Shallow seismic reflection imaging of the Inabanga–Clarin portion of the North Bohol Fault, Central Visayas, Philippines

Abstract

On 15 October 2013, a magnitude 7.2 earthquake was generated from a previously unidentified fault in the island of Bohol. This fault was named the North Bohol Fault (NBF) by authorities. We investigated the geometry of the Inabanga–Clarin portion of the NBF using three high-resolution shallow seismic reflection profiles to image sections of the fault up to 150 m depth not seen in trenching and regional offshore seismic profiles. These seismic profiles are along the Calubian, Napo, and Caluwasan transects which run perpendicular to the N40 ∘E strike of the NBF. Reverse faults were identified in the Calubian and Napo profiles, whereas a positive flower structure was seen in the Caluwasan profile. Normal faults were also identified in the Caluwasan and Napo profiles. This study corroborates the observations in earlier trenching studies that measured the reverse fault dip angle and direction of the NBF at 70 ∘SE. It also demonstrates that topographic flexures are the surface manifestation of steeply dipping faults. The downthrown block of the reverse faults in the Calubian profile defines a depression on the surface; the Napo seismic profile displacement of 3 m is consistent with the 3-m-high surface rupture in Barangay Anonang; and the flower structure in the Caluwasan profile is related to the pressure ridge and right lateral offset stream on the surface. Furthermore, the presence of normal faults as well as the other deformational features is consistent with the transpressional regime described in the literature, wherein the principal horizontal stress is oriented NW–SE. These findings complement earlier geomorphic and trenching investigations of the NBF and demonstrate the application of a tool to image the subsurface and characterize undescribed or hidden faults, which is necessary for earthquake hazard assessment and attendant risk mitigation and prevention planning.