Seismic Swarm S20230519.1 Southeast of the Loyalty Islands

Seismic swarm S20230519.1 occurred southeast of the Loyalty Islands, New Caledonia, in the southwestern Pacific Ocean. The sequence began at 03:40 on 19 May 2023 and concluded at 02:25 on 21 May 2023, lasting 46 hours and 45 minutes. During this period, 34 earthquakes were recorded, with magnitudes ranging from 4.1 to 4.9 and focal depths predominantly near 10 km, though several events reached depths of 23–35 km.

The Loyalty Islands lie along the Loyalty Ridge, a submerged volcanic arc influenced by the complex tectonics of the New Hebrides subduction zone. Here, the Australian Plate converges with the Pacific Plate, generating frequent seismicity. This tectonic setting produces both interplate thrust events and intraplate normal or strike-slip earthquakes within the overriding plate. The swarm's shallow depths align with typical activity in the upper crust of this region, where stress accumulation along faults is relieved through clustered events rather than a single large rupture.

Analysis of the sequence shows a rapid onset with multiple events of magnitude 4.9 within the first hours, followed by a sustained series of magnitude 4.4–4.8 shocks throughout 19 May. Activity continued at a lower rate on 20 May before tapering off early on 21 May. Depths remained shallow for most events, consistent with brittle failure in the volcanic and sedimentary rocks of the Loyalty Ridge. No clear mainshock-aftershock pattern emerged, characteristic of swarm behavior driven by fluid migration or aseismic slip rather than a dominant rupture.

The Loyalty Islands region has experienced recurrent swarms. Since 1 January 2000, eight such episodes have been documented, occurring in 2008 (one swarm), 2009 (two swarms), 2021 (three swarms), 2022 (one swarm), and 2023 (one swarm, the present event). These clusters highlight the area's propensity for episodic seismic release without producing great earthquakes.

Such swarms contribute to ongoing monitoring of strain accumulation along the plate boundary. While individual events in S20230519.1 posed limited hazard due to their moderate magnitudes and offshore location, they underscore the persistent seismic risk in this tectonically active margin.

References

USGS Earthquake Catalog
Global CMT Project
Geoscience Australia Seismic Database