Seismic Swarm PS20240803.1 in Mindanao, Philippines

Mindanao, the southernmost major island of the Philippines, lies within the tectonically active Pacific Ring of Fire. The region experiences frequent seismic activity due to the complex interactions among the Philippine Sea Plate, the Sunda Plate, and smaller crustal blocks. Subduction along the Philippine Trench to the east and the Cotabato Trench to the west, combined with strike-slip faulting on the Philippine Fault system, drives most earthquakes. Depths typically range from shallow crustal events to intermediate depths exceeding 70 km, reflecting both subduction-related and intra-plate deformation.

On 2 August 2024 at 22:23 UTC, seismic swarm PS20240803.1 began near Mindanao and concluded at 04:20 on 3 August 2024, spanning 5 hours and 57 minutes. Seven earthquakes were recorded during this interval. The sequence initiated with a magnitude 6.8 event at 32 km depth, followed within minutes by a magnitude 5.3 shock at 76 km. Subsequent events included magnitudes 5.0 at 45 km, 3.8 at 10 km, 5.2 at 53 km, 5.3 at 83 km, and a final magnitude 6.3 event at 15 km depth. The temporal clustering and varying focal depths illustrate typical swarm behavior, where stress redistribution along local faults triggers multiple ruptures without a single dominant mainshock-aftershock pattern.

Such swarms are not uncommon in Mindanao. Since 2000, fifteen swarms have been documented in the area, occurring in 2001 (1), 2009 (1), 2014 (2), 2018 (1), 2019 (2), 2022 (1), and 2023 (7). These episodes reflect episodic strain release within the island’s fragmented fault network. The 2024 swarm’s maximum magnitude of 6.8 aligns with the upper range observed in prior events, underscoring the potential for moderate-to-strong shaking even in short-duration sequences.

Two notable strong earthquakes have occurred nearby since 2000. A magnitude 7.6 event struck 19 km east of Gamut on 2 December 2023, approximately 62 km from the swarm center. Another magnitude 7.4 earthquake occurred 12 km east of Santiago on 10 October 2025, roughly 93 km from the same location. Both events highlight the region’s capacity for generating destructive shaking and associated hazards such as landslides and tsunamis.

The combination of historical swarm frequency and proximity to large-magnitude ruptures indicates that Mindanao remains under elevated seismic risk. Continued monitoring of depth distributions and temporal patterns can improve short-term hazard assessment for local populations and infrastructure.

References

USGS Earthquake Catalog
Philippine Institute of Volcanology and Seismology (PHIVOLCS) Annual Reports
SeismoSight Internal Swarm Database