Seismic Swarm PS20020306.1: Analysis of Events Near Daliao, Philippines

A seismic swarm designated PS20020306.1 occurred 5 km north of Daliao in the Philippines between 21:16 on 5 March 2002 and 19:19 on 6 March 2002. In 22 hours and 3 minutes, six earthquakes were recorded. The sequence began with a magnitude 7.5 event at 31 km depth, followed by five additional shocks ranging from magnitude 4.4 to 5.4, all at depths near 33 km.

The initial magnitude 7.5 earthquake struck at 21:16:09 on 5 March 2002. Subsequent events included a magnitude 5.0 at 21:39:10, magnitude 5.3 at 21:48:26, magnitude 4.4 at 14:29:06 on 6 March, magnitude 5.4 at 14:36:55, and magnitude 5.1 at 19:19:52. Depths remained consistent around 31–33 km, indicating activity within the mid-crustal region.

This swarm took place in a tectonically active zone of Mindanao, part of the Philippine archipelago situated along the Pacific Ring of Fire. The region experiences frequent seismicity due to the convergence of the Philippine Sea Plate and the Eurasian Plate, with the Philippine Fault and associated subduction zones driving deformation. Historical records show repeated moderate-to-large earthquakes in southern Mindanao, reflecting ongoing plate boundary interactions that have shaped the island’s geology over millions of years.

Notable strong events since 2000 include a magnitude 7.5 earthquake in Mindanao on 5 March 2002, located 46 km from the swarm center, and a magnitude 7.8 event 26 km southwest of Kablalan on 7 June 2026, situated 54 km from the swarm epicenter. These occurrences underscore the persistent seismic hazard in the area.

The swarm provides insight into clustered seismic behavior, where an initial large shock triggered a series of smaller events along nearby fault segments. Depths clustered near 33 km suggest rupture within a stable crustal layer influenced by regional compression. Such sequences are common in subduction-related settings and help delineate active fault structures without producing surface rupture.

Philippine seismic monitoring networks have improved detection of these patterns since the early 2000s, aiding in refined hazard assessments for Mindanao communities. Continued observation supports better understanding of how swarms relate to larger tectonic cycles in the archipelago.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Philippine Institute of Volcanology and Seismology (phivolcs.dost.gov.ph)