Geometry and structure of the offshore segments of the Philippine Fault in Central Philippines

The Philippine Fault, an ~1500 left-lateral strike-slip fault, traverses the entire Philippine archipelago. Previous studies on the Philippine Fault are centered on the characteristics of its onshore traces. However, many of its segments are located offshore and are still unmapped. Moreover, recent surface-rupturing earthquakes documented along the Philippine Fault nucleated along the offshore segments and propagated onshore. Among these earthquakes is the most recent 2020 Mw 6.6 Masbate earthquake. The occurrence of these earthquakes emphasizes the necessity to understand the characteristics of the offshore segments. In this study, we investigated the geometry and structure of the offshore segments of the Philippine Fault in Masbate-Leyte Sea, Cataingan Bay, Naro Bay, Masbate Pass, and the Sibuyan Sea using marine geophysical datasets, including multi-beam bathymetry, sub-bottom acoustic profiles, and deep seismic profiles. We conducted detailed mapping and refinement of the offshore segments of the Philippine Fault through morphotectonic and structural analysis. The offshore segments of the Philippine Fault are characterized by pull-apart basins, linear ridges, and valleys, offset submarine channels, fault scarps, and releasing bends and steps. The subsurface structure indicates that the left-lateral movement is accompanied by significant normal faulting. These suggest that the central Philippine Fault is dominated by transtensional deformation which accommodates the changes in strike resulting from its arcuate trace. Correlating with earthquake data, we were also able to constrain the extent of the source fault of the 2020 M w 6.6 Masbate earthquakes. Such efforts are crucial to better understand the overall dynamics of the Philippine Fault and accurately assess its seismogenic potential.