Seismic Swarm PS20120206.1 Near Amdos, Philippines

Seismic swarm PS20120206.1 was recorded northwest of Amdos, Philippines, commencing at 03:49 on 6 February 2012 and concluding at 04:05 on 7 February 2012. Within this 24-hour and 16-minute interval, eight earthquakes were registered, providing a snapshot of clustered seismic activity in a tectonically active region.

The sequence began with a magnitude 6.7 event at 11 km depth, followed by subsequent shocks including magnitudes 5.6, 6.0, 6.1, 5.0, 5.8, 5.3, and a final 5.0 event. Depths ranged primarily between 9 and 15 km, with one outlier at 50 km. This pattern reflects typical swarm behavior, where multiple events occur without a single dominant mainshock-aftershock progression.

The Philippines lies within the Pacific Ring of Fire, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Philippine Trench. This convergence drives frequent seismicity, augmented by the left-lateral Philippine Fault system that transects the archipelago. Amdos is situated in a zone influenced by both subduction-related thrusting and strike-slip faulting, contributing to shallow crustal earthquakes.

Historical records since 2000 indicate limited swarm occurrences in this locality, with only one prior swarm documented in 2011. The 2013 magnitude 7.1 earthquake centered 4 km southeast of Sagbayan, approximately 83 km from the swarm epicenter, underscores the region's capacity for larger events along nearby fault segments.

Such swarms offer insights into stress redistribution along local faults without necessarily signaling an imminent major rupture. Monitoring these episodes aids in refining seismic hazard assessments for nearby communities.

References

USGS Earthquake Catalog
Philippine Institute of Volcanology and Seismology (PHIVOLCS) Annual Reports
Global CMT Catalog for tectonic setting verification