Seismic Swarm PS20230519.1 Southeast of the Loyalty Islands

The Loyalty Islands region, located in the southwestern Pacific Ocean as part of New Caledonia, sits within a tectonically active zone influenced by the convergence of the Australian and Pacific plates. Subduction along the nearby New Hebrides Trench generates frequent earthquakes and occasional seismic swarms. The area features a complex arrangement of ridges, basins, and volcanic arcs that contribute to elevated seismic hazard.

Seismic swarms have occurred regularly in this setting. Records since 2000 document 19 such events, distributed across years including 2001, 2003, 2004, 2005, 2008, 2009, 2011, 2015, 2018, 2019, 2021, and 2022. These swarms typically involve clusters of moderate-magnitude events without a single dominant mainshock, reflecting fluid migration or stress redistribution along local faults.

Swarm PS20230519.1 began at 02:57 on 19 May 2023 and concluded at 18:07 on 20 May 2023, lasting 39 hours and 10 minutes. During this interval, 44 earthquakes were recorded southeast of the Loyalty Islands. The sequence opened with a magnitude 7.7 event at 18 km depth, followed by numerous aftershocks with magnitudes between 5.0 and 5.9 and focal depths ranging from 8 km to 49 km. A magnitude 7.1 earthquake occurred at 27 km depth on 20 May, accompanied by additional events up to magnitude 6.5. Depths clustered predominantly between 10 km and 36 km, consistent with activity in the upper crust and shallow subduction interface.

Notable recent strong earthquakes near the swarm center include the magnitude 7.7 event of 19 May 2023 (38 km distant), the magnitude 7.1 event of 20 May 2023 (29 km distant), a magnitude 7.0 earthquake on 31 March 2022 (51 km distant), and a magnitude 7.7 shock on 10 February 2021 (93 km distant). These events underscore the region’s capacity for large-magnitude ruptures.

The swarm illustrates typical patterns of clustered seismicity in subduction-related environments, where initial large events trigger prolonged sequences of smaller shocks. Depths and magnitudes align with known characteristics of the Loyalty Islands forearc and trench slope.

References

USGS Earthquake Catalog
Global CMT Catalog
Geoscience Australia Earthquake Database