8/17/2019 Neotectonics and Paleoseismology of a Previously Unmapped Reverse Fault in Central Philippines

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Neotectonics and paleoseismology of a previously unmapped reverse fault incentral Philippines―insights f rom the magnitude Mw 7.2 Bohol Earthquake of 15October 2013

Jeremy Rimando, Mario Aurelio, John Dale Dianala, Kristine Joy Taguibao 

The 2013 Mw 7.2 Bohol earthquake is considered, historically, the strongest tohit the island of Bohol. It is also very significant because it is associated with a previouslyunmapped fault and because there is a rarity in documented ground rupture associatedwith earthquakes generated by reverse faulting mechanism in the Philippines. Thetentatively 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 dipsapproximately 45°SE, is a reverse fault which is is aligned to the ENE-trending foothills inthe municipalities of Inabanga and Buenavista, Bohol. The geometry and kinematics ofthe NBF was examined through documentation of fault trace patterns, analysis of along-strike variations in scarp morphologies and heights, and through observations of faultcharacteristics from natural exposures. Paleoseismic trenching in Inabanga reveals thatat least three large-magnitude earthquakes occurred in the past ~12,000 years. The longreturn period seems to be consistent with the subtleness of the pre-existingmorphotectonic features that are associated with the 2013 ground rupture. Transectsconstructed from onshore structural readings and analysis of offshore seismic reflectionprofiles 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-unmappedSW extent of the NBF. The long return period for earthquakes can be explained byaccommodation of slip in the Visayan Sea Basin partially along NE-SW trending folds inBohol. Several other possibilities are also discussed to explain why the earthquake returnperiod could be even shorter than estimated. Lastly, this study emphasizes theimportance of using a wide-array of methods in mapping of tectonic structures tothe effective evaluation of seismic hazards in the region.