FOCAL MECHANISMS AND TECTONICS IN THE TAIWAN-PHILIPPINE REGION

Abstract

Seismic activity and focal mechanisms in the vicinity of Taiwan and the Philippines are studied to elucidate the tectonics in this complex region. Epicentral distribution and vertical profiles of earthquake foci support the idea that the entire Philippines is not a part of the Eurasian plate, but another block of lithosphere and the relative motion between the Philippine Sea and the Eurasian plates is shared by the subduction along the two boundaries, west and east of the Philippines. Thrust type of mechanisms are the dominant mode of deformation along the eastern margin of the Philippines; in contrast, no thrust type of solutions are obtained along the western margin of this islands. Between Taiwan and Luzon, mode of plate consumption is most complex. Seismic activity is mostly shallow and diffuse in a 200km wide zone. Reverse faultings along the eastern margin of Taiwan, strike-slip faultings off the southeastern coast of Taiwan, and normal faultings between the Manila trench and the North-Luzon trough are the major mode of deformation. We believe that the region between Taiwan and Luzon constitutes a left-lateral shear zone due to the gradual transition of site of plate consumption from eastern margin of Taiwan to east Luzon. Subduction under the west-facing Luzon arc in this region is likely to be ending now; this might be causing the opening of the sedimentary wedge behind the Manila trench. Normal faultings in this study probably indicate the initial phase or prespreading phase of opening of the area behind the Manila trench.