Seismic Swarm PS20210214.1 Southeast of the Loyalty Islands

The Loyalty Islands region, located in the southwestern Pacific Ocean as part of New Caledonia, sits at a complex tectonic boundary where the Australian Plate subducts beneath the Pacific Plate. This interaction along the Vanuatu subduction zone and adjacent features such as the Loyalty Ridge produces elevated seismicity, including frequent earthquake swarms. The ridge itself represents an ancient volcanic arc system influenced by back-arc spreading in the North Fiji Basin and strike-slip faulting along the Hunter Fracture Zone. These geological conditions have generated recurrent seismic episodes documented over decades.

Seismic swarm PS20210214.1 began at 21:48 on 13 February 2021 and concluded at 21:26 on 15 February 2021, lasting 47 hours and 38 minutes. During this interval, 14 earthquakes were recorded southeast of the Loyalty Islands. Event magnitudes ranged from 4.5 to 5.7, with the majority occurring at shallow depths of 10 km. Notable events included a magnitude 5.3 quake at 08:09 on 14 February, a magnitude 5.7 event at 15:01 the same day, and a magnitude 5.2 shock at 21:21 on 15 February. Two events reached greater depths of 52 km and 108 km, indicating activity across multiple crustal levels.

This swarm occurred shortly after a magnitude 7.7 earthquake on 10 February 2021, centered only 9 km from the swarm epicentral area. Such proximity suggests possible triggering through static stress transfer or aftershock processes within the same fault network. A subsequent magnitude 7.7 event struck the same region on 19 May 2023, 93 km from the swarm center, underscoring the persistent seismic hazard.

Historical records indicate 12 swarms in the area since 1 January 2000. These occurred in 2001, 2003, 2004, 2005, 2008, 2009 (two swarms), 2011, 2016, 2018, and 2021 (two swarms). The clustering pattern reflects episodic release of strain along the plate interface and subsidiary faults, a characteristic behavior in this subduction-influenced setting.

The February 2021 swarm highlights the dynamic nature of seismicity southeast of the Loyalty Islands. Continued monitoring remains essential given the region's tectonic framework and history of large-magnitude events.

References

USGS Earthquake Catalog
SeismoSight internal swarm classification PS20210214.1