Seismic Swarm PS20181016.1 Near New Caledonia

The seismic swarm designated PS20181016.1 occurred in a tectonically active zone 171 km east of Tadine, New Caledonia. It began at 00:28 on 16 October 2018 and concluded at 01:23 on 17 October 2018, spanning 24 hours and 54 minutes. During this interval, 19 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.5 and focal depths primarily between 10 and 17 km.

The sequence opened with a magnitude 6.3 event at 12 km depth, followed rapidly by a magnitude 6.5 shock at 17 km depth. Subsequent activity consisted mainly of magnitude 5.0–5.6 events clustered at approximately 10 km depth. The final recorded event reached magnitude 5.3 at 10 km depth. This pattern illustrates a classic foreshock-mainshock-aftershock progression within a short temporal window, typical of swarm behavior in subduction-related settings.

The location lies along the convergent boundary between the Australian and Pacific plates, where the Australian Plate subducts eastward beneath the Pacific Plate along the New Hebrides Trench system. This tectonic regime produces frequent moderate-to-large earthquakes and episodic swarms due to stress accumulation and release along the megathrust and associated crustal faults. Depths of 10–17 km align with the upper crustal portion of the overriding plate and the shallow megathrust interface.

Since 1 January 2000, 25 swarms have been documented in the same general area. These events cluster in specific years, including multiple occurrences in 2007, 2008, 2011, and 2017. The 2018 swarm represents one of the more energetic episodes in the recent record. Nearby strong earthquakes since 2000 include a magnitude 7.3 event 33 km from the swarm center in December 2003, a magnitude 7.1 event 53 km distant in January 2004, a magnitude 7.1 event 64 km away in August 2018, a magnitude 7.5 event only 22 km distant in December 2018, and a magnitude 7.0 event 98 km away in September 2022. Such proximity underscores the persistent seismic hazard in the Loyalty Islands–Vanuatu segment of the plate boundary.

Overall, swarm PS20181016.1 fits within the established pattern of clustered seismicity driven by ongoing subduction. Continued monitoring remains essential given the region’s history of both swarms and larger mainshock events.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification records