The Ryukyu subduction zone, comprised of the Ryukyu trench, the Ryukyu arc and the Okinawa trough, extends between southern Japan and northern Taiwan. This subduction zone is produced by the northwestward subduction of the Philippine Sea plate beneath the Eurasian plate along the Ryukyu trench. The motion of the Philippine Sea plate is, however, impeded at its northwestern corner by the island of Taiwan, which is reflected in the complicated structure of the Philippine Sea plate at the western end of the Ryukyu subduction zone. Using seismicity, as well as other geophysical and geological data in the Taiwan region, we have investigated the lithospheric structure of the Philippine Sea plate beneath the westhern end of the Ryukyu subduction zone and its tectonic effects.

Although the Philippine Sea plate moves northwestward in general, at its northwestern corner the direction of subduction changes from the northwest to the north. The lithosphere of this northwestern corner of the plate underthrusts and extends westward beneath the island of Taiwan. A western boundary of the lithosphere exists beneath Taiwan. The boundary lies almost along the 121.°E meridian line. The boundary is migrating westward at a rate of about 6 cm/y along with the movement of the Philippine Sea plate. The westward migration of the lithospheric boundary of the plate causes the westward extension of the Okinawa trough, which lags behind the former by about 600,000 years (-35 km). The initial stage of the formation of the Okinawa trough, in turn, put the whole northern Taiwan area under tensional stress.