Seismic Swarm PS20171116.1 in the Loyalty Islands

A seismic swarm designated PS20171116.1 occurred in the Loyalty Islands region of New Caledonia between 20:23 on 15 November 2017 and 14:52 on 16 November 2017. In 18 hours and 29 minutes, 12 earthquakes were recorded with magnitudes ranging from 4.9 to 5.9 and focal depths between 8 and 13 km. The sequence began with a magnitude 5.4 event at 20:23 on 15 November, followed by additional events of magnitude 5.0 at 21:10 and 21:42. Activity continued into 16 November with events at 03:00 (M4.9), 03:27 (M5.1), 03:28 (M5.4), 03:35 (M5.0), 07:02 (M5.9), 07:03 (M5.4), 07:37 (M5.3), 09:54 (M5.0), and concluded with a magnitude 5.2 event at 14:52.

This swarm reflects typical clustered seismicity in an area characterized by active subduction-related tectonics. The Loyalty Islands lie along the Loyalty Ridge, where the Australian Plate interacts with the Pacific Plate near the Vanuatu subduction zone. Shallow crustal depths of the recorded events align with regional patterns of brittle failure in the upper lithosphere influenced by oblique convergence and back-arc extension.

Seismic swarms have occurred frequently in the Loyalty Islands since 2000, with a total of 19 documented episodes. These took place in 2001, 2003, 2004, 2005 (two swarms), 2006, 2007 (four swarms), 2008 (two swarms), 2011 (three swarms), 2015, 2016, and 2017 (two swarms). Such recurrent activity underscores the region's persistent strain accumulation along plate boundary structures.

Notable larger earthquakes have also struck nearby since 2000. A magnitude 7.0 event occurred 85 km ENE of Tadine on 19 November 2017, approximately 41 km from the swarm center. Another magnitude 7.5 earthquake took place 166 km ESE of Tadine on 5 December 2018, 98 km from the swarm location. The timing of the November 2017 swarm immediately preceded the magnitude 7.0 mainshock, suggesting possible stress transfer within the same fault system.

Insights from the swarm data indicate a rapid onset and decay typical of fluid-influenced or triggered sequences in subduction forearc settings. Most events clustered at depths of 10 km, with the two largest (magnitudes 5.4 and 5.9) occurring at 8–10 km. This pattern is consistent with historical swarm behavior in the Loyalty Islands, where moderate-magnitude clusters often serve as indicators of ongoing tectonic adjustment without producing widespread surface rupture.

References

• SeismoSight internal swarm classification records
• USGS earthquake catalog (global events since 2000)