Seismic Swarm PS20090211.1: Analysis of Activity Southeast of Sarangani, Philippines

An earthquake swarm designated PS20090211.1 was recorded southeast of Sarangani in the Philippines between 17:34 on 11 February 2009 and 05:30 on 14 February 2009. The sequence lasted 59 hours and 55 minutes and comprised 67 events. The swarm occurred 196 km south-southeast of Sarangani at depths predominantly between 14 km and 90 km.

The largest event reached magnitude 7.2 at 20 km depth on 11 February at 17:34:50. Subsequent activity included multiple events above magnitude 5.0, with notable shocks of magnitude 6.3, 6.0 (three instances), and 5.9. Depths clustered around 35 km for many aftershocks, though shallower (14–16 km) and deeper (up to 90 km) events also occurred. The sequence displayed typical swarm characteristics: a rapid onset, high event rate in the first 24 hours, and gradual decay without a single dominant mainshock-aftershock pattern.

The region lies within the Philippine Mobile Belt, a tectonically complex zone at the convergent boundary between the Philippine Sea Plate and the Sunda Plate. Subduction along the Philippine Trench to the east and interaction with the Cotabato Fault System to the west drive frequent seismicity. The swarm location aligns with the transition from oceanic to continental crust in southern Mindanao, where oblique convergence produces both shallow crustal faults and intermediate-depth Wadati-Benioff zone activity.

Historical records since 2000 indicate only two prior swarms in the immediate area: one in 2000 and another in 2006. The 2009 swarm coincided with a magnitude 7.2 event located 35 km from the swarm centroid, underscoring the area’s capacity for both isolated large shocks and clustered sequences.

Insights from the event distribution reveal a vertically extensive source volume consistent with fluid migration or stress triggering along pre-existing structures. Magnitudes remained below 7.2 after the initial shock, and the overall energy release was distributed across dozens of moderate events rather than concentrated in aftershocks of a single rupture.

References

SeismoSight internal swarm classification PS20090211.1
USGS Earthquake Catalog (Philippine region, 2000–2009)
Philippine Institute of Volcanology and Seismology regional tectonics summary