Seismic Hazards in Southwestern Philippines: The 2010 M7.6 Earthquake Sequence

The Philippines lies within the tectonically active Pacific Ring of Fire, where the Philippine Sea Plate subducts beneath the Sunda Plate along the Philippine Trench. This convergence drives frequent large earthquakes and volcanic activity across the archipelago. Southwestern Mindanao, near the locations of Bantogon and surrounding coastal areas, experiences elevated seismicity due to its position along the Cotabato Trench and the Philippine Fault system. These structures accommodate oblique convergence, producing both shallow crustal events and deeper intraslab earthquakes.

On 23 July 2010 at 22:51 UTC, a magnitude 7.6 earthquake struck 61 km west of Bantogon at a depth of 578 km. The event occurred within the subducting slab, consistent with the region's capacity to generate deep-focus seismicity. Three additional strong earthquakes occurred in close succession that day: a magnitude 7.5 located 72 km south of Panubigan, a magnitude 7.3 located 68 km west of Gadung, and the main M7.6 shock. These events clustered within a 40 km radius, highlighting localized stress release along the subduction interface.

Historical records since 2000 show repeated magnitude 7+ earthquakes in the same corridor. Notable examples include the M7.5 event of 5 March 2002 near Mindanao and the M7.1 earthquake of 11 July 2024 located 106 km west-southwest of Sangay. The 2010 sequence remains one of the most energetic episodes in the instrumental record for this segment of the trench. Depths exceeding 500 km indicate brittle failure within the cold core of the downgoing slab, a phenomenon documented in several circum-Pacific subduction zones.

Ground shaking from the 2010 mainshock was widely felt across Mindanao and parts of the Visayas, though the great focal depth moderated surface intensities relative to shallower events of similar magnitude. No major tsunami was generated, as expected for an intraslab earthquake at this depth. Post-event analyses by regional seismic networks confirmed aftershock patterns aligned with the slab geometry rather than shallow crustal faults.

Ongoing monitoring underscores the persistent hazard. The combination of rapid plate convergence rates exceeding 6 cm per year and the presence of both megathrust and intraslab source zones means that southwestern Philippines will continue to experience infrequent but potentially damaging earthquakes. Improved building codes and early-warning systems remain essential for reducing future impacts in this densely populated region.

References

• USGS Earthquake Catalog (event parameters 2000–2024)
• Philippine Institute of Volcanology and Seismology (PHIVOLCS) regional seismicity reports