Seismic Swarm PS20210812.1: Geological Context and Event Analysis Near Pondaguitan, Philippines

The seismic swarm designated PS20210812.1 occurred in a tectonically active region of southern Mindanao, Philippines, centered approximately 106 km east-southeast of Pondaguitan in Davao Occidental. This area lies within the Philippine Mobile Belt, where complex interactions between the Philippine Sea Plate and the Sunda Plate drive frequent seismic activity. Subduction along the Philippine Trench to the east, combined with strike-slip motion along the Philippine Fault system, generates significant crustal stress. Depths of recorded events in the swarm ranged from 10 km to 55 km, consistent with both shallow crustal faulting and deeper subduction-related processes characteristic of the region.

The swarm initiated at 17:46 on 11 August 2021 with a magnitude 7.1 earthquake at 55 km depth. Over the subsequent 27 hours and 17 minutes, seven additional events were registered, concluding at 21:03 on 12 August 2021. The sequence included a magnitude 5.2 event at 35 km depth early on 12 August, followed by a cluster of activity in the afternoon featuring magnitudes 5.1 (35 km), 5.7 (10 km), and 4.7 (42 km). Evening events comprised magnitudes 5.2 (54 km), 5.0 (53 km), and a final 5.3 (47 km). These events reflect progressive stress release along local fault structures, with the initial mainshock likely triggering aftershocks at varying depths.

Historical records indicate this is the fourth swarm in the vicinity since 2000, following single-swarm episodes in 2001, 2009, and 2017. The region has also experienced notable strong earthquakes, including a magnitude 7.0 event on 29 December 2018 located 96 km east-southeast of Pondaguitan, approximately 30 km from the 2021 swarm center. The 2021 magnitude 7.1 mainshock itself was recorded 60 km east-northeast of Pondaguitan. Such recurrent activity underscores the persistent tectonic loading in Mindanao, where convergence rates exceed 7 cm per year.

This swarm provides insight into episodic seismic behavior in a subduction-influenced setting. The distribution of magnitudes and depths suggests a combination of mainshock-aftershock dynamics and possible swarm-like triggering from fluid migration or aseismic slip, common in Philippine arc environments. Monitoring such sequences aids in understanding regional hazard patterns, as the area remains susceptible to future events due to ongoing plate boundary deformation.

References

• Philippine Institute of Volcanology and Seismology (PHIVOLCS) earthquake catalogs
• United States Geological Survey (USGS) earthquake data archives
• SeismoSight internal swarm classification records