Spatial heterogeneous stress orientation in the Philippines caused by the slip heterogeneity along the Philippines fault

Abstract:

In order to understand the complex tectonic interactions between subduction margins and great transcurrent faults, we investigated the spatial distribution of stress orientations in the Philippine crust, by using moment tensor solutions derived from waveform data of regional broadband seismic stations, as well as GCMT solutions. We investigated the detailed spatial distribution of stress by dividing the region into a zone containing the PF, as well as the areas east and west of the PF zone. Our results of stress tensor inversions show that orientations of -axis are nearly parallel to those of relative plate motions at the PF zone and eastern regions. On the other hand, at the western region, orientations of -axis are significantly different from those of the relative plate movements. In particular, the orientations of -axis in a southern part (Bohol region) are substantially different (> ~ 60°) from those of the relative plate motions. Also, the orientations of -axis in northern part (Mindoro region) are different (~ 30°) from those in the entire Philippines. The -axes and -axes in the Bohol and Mindoro regions are almost opposite, and they are parallel to the strike of the PF. This result strongly suggests that observed heterogeneous stress orientations west from the PF are caused by the left-lateral slip on a central part of the fault. This region approximately corresponds to the Masbate fault, a large slipping segment of the PF which is characterized by the occurrence of a large number of moderate-sized events and creeps. Numerical model calculations incorporating the slip on the Masbate fault successfully reproduced the observed features of stress orientations, and imply the possibility of a low coupling at the Philippines and Negros subduction zones.