Seismic Swarm PS20181216.1 Near Lakatoro, Vanuatu

A seismic swarm designated PS20181216.1 occurred approximately 75 km southeast of Lakatoro, Vanuatu, from 20:21 on 15 December 2018 to 16:15 on 16 December 2018. Over 19 hours and 53 minutes, eight earthquakes were recorded with magnitudes ranging from 4.4 to 5.5. The events clustered tightly in both time and space, characteristic of swarm activity rather than a typical mainshock-aftershock sequence.

The sequence began with a magnitude 5.4 event at 11 km depth on 15 December at 20:21:54. Subsequent shocks included a magnitude 5.2 at 10 km depth and a magnitude 5.1 at 60 km depth early on 16 December, followed by additional events at shallow depths around 9–10 km, including a magnitude 5.5 at 9 km. Two magnitude 5.0 events concluded the swarm near 16:15, one at 10 km and one at 58 km depth. Most activity concentrated in the upper crust, consistent with tectonic stress release along regional fault systems.

Vanuatu lies along the New Hebrides subduction zone, where the Australian Plate converges with and subducts beneath the Pacific Plate at rates exceeding 10 cm per year. This tectonic setting produces frequent seismicity and volcanic activity throughout the archipelago. The swarm location near Malekula Island aligns with the broader deformation zone influenced by the subduction interface and associated crustal faults. Shallow depths dominate regional events, though deeper activity can occur within the subducting slab.

Since 1 January 2000, twelve seismic swarms have been documented in the area, occurring in 2002 (two swarms), 2004, 2005, 2008, 2009 (three swarms), 2010, 2012, 2015, and 2016. These episodes highlight recurrent episodic strain release without a single dominant rupture. The strongest regional earthquake since 2000 was a magnitude 7.0 event on 28 April 2016 located 3 km northwest of Norsup, approximately 97 km from the 2018 swarm center. That event underscored the potential for larger individual shocks within the same tectonic framework.

The 2018 swarm fits the established pattern of clustered moderate-magnitude activity driven by plate-boundary stresses. No escalation to a major event was observed, and the sequence remained confined to magnitudes below 6.0. Continued monitoring of the New Hebrides region remains essential given its position on the Pacific Ring of Fire and history of both seismic and volcanic hazards.

References
SeismoSight internal swarm classification PS20181216.1 (2018 data).
USGS Earthquake Catalog (regional events since 2000).
Global CMT catalog (tectonic parameters for Vanuatu subduction zone).