Neotectonics and Paleoseismology of a Previously Unmapped Reverse Fault in Central Philippines―Insights from the Magnitude Mw 7.2 Bohol Earthquake of 15 October 2013

Abstract:

The 2013 Mw 7.2 Bohol earthquake is considered, historically, the strongest to hit the island of Bohol. It is also very significant because it is associated with a previously unmapped fault and because there is a rarity in documented ground rupture associated with earthquakes generated by reverse faulting mechanism in the Philippines. The tentatively 6 km-long ground rupture, now known as the North Bohol Fault (NBF), is situated on the northern sector of the island. The NBF, which trends N40°-60°E and dips approximately 45°SE, is a reverse fault which is is aligned to the ENE-trending foothills in the municipalities of Inabanga and Buenavista, Bohol. The geometry and kinematics of the NBF was examined through documentation of fault trace patterns, analysis of along-strike variations in scarp morphologies and heights, and through observations of fault characteristics from natural exposures. Paleoseismic trenching in Inabanga reveals that at least three large-magnitude earthquakes occurred in the past ~12,000 years. The long return period seems to be consistent with the subtleness of the pre-existing morphotectonic features that are associated with the 2013 ground rupture. Transects constructed from onshore structural readings and analysis of offshore seismic reflection profiles reveal the structural setting of the island and the possible relation of pre-existing offshore fault-fold structures in the western portion of Bohol Island to the still-unmapped SW extent of the NBF. The long return period for earthquakes can be explained by accommodation of slip in the Visayan Sea Basin partially along NE-SW trending folds in Bohol. Several other possibilities are also discussed to explain why the earthquake return period could be even shorter than estimated. Lastly, this study emphasizes the importance of using a wide-array of methods in mapping of tectonic structures to the effective evaluation of seismic hazards in the region.