Seismic Swarm PS20170410.1: Analysis of the April 2017 Event Offshore from Pondaguitan, Philippines

The seismic swarm designated PS20170410.1 began at 10:38 on 10 April 2017 and ended at 18:08 on 11 April 2017. Centered 120 km east-southeast of Pondaguitan in the Philippines, the sequence produced 14 earthquakes within 31 hours and 30 minutes. Events ranged in magnitude from 4.6 to 5.8, with focal depths between 9 km and 69 km. The initial pair of events occurred within seconds of each other at 10:38:47 and 10:38:53 UTC, registering magnitudes 5.8 and 5.6 respectively. Subsequent activity included a magnitude 5.1 at 11:08:11 followed by a 5.2 at 11:08:18, then a 4.6 at 11:22:35. On 11 April, additional events clustered around 02:44–02:50, 11:22, 14:42–14:47, 15:55, and 17:41–18:08, with the strongest late-stage shock reaching magnitude 5.7 at 14:42:16.

This swarm occurred in a tectonically active offshore region east of Mindanao. The Philippine archipelago forms part of the Pacific Ring of Fire, where the Philippine Sea Plate converges with the Sunda Plate along the Philippine Trench. Subduction generates frequent moderate-to-large earthquakes, while strike-slip motion along the Philippine Fault and its offshore extensions contributes to shallow crustal seismicity. Depths recorded during the swarm span both crustal and upper-mantle levels, consistent with the transition from interplate thrust events to deeper intraslab activity near the trench.

Historical records maintained since 1 January 2000 indicate only two prior swarms in the same general area: one in 2001 and another in 2009. In contrast, strong individual earthquakes have been more common. A magnitude 7.0 event occurred 96 km east-southeast of Pondaguitan on 29 December 2018, approximately 44 km from the 2017 swarm centroid. Another magnitude 7.0 shock struck 211 km southeast of Pondaguitan on 21 January 2021, 87 km from the swarm center. These larger events illustrate the region’s capacity for both clustered moderate activity and isolated great earthquakes driven by ongoing plate convergence.

Swarm sequences such as PS20170410.1 typically reflect fluid migration or aseismic slip that transiently increases stress on nearby faults without culminating in a single dominant mainshock. The rapid succession of events with mixed depths observed here aligns with patterns documented along subduction margins, where small stress perturbations can trigger multiple ruptures within hours to days. No damage or casualties were associated with this particular swarm.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Philippine Institute of Volcanology and Seismology (PHIVOLCS) regional seismicity summaries
Global CMT Project focal-mechanism database