Seismic Swarm PS20210203.1 Southeast of the Loyalty Islands

A seismic swarm designated PS20210203.1 occurred southeast of the Loyalty Islands between 23:58 UTC on 2 February 2021 and 01:44 UTC on 3 February 2021. Eight earthquakes were recorded during this 1-hour 45-minute interval. The events clustered tightly in both space and time, with hypocenters at depths of 9–10 km and magnitudes ranging from 5.0 to 5.8.

The sequence began with a magnitude 5.8 event at 23:58:34 on 2 February, followed within minutes by a magnitude 5.2 shock. Subsequent events included magnitudes 5.4, two 5.6 events occurring two seconds apart, and three additional shocks of magnitudes 5.0, 5.3, and 5.0. All but the first event originated at 10 km depth, indicating a shallow crustal source.

The Loyalty Islands lie within the southwestern Pacific plate boundary zone, where the Australian plate subducts beneath the Pacific plate along the New Hebrides Trench system. This tectonic setting produces frequent moderate-to-large earthquakes and occasional earthquake swarms driven by stress transfer along the slab interface and overlying crust. The February 2021 swarm occurred in a region of elevated background seismicity consistent with ongoing subduction-related deformation.

Historical records since 2000 document eleven prior swarms in the same area, occurring in 2001, 2003, 2004, 2005, 2008, 2009 (two swarms), 2011, 2016, 2018, and 2019. These episodes demonstrate recurrent swarm behavior without a clear long-term increase in frequency.

On 10 February 2021, a magnitude 7.7 earthquake struck approximately 27 km from the swarm centroid. This larger event followed the swarm by one week and may reflect continued strain release along the same tectonic structures.

SeismoSight internal classification
USGS earthquake catalog (regional tectonics)