Seismic Swarm PS20251010.1 Near San Ignacio, Philippines: Geological Context and Event Analysis

The earthquake swarm designated PS20251010.1 occurred 26 km east-southeast of San Ignacio, Philippines, from 01:43 on 10 October 2025 to 10:27 on 11 October 2025. Over 32 hours and 43 minutes, 21 earthquakes were recorded, highlighting the region's persistent tectonic instability.

The sequence began with a magnitude 7.4 mainshock at 59 km depth. Subsequent events included multiple magnitude 5+ shocks, with the largest aftershock reaching magnitude 6.7 at 47 km depth. Depths varied widely from 6 km to 86 km, indicating activity across both shallow crustal and deeper subduction-related structures. Key events encompassed magnitudes of 5.9, 5.6, 5.5, and several 5.1–5.3 shocks clustered within the first 12 hours.

This swarm aligns with the Philippines' position along the Pacific Ring of Fire, where the Philippine Sea Plate subducts beneath the Eurasian Plate. The area near San Ignacio lies close to the Philippine Fault Zone and associated thrust systems, which accommodate oblique convergence and generate frequent seismic sequences. Historical data show 11 prior swarms since 2000, concentrated in years such as 2023 (four events), with single occurrences in 2001, 2009, 2017, 2018, 2021, 2022, and 2024.

Notable strong earthquakes nearby include the magnitude 7.4 event of 10 October 2025 located 12 km east of Santiago (24 km from the swarm center), the magnitude 7.1 shock of 11 August 2021 (94 km distant), and the magnitude 7.5 event of 1 January 2001 (58 km distant). These reflect the high strain accumulation along regional faults.

Swarm activity of this type often signals fluid migration or stress transfer within the fault network rather than a single large rupture. Depths spanning the seismogenic zone underscore the complex interplay between the subducting slab and overlying crust in this segment of Mindanao.

References

SeismoSight internal swarm classification PS20251010.1 (2025).
USGS Earthquake Catalog for regional tectonics and historical events (updated through 2025).
Philippine Institute of Volcanology and Seismology (PHIVOLCS) tectonic framework reports.