Seismic Swarm PS20181205.1: Earthquake Activity Southeast of New Caledonia

Seismic swarm PS20181205.1 occurred in a tectonically active region 166 km east-southeast of Tadine on the island of Maré in New Caledonia’s Loyalty Islands. The swarm began at 04:14 on 5 December 2018 and concluded at 07:07 on 8 December 2018, spanning 74 hours and 52 minutes. During this interval, 42 earthquakes were recorded, with the largest event reaching magnitude 7.5 at a depth of 10 km.

The area lies within the influence of the Vanuatu subduction zone, where the Australian plate converges with the Pacific plate. This setting produces frequent seismic activity along the New Hebrides Trench and associated back-arc structures. Shallow crustal depths typical of the recorded events reflect normal faulting and strike-slip mechanisms common in the overriding plate near the Loyalty Ridge.

The swarm initiated with a rapid succession of events on 5 December, including the magnitude 7.5 mainshock followed within minutes by several magnitude 5.9 aftershocks. Subsequent activity included a magnitude 6.6 event later that morning and a magnitude 5.9 shock on 6 December at 9 km depth. Event depths remained predominantly at 10 km until the final recorded earthquake on 8 December, which occurred at 36 km.

New Caledonia and its offshore regions have experienced recurrent earthquake swarms since 2000, with 23 documented episodes. These swarms cluster in years such as 2007 (four swarms), 2017 (four swarms), and 2008 (three swarms). The 2018 sequence followed another swarm earlier that year and was located near the epicenters of prior strong earthquakes, including magnitude 7.1 events in August 2018 and January 2004.

Strong earthquakes exceeding magnitude 7.0 have struck within 75 km of the swarm centroid since 2000. The magnitude 7.5 event of 5 December 2018 occurred only 3 km from the swarm center, underscoring the persistent seismic hazard along this segment of the plate boundary.

Swarm sequences in this region typically feature a high proportion of moderate-magnitude events clustered over several days, consistent with fluid migration or stress triggering along pre-existing faults rather than a single large rupture. The 2018 swarm exhibited this pattern, with 39 of the 42 events falling between magnitude 5.0 and 6.6.

Ongoing monitoring by regional seismic networks continues to track aftershock decay and background seismicity in the Loyalty Islands area. Such data contribute to refined hazard assessments for New Caledonia’s eastern offshore zones.

References

USGS Earthquake Catalog (events 2000–2018)
Geoscience Australia Regional Seismicity Reports
IRD New Caledonia Geophysical Observatory Publications