Seismic Swarm PS20190426.1 Near Union, Philippines: Geological Context and Event Analysis

On 26 April 2019, a seismic swarm designated PS20190426.1 was recorded 41 km east of Union, Philippines. The sequence began at 05:26 and concluded at 20:28 local time, encompassing nine earthquakes over 15 hours and one minute. This activity aligns with the region's established pattern of clustered seismicity within the tectonically dynamic Philippine archipelago.

The swarm comprised the following events with their recorded times, magnitudes, and depths: at 05:26:59 a magnitude 5.4 earthquake at 10 km depth; at 05:27:08 a magnitude 5.2 event at 80 km; at 06:04:20 a magnitude 5.1 quake at 10 km; at 06:04:27 a magnitude 5.0 shock at 69 km; at 06:29:13 a magnitude 4.6 event at 10 km; at 08:04:23 a magnitude 5.3 earthquake at 10 km; at 08:04:30 a magnitude 5.3 event at 69 km; at 10:54:50 a magnitude 5.0 quake at 10 km; and at 20:28:30 a magnitude 5.3 shock at 16 km. Multiple events shared shallow focal depths around 10 km, while others occurred at intermediate depths near 69–80 km, indicating varied rupture mechanisms within the local crust and upper mantle.

The Philippines lies within the Pacific Ring of Fire, where the Philippine Sea Plate subducts beneath the Eurasian Plate along major boundaries including the Philippine Trench to the east and the Manila Trench to the west. This subduction drives frequent seismic activity, supplemented by strike-slip motion along the Philippine Fault system. Historical records since 2000 document seven swarms in the vicinity of Union, occurring in 2012 (three swarms), 2014 (two swarms), 2018 (one swarm), and 2019 (one swarm). These clusters reflect episodic stress release along fault segments influenced by both subduction and intra-arc deformation.

Such swarms typically arise from fluid migration or aseismic slip triggering successive failures, a process common in subduction-related settings. Depths spanning 10–80 km suggest involvement of both crustal faults and the subducting slab interface. No surface rupture or significant damage was associated with this sequence, consistent with moderate magnitudes and variable depths.

Ongoing monitoring by regional networks supports improved understanding of swarm recurrence, aiding hazard assessment in this high-seismicity zone.

References

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