Seismic Activity in Leyte, Philippines: The August 2020 Earthquake Swarm

Leyte lies within the tectonically active Philippine Mobile Belt, where the Philippine Sea Plate subducts beneath the Sunda Plate along the Philippine Trench to the east. The island is also transected by the Philippine Fault, a major left-lateral strike-slip system that accommodates oblique convergence. This setting produces frequent moderate-to-large earthquakes, with historical events including the 1948 M7.0 and 2017 M6.5 Leyte quakes that caused significant ground rupture along fault segments.

The S20200818.1 swarm began at 00:03 UTC on 18 August 2020 with a magnitude 6.6 event at 10 km depth. Over the following 62 hours and 4 minutes, 46 earthquakes were recorded, concluding at 14:07 on 20 August. Depths ranged from 1 km to 22 km, with the majority occurring at shallow crustal levels between 5 km and 15 km. Magnitudes clustered between 3.0 and 4.8 after the initial mainshock, indicating a rapid decay in energy release typical of aftershock sequences within a swarm-like pattern.

The sequence showed two notable peaks of activity on 18 August, with events at 10:18 (M4.4), 11:21 (M4.6), and 21:50 (M4.8) representing the largest aftershocks. Subsequent days recorded progressively smaller events, consistent with Omori-law decay. Shallow depths suggest rupture within the upper crust along or near the Philippine Fault trace in central Leyte.

Such swarms contribute to long-term strain release along the fault but rarely exceed the energy of the triggering mainshock. Regional monitoring by PHIVOLCS confirms that Leyte experiences elevated seismicity rates compared with neighboring islands, reflecting its position at the intersection of subduction and strike-slip regimes. Updated hazard assessments continue to identify the northern and central segments of the Philippine Fault as capable of producing future events of M6.5 or greater.

References
Philippine Institute of Volcanology and Seismology (PHIVOLCS) – Leyte Fault Zone reports
USGS Earthquake Catalog – Regional seismicity data
Scientific literature on Philippine Fault kinematics (2020 updates)