M 7.0; 85 km ENE of Tadine, New Caledonia; (19 Nov 2017) (98km from the swarm center)
M 7.0; 133 km SSE of Isangel, Vanuatu; (3 Sep 2011) (96km from the swarm center)
M 7.0; 118 km NNE of Tadine, New Caledonia; (13 Jan 2011) (41km from the swarm center)
M 7.3; LOYALTY ISLANDS; (9 Apr 2008) (35km from the swarm center)
M 7.3; 71 km SW of Isangel, Vanuatu; (9 Apr 2008) (42km from the swarm center)
M 7.1; 119 km S of Isangel, Vanuatu; (25 Mar 2007) (58km from the swarm center)
Seismic Swarm PS20231224.2: Analysis of Activity near Isangel, Vanuatu
Seismic swarm PS20231224.2 occurred approximately 117 km south-southwest of Isangel, Vanuatu, in a tectonically active segment of the southwest Pacific. The sequence began at 20:43 on 23 December 2023 and concluded at 17:32 on 24 December 2023, spanning 20 hours and 49 minutes. Five earthquakes were recorded during this interval, with all events located at a focal depth of 10 km.
The sequence initiated with a magnitude 5.2 event at 20:43:45 on 23 December. Subsequent activity included a magnitude 5.9 earthquake at 01:59:39 on 24 December, followed closely by a magnitude 5.8 event at 02:00:32. Later shocks comprised a magnitude 4.5 earthquake at 17:02:48 and a magnitude 5.0 event at 17:32:47 on the same day. These events illustrate a typical swarm pattern of clustered moderate-magnitude shocks without a single dominant mainshock.
The region forms part of the Vanuatu subduction zone, where the Australian Plate converges with the Pacific Plate along the New Hebrides Trench. This setting produces frequent shallow seismicity and occasional deeper events associated with slab subduction. Historical records indicate 24 prior swarms in the area since 2000, with notable concentrations in 2011 (five swarms) and 2017 (four swarms). Earlier episodes occurred in 2006, 2007, 2008, 2010, 2015, 2016, 2018, and 2019.
Strong regional earthquakes since 2000 further underscore the zone’s activity. These include a magnitude 7.1 event 118 km south of Isangel on 7 December 2023, a magnitude 7.0 shock 85 km east-northeast of Tadine, New Caledonia, on 19 November 2017, and multiple magnitude 7.0–7.3 events in 2011, 2008, and 2007, all located within 35–98 km of the December 2023 swarm centroid.
Such swarms are characteristic of stress redistribution along the plate interface and associated crustal faults. The shallow depths recorded in the present sequence align with the predominantly crustal seismicity observed throughout the Vanuatu arc.
References: USGS Earthquake Catalog (earthquake.usgs.gov) Global CMT Catalog Pacific Island Seismicity Reports, Geoscience Australia