Seismic Swarm Activity in Mindanao, Philippines, December 2023

Mindanao, the southernmost major island of the Philippine archipelago, lies within the tectonically complex Pacific Ring of Fire. The region experiences frequent seismic activity due to the interaction of the Philippine Sea Plate and the Sunda Plate along the Philippine Trench and the Philippine Fault system. Subduction processes and strike-slip faulting create a high level of crustal stress, resulting in both deep and shallow earthquakes as well as occasional swarm sequences.

Between 23:43 on 11 December 2023 and 13:58 on 12 December 2023, a seismic swarm designated PS20231212.1 was recorded in Mindanao. Five earthquakes occurred over 14 hours and 15 minutes. The sequence began with a magnitude 5.0 event at 10 km depth, followed by two additional magnitude 5.0 shocks at 35 km and 10 km depths early on 12 December. A smaller magnitude 3.3 event at 12 km depth preceded the final magnitude 5.1 earthquake at 42 km depth.

Such swarms represent clusters of events closely spaced in time and location without a single dominant mainshock. In tectonically active subduction settings like Mindanao, they often reflect localized stress adjustments along fault segments or fluid migration within the crust. The December 2023 swarm occurred shortly after a magnitude 7.6 earthquake on 2 December 2023 located 19 km east of Gamut, approximately 44 km from the swarm center, suggesting possible stress transfer in the surrounding fault network.

Historical records since 2000 indicate 11 prior swarms in the same general area, with occurrences in 2009 (one swarm), 2014 (two swarms), 2018 (one swarm), 2019 (two swarms), and five swarms in 2023. This pattern underscores Mindanao’s persistent seismic productivity, driven by ongoing plate convergence at rates exceeding 7 cm per year along regional boundaries.

The December 2023 swarm, while moderate in magnitude, illustrates the value of continuous monitoring in understanding short-term seismic clustering. No significant damage or casualties were associated with these events, consistent with their moderate sizes and varying focal depths. Continued observation remains essential for assessing any evolving seismic hazard in this geologically dynamic region.