continental shelf, and an island arc, represented by the northern extension of the Luzon arc on the Philippine Sea plate. To understand better the evolution and current tectonics of this collision, we selected 1260 well-recorded earthquakes from an initial set of 50,000 located by the Taiwan Telemetered Seismograph Network for the determination of one- and three-dimensional P and S wave velocity structures beneath the island. The results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones. In the east, the Philippine oceanic plate is underthrusting the Eurasian plate east of a north-south boundary that is well defined by both seismic activity and a region of high velocity. In the south, the Eurasian continental plate is underthrusting the Philippine Sea plate south of an east-west boundary at about 23 ¿N that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities.

The dip of the subducted continent is shallow until it reaches the Luzon island arc 50 km east of the main island. Structure under the main part of the island north of 23 ¿N reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range. The dip of this low-velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km. This seismic zone lies in a velocity ''saddle'' and marks an apparent offset in the central low-velocity region at 24 ¿N. Evidence for the subduction of the Eurasian continent beneath Taiwan therefore exists everywhere beneath the island, and the low-velocity regions in the mantle support, but do not require, the subduction of some 6--16 km of lower continental crust to depths of at least 50 km. The distinct change in both the velocity structure and the seismic activity at 23 ¿N and the offset of the low-velocity regions at 24 ¿N argue for abrupt changes in the nature of subduction across these latitudes.

These changes may be caused by an interaction of the Luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface. Finally, the velocity structure of the subducting Phillippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest-velocity regions of the plate, but are probably located near the upper edge of the plate. There is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented, and that subduction is presently occurring as far south as 22¿N. ¿ American Geophysical Union 1987