Unusual stress rotations within the Philippines possibly caused by slip heterogeneity along the Philippine fault

To understand the complex tectonic interactions between subduction margins and transcurrent faults, we investigated the spatial distribution of stress orientations in the Philippines by using focal mechanisms derived from the waveform data of regional broadband seismic stations and Global Centroid Moment Tensor solutions. We investigated the spatial distribution of stress orientations by dividing the region into a central region containing the Philippine fault (PF) and regions to the east and west of the PF. Our stress tensor inversion results show that the σ₁ axes in the central and eastern regions are WNW-ESE oriented and parallel to the relative plate motion orientations. However, in the western region, the orientations of σ₁ axes were different. In particular, the orientations of the σ₁ axes in the southern part (Bohol region) differ substantially (differences exceed approximately 60°) from those of the relative plate motion. Furthermore, the orientations of the σ₁ axes in the northern part (Mindoro and southwestern Luzon regions) differ by approximately 30° from those found throughout the Philippines. The σ₁ axes in the Bohol region and the σ₃ axes in the Mindoro and southwestern Luzon regions are parallel to the strike of the PF. It was demonstrated that our numerical model incorporating slip on the PF and interplate coupling is able to successfully reproduce the features of the observed stress pattern.