Seismic Swarm PS20110113.1: Insights into Activity Near the Loyalty Islands

The seismic swarm designated PS20110113.1 occurred in a tectonically dynamic region approximately 118 km north-northeast of Tadine in New Caledonia's Loyalty Islands. This area lies within the broader Vanuatu subduction zone, where the Australian Plate converges with the Pacific Plate, driving frequent seismic events along the New Hebrides Trench and associated ridges. The swarm spanned 33 hours and 23 minutes from 16:16 on 13 January 2011 to 01:39 on 15 January 2011, registering 18 earthquakes. Such swarms reflect episodic stress release in a setting characterized by oblique subduction and back-arc spreading.

The sequence began with a magnitude 7.0 event at a depth of 9 km. Subsequent shocks included multiple events of magnitude 5.0 to 5.9, predominantly at depths of 10 km, with a few deeper occurrences reaching 23 km and 40 km. Activity clustered tightly in time during the first day before tapering, consistent with swarm patterns where no single mainshock dominates but rather a series of comparably sized events occurs. This distribution underscores the region's capacity for rapid, distributed seismicity influenced by local fault networks and fluid migration along the plate interface.

Geologically, the Loyalty Islands sit atop the Loyalty Ridge, an uplifted feature resulting from Miocene to recent tectonic interactions. The ridge experiences compression and extension due to the subduction dynamics, with historical records showing elevated seismicity since at least the early 2000s. Since 1 January 2000, ten swarms have been documented in the vicinity, occurring in 2006 (one swarm), 2007 (four swarms), 2008 (two swarms), 2010 (two swarms), and 2011 (this event). These episodes highlight recurring patterns of clustered activity amid the ongoing plate convergence.

Notable strong earthquakes in the same period further illustrate the hazard potential. A magnitude 7.1 event struck 118 km south of Isangel, Vanuatu, on 7 December 2023, located 64 km from the swarm center. Earlier, a magnitude 7.0 quake occurred 85 km east-northeast of Tadine on 19 November 2017, 75 km from the center. The 13 January 2011 magnitude 7.0 event itself registered 22 km from the swarm epicenter. Additional magnitude 7.3 events on 9 April 2008 were centered in the Loyalty Islands and 71 km southwest of Isangel, at distances of 59 km and 67 km respectively. A magnitude 7.1 shock on 25 March 2007 lay 119 km south of Isangel, 69 km from the swarm focus.

These events collectively demonstrate the persistent seismic productivity of the region, where subduction-related megathrust and intraslab processes generate both isolated large quakes and swarm sequences. Depths predominantly in the upper crust align with typical rupture zones in this arc setting. Monitoring such swarms provides valuable data on stress accumulation and release along the plate boundary.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical events).
Geological Survey of New Caledonia tectonic summaries.