Seismic Swarm PS20220331.1: March–April 2022 Events Southeast of the Loyalty Islands

A seismic swarm designated PS20220331.1 occurred southeast of the Loyalty Islands, New Caledonia, beginning at 20:56 on 30 March 2022 and concluding at 13:05 on 1 April 2022. Over 40 hours and 8 minutes, 16 earthquakes were recorded at a location 254 km east of Tadine. The sequence featured two events exceeding magnitude 6.9, with the majority occurring at depths of 10 km and two slightly deeper shocks at 16 km and 20 km.

The swarm initiated with a magnitude 6.9 earthquake at 20:56 on 30 March, followed within hours by a magnitude 5.1 event. Activity intensified on 31 March, including a magnitude 7.0 shock at 05:44 and a magnitude 6.4 event at 19:50. Additional events ranged from magnitude 4.4 to 5.7, with 13 of the 16 total shocks clustered between 03:06 and 21:59 on 31 March. The final magnitude 5.5 earthquake occurred on 1 April. Depths remained predominantly shallow, consistent with regional crustal dynamics.

The Loyalty Islands region lies within the southwest Pacific plate boundary zone, where convergence between the Australian and Pacific plates drives frequent seismicity. This tectonic setting produces both mainshock-aftershock sequences and episodic swarms along the Loyalty Ridge and adjacent subduction-related structures. Shallow focal depths of 10–20 km align with known patterns of upper-plate deformation in the area.

Since 1 January 2000, 23 seismic swarms have been documented in the region. These occurred in the following years with the indicated counts: 2001 (1), 2003 (2), 2004 (1), 2005 (2), 2007 (3), 2008 (1), 2009 (2), 2011 (1), 2015 (1), 2017 (2), 2018 (3), 2019 (1), and 2021 (3). Strong earthquakes of magnitude 7.0 or greater since 2000 have also clustered nearby, including events of M 7.3 (27 December 2003), M 7.1 (3 January 2004), M 7.1 (29 August 2018), M 7.0 (31 March 2022), M 7.7 (19 May 2023), and M 7.1 (20 May 2023). Several of these large events nucleated within 100 km of the PS20220331.1 swarm centroid.

The 2022 swarm shares temporal proximity with the M 7.0 earthquake of 31 March 2022, located only 11 km from the swarm center, suggesting possible triggering or stress redistribution effects. Later 2023 events at 50–73 km distance further illustrate the persistent seismic productivity of this segment of the plate boundary.

• USGS Earthquake Catalog (earthquake.usgs.gov)
• SeismoSight internal swarm classification records