Seismic Swarm Activity in Mindanao, Philippines: December 2023

Mindanao, the southernmost major island of the Philippines, occupies a complex tectonic setting at the convergence of the Philippine Sea Plate and the Eurasian Plate. The region experiences frequent seismicity due to subduction along the Philippine Trench to the east and strike-slip motion along the Philippine Fault. These features have shaped Mindanao’s geological history, producing both deep-focus events and shallower crustal earthquakes throughout recorded time. Between 15:20 on 2 December 2023 and 23:27 on 5 December 2023, SeismoSight recorded Swarm S20231203.1 in Mindanao. Over 80 hours and 6 minutes the swarm comprised 49 earthquakes. Magnitudes ranged from 3.0 to 4.9, with the largest events occurring early in the sequence. Focal depths varied widely, from near-surface levels of 1 km to 88 km, indicating activity across both crustal and subducting-slab environments. The temporal distribution showed the highest rate of events during the first 12 hours, followed by a gradual decline. Several magnitude 4.9 earthquakes clustered between 15:20 and 22:02 on 2 December, while later days featured more frequent smaller-magnitude events. Depths remained predominantly between 35 km and 70 km, consistent with the geometry of the subducting slab beneath eastern Mindanao. Seismic swarms are sequences of earthquakes lacking a single dominant mainshock. In subduction-zone settings such as Mindanao they commonly arise from fluid migration, aseismic slip, or stress transfer along the plate interface. The December 2023 swarm fits this pattern, with no event clearly triggering the others and a relatively narrow magnitude range. Historical records since 1 January 2000 document only four comparable swarms in the same region: one in 2009, one in 2018, and two in 2023. This low frequency underscores that intense, short-lived clusters remain uncommon relative to the background rate of isolated earthquakes. The December swarm produced no reported damage or casualties, consistent with the moderate magnitudes and depths involved. Continued monitoring by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) remains essential, as Mindanao’s tectonic environment can generate larger events capable of significant impact. References
SeismoSight internal swarm catalogue S20231203.1
PHIVOLCS seismic bulletins
USGS Earthquake Catalog (historical regional events)