Uplifted late Quaternary marine terraces along the southern coast of Bohol Island, central Philippines, and their active tectonic implications

Abstract The island of Bohol in central Philippines is tectonically active with frequent seismic activities. The disastrous October 2013 earthquake, which occurred on a previously unmapped active fault, shows the urgent need for better understanding of potential seismogenic structures in and around the island. Toward this end, we mapped and analyzed the late Quaternary marine terraces along the southern coast of Bohol Island. Our results show that at least six levels of marine terraces are preserved in southern Bohol. Among them, Terrace 3 was likely formed during the MIS (Marine Isotope Stage) 5e, and is distributed throughout the coast. Three higher terraces are present west of the Loboc River and may have been formed during older sea-level highstands, and two lower terraces are found east of the river, which may correlate with MIS 5a and 5c. Based on the proposed ages and the elevation of the terraces, we calculated the late Quaternary uplift rates of the southern Bohol coast to be between 0.1 and 0.4 mm/yr. We propose the coastal uplift is produced by the activity of the North Mindanao Sea fault offshore southern Bohol, which appears to be a major active fault in the area. The prominent difference of terrace distribution patterns and uplift rates across the Loboc River indicates a potential active backthrust there, probably branching out from the North Mindanao Sea fault, rather than the previously mapped East Bohol fault. The North Mindanao Sea fault poses significant earthquake and tsunami hazard for both Bohol and surrounding islands, making it crucial to further analyze the characteristics of this fault and its paleoseismological records.