Seismic Swarm PS20100723.1: Deep-Focus Activity West of Gadung, Philippines

A notable seismic swarm designated PS20100723.1 occurred 68 km west of Gadung in the southern Philippines. The sequence began at 22:08 UTC on 23 July 2010 and concluded at 11:56 UTC on 24 July 2010, spanning 13 hours and 48 minutes. During this interval, 11 earthquakes were recorded, with focal depths ranging from 553 km to 640 km. These events clustered tightly in both space and time, characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

The largest events reached magnitudes of 7.6, 7.5, and 7.3, all at depths exceeding 570 km. Such deep hypocenters place the activity well within the mantle transition zone, consistent with rupture along a subducting slab. The remaining events ranged from magnitude 5.0 to 6.6 and exhibited similar depth distributions, indicating repeated failure at comparable levels within the slab.

The southern Philippines lies at the complex junction of the Philippine Sea Plate, the Sunda Plate, and smaller microplates. Convergence is accommodated primarily along the Philippine Trench to the east and the Cotabato Trench to the west. Deep-focus seismicity in this region arises from the descent of oceanic lithosphere into the mantle, where phase transitions and thermal stresses can trigger brittle failure even at extreme pressures. Earthquakes between 500 km and 650 km depth are relatively common in the western Pacific subduction zones and often occur in clusters when slab geometry or mineralogical boundaries favor localized stress accumulation.

Historical records since 2000 show only one prior swarm in the immediate vicinity, occurring in 2002. That earlier episode was also dominated by deep events. In addition, several strong earthquakes have been documented within 100 km of the swarm centroid, including an M 7.6 event 61 km west of Bantogon and an M 7.5 event 72 km south of Panubigan on the same day in 2010. An M 7.1 earthquake occurred 106 km west-southwest of Sangay in July 2024, underscoring the persistent seismic productivity of the area. An earlier M 7.5 event near Mindanao in March 2002 further illustrates the recurrence of large deep-focus shocks.

These observations align with the tectonic framework of the Philippine archipelago, which experiences frequent deep seismicity due to ongoing subduction. The 2010 swarm highlights how transient stress perturbations within the slab can produce multiple large-magnitude ruptures over a short period without generating a classical aftershock decay pattern. Continued monitoring of deep seismicity in this region contributes to improved understanding of slab dynamics and associated hazards at intermediate and great depths.

References

SeismoSight internal swarm catalog PS20100723.1
USGS Earthquake Catalog (historical events since 2000)
Philippine Institute of Volcanology and Seismology (PHIVOLCS) tectonic summaries