The 2013 Sagbayan Earthquake and Bohol's Tectonic Setting

On 15 October 2013 at 00:12 local time, a magnitude 7.1 earthquake struck 4 km southeast of Sagbayan in Bohol province, Philippines. The event originated at a depth of 19 km and remains the sole strong earthquake of this scale recorded in the immediate area since 2000. Bohol lies within the tectonically active Philippine Mobile Belt, where convergence between the Philippine Sea Plate and the Eurasian Plate drives regional deformation. The island's geology features extensive karst landscapes formed from Eocene to Miocene limestone sequences overlying older volcanic and sedimentary basement rocks. These formations reflect long-term uplift and erosion linked to ongoing plate-boundary forces. The 2013 rupture occurred along a previously unrecognized reverse fault now identified as the North Bohol Fault. This structure trends northwest-southeast and accommodates compressional strain associated with the broader Philippine Fault System. Although the island had experienced moderate seismicity in prior decades, the event highlighted the presence of blind thrust faults capable of producing significant ground shaking and surface deformation, including the emergence of a coastal uplift zone. Historical records indicate that Bohol has been subject to occasional large earthquakes linked to regional fault networks, yet none matching the 2013 magnitude had occurred nearby in the preceding thirteen years. Post-event studies confirmed that the mainshock was followed by a productive aftershock sequence distributed along the newly recognized fault plane. The earthquake underscored the importance of detailed fault mapping in densely populated island arcs. Ongoing monitoring by the Philippine Institute of Volcanology and Seismology continues to refine models of strain accumulation along Bohol's active structures, contributing to improved seismic hazard assessment for the central Philippines.

References

USGS Earthquake Catalog Philippine Institute of Volcanology and Seismology (PHIVOLCS) reports