Westward Extension of the Okinawa Trough at its Western End in the Northern Taiwan Area: Bathymetric and Seismological Evidence

In this paper, we used detailed bathymetry, earthquake distribution and focal mechanisms to study the phenomenon of active westward exten sion of the Okinawa trough in the northern Taiwan area.We found a dis tinguishable subsiding and collapsing area on the continental shelf edge and the continental slope on the northern side of the Okinawa trough.This area extends westwards to at least 121.5°E and includes several morpho logical units related to the existence and formation of three major canyons.The canyons and the morphological units are still evolving through the sedi ment transport and through the subsidence and collapse of material due to the formation of the Okinawa trough.According to the degree of develop ment, we found that these morphological units have developed from the east to the west.There are two parallel E-W trending central graben at the westernmost part of the Okinawa trough, with each corresponding to a narrow shallow seismic belt.The widths of the central graben are 10-15 km.There is geophysical and geological evidence that the formation of these central graben has been extended westwards to the onland area of Taiwan.Focal mechanisms of earthquakes and the topographic features show that the formation of the Okinawa trough is associated with the down-dip ex tensional stress along the subducting slab of the Philippine Sea plate, and most of northern Taiwan and all the northeastern offshore area of Taiwan are under tensional stress.New portions of the Okinawa trough have been forming across the whole width at its western end through subsidence in the continental shelf, the continental slope and the traditionally recognized area of the Okinawa trough in the northeastern Taiwan area, to make the Okinawa trough develop gradually and extend westwards.