Seismic Swarm PS20140531.1: Analysis of Earthquake Activity Near Cortes, Philippines

A seismic swarm designated PS20140531.1 occurred on 31 May 2014, centered 37 km east-northeast of Cortes in Bohol province, Philippines. The sequence began at 18:13 UTC and concluded at 19:54 UTC, lasting one hour and forty minutes. During this period, nine earthquakes were recorded with magnitudes ranging from 4.6 to 5.4 and focal depths between 9 km and 15 km.

The sequence opened with a magnitude 4.6 event at 18:13:50, followed rapidly by stronger shocks. Peak activity included a magnitude 5.2 quake at 18:46:48 and a magnitude 5.4 event at 19:54:15 that marked the swarm’s close. Most events clustered at depths of 9–11 km, consistent with shallow crustal faulting typical of the region.

The Philippines lies within the Pacific Ring of Fire, where complex interactions between the Philippine Sea Plate and the Sunda Plate drive frequent seismicity. Bohol Island sits amid the Philippine Mobile Belt, influenced by the Philippine Fault and regional thrust systems. These structures accommodate oblique convergence and produce both strike-slip and reverse-faulting earthquakes. Historical records document recurrent moderate-magnitude swarms in the vicinity, reflecting episodic strain release along secondary faults.

Since 2000, four swarms have been identified in the area: one in 2009, two in 2012, and the 2014 event analyzed here. Such clustering indicates localized stress accumulation and release without progression to a single mainshock. The most recent strong regional earthquake, a magnitude 7.6 event on 2 December 2023 located 19 km east of Gamut, occurred approximately 89 km from the 2014 swarm center, underscoring ongoing tectonic activity across the broader Mindanao–Bohol corridor.

Seismic swarms like PS20140531.1 provide valuable data for understanding short-term fault behavior and refining regional hazard assessments. Continued monitoring remains essential given the archipelago’s high seismicity and population exposure.