Seismic Swarm PS20110510.1 in the Loyalty Islands Region

The seismic swarm PS20110510.1 occurred 122 km northeast of Wé, New Caledonia, in the Loyalty Islands area. It began at 08:55 on 10 May 2011 and concluded at 10:32 on 11 May 2011, spanning 25 hours and 37 minutes. During this period, 24 earthquakes were recorded, with the largest event registering magnitude 6.8 at a depth of 11 km.

This region lies along the convergent boundary between the Australian and Pacific plates. The Australian plate subducts eastward beneath the Pacific plate at rates of approximately 5–8 cm per year, generating frequent seismic activity. The Loyalty Islands sit near the Vanuatu subduction zone, where intermediate-depth and shallow crustal earthquakes are common due to plate interaction and associated faulting.

The swarm initiated with the magnitude 6.8 mainshock, followed by a sequence of aftershocks predominantly between magnitudes 5.0 and 5.9. Depths clustered around 10 km, with one outlier at 60 km. Subsequent events included magnitudes 5.9, 5.6, and multiple 5.3 events, reflecting typical aftershock decay patterns in a subduction-related setting.

Historical data indicate 12 swarms in the area since 2000. These occurred in 2006 (1 swarm), 2007 (4 swarms), 2008 (2 swarms), 2010 (2 swarms), and 2011 (3 swarms). Such clustering aligns with the tectonic regime, where stress accumulation along the subduction interface periodically triggers episodic seismicity.

Notable strong earthquakes since 2000 further illustrate regional hazard. A magnitude 7.0 event struck 118 km NNE of Tadine, New Caledonia, on 13 January 2011, only 26 km from the swarm center. Earlier, magnitude 7.3 events occurred on 25 December 2010 (91 km from center), 9 April 2008 (54 km and 61 km from center), and a magnitude 7.1 on 7 December 2023 (95 km from center). These events underscore persistent seismic productivity driven by ongoing subduction.

The May 2011 swarm exemplifies how moderate-to-large events can initiate prolonged sequences in this environment. Depths near 10 km suggest activity within the upper crust or along the plate interface, consistent with the area's geological structure. No surface rupture or significant tsunami was associated with this particular sequence.

Ongoing monitoring by regional seismic networks continues to track activity in the Loyalty Islands, supporting improved understanding of subduction zone dynamics and seismic hazard assessment for New Caledonia and nearby Vanuatu.

References

USGS Earthquake Catalog
Global CMT Catalog
Geoscience Australia Earthquake Database