Seismic Swarm PS20191215.1: Analysis of Earthquake Activity Near New Baclayon, Philippines

On 15 December 2019, a seismic swarm designated PS20191215.1 was recorded 7 km SSW of New Baclayon in the Philippines. The sequence began at 06:11 and concluded at 12:38, spanning 6 hours and 27 minutes during which eight earthquakes occurred. The events exhibited magnitudes ranging from 4.5 to 6.8 and focal depths between 10 and 18 km, consistent with shallow crustal activity in a tectonically active zone.

The strongest event reached magnitude 6.8 at 06:11:51 with a depth of 18 km. Subsequent shocks included a magnitude 5.6 at 07:09:20 (10 km depth) and a magnitude 5.3 at 12:38:53 (10 km depth). Intermediate events registered magnitudes of 5.0 at depths of 11–15 km, alongside a magnitude 4.5 shock at 10 km. These parameters indicate a compact cluster of energy release without a single dominant mainshock, characteristic of swarm behavior.

The Philippines lies within the Pacific Ring of Fire, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Philippine Trench. This convergence drives frequent seismic activity across the archipelago, including strike-slip faulting along the Philippine Fault system. Historical records document major events such as the 1976 Moro Gulf earthquake and the 2013 Bohol earthquake, underscoring the region's persistent tectonic strain accumulation and release.

Swarm activity in this area aligns with broader patterns observed since 2000. Six distinct swarms have occurred in the years 2001, 2002, 2006, 2009, 2010, and 2019. Such sequences often reflect fluid migration or localized stress adjustments along fault segments rather than progressive mainshock-aftershock decay. Depths in the 10–18 km range for the 2019 swarm suggest involvement of the upper crust, where brittle failure predominates.

This event contributes to understanding episodic seismicity in the central Philippines. Continued monitoring by agencies such as PHIVOLCS supports hazard assessment in proximity to populated centers, where even moderate-magnitude swarms can produce perceptible shaking and prompt public safety measures.