Seismic Swarm PS20050211.1: Analysis of Activity East of New Caledonia

A seismic swarm designated PS20050211.1 occurred approximately 160 km east-southeast of Tadine, New Caledonia, between 16:53 on 10 February 2005 and 07:42 on 11 February 2005. The sequence lasted 14 hours and 49 minutes and comprised five earthquakes. This event took place in a tectonically active segment of the southwest Pacific, where the interaction between the Australian and Pacific plates generates frequent seismicity along subduction-related structures.

The individual events recorded during the swarm are as follows: a magnitude 6.3 earthquake at 9 km depth at 16:53:19 on 10 February; a magnitude 6.4 event at 33 km depth two seconds later; a magnitude 5.2 shock at 10 km depth at 17:12:45; a magnitude 5.3 event at 10 km depth at 21:52:20; and a final magnitude 3.9 earthquake at 10 km depth at 07:42:49 on 11 February. The shallow focal depths of most events indicate activity within the upper crust or near the plate interface.

New Caledonia lies on the Norfolk Ridge, a submerged continental fragment situated between the New Caledonia Basin and the Loyalty Basin. To the east, the region is influenced by the Vanuatu subduction zone, where the Australian Plate descends beneath the Pacific Plate. This convergent margin produces intermediate-depth and shallow crustal earthquakes, with occasional clusters reflecting stress release along faults or slab segments. Historical records show that swarm-type sequences are uncommon but documented in the area since 2000, with three prior episodes recorded in 2001, 2003, and 2004.

Stronger individual earthquakes have also occurred nearby. A magnitude 7.5 event struck 166 km east-southeast of Tadine on 5 December 2018, centered 77 km from the 2005 swarm location. Earlier activity included a magnitude 7.1 earthquake 202 km east-southeast of Tadine on 3 January 2004 (62 km from the swarm center) and a magnitude 7.3 shock 201 km east-southeast of Tadine on 27 December 2003 (87 km from the swarm center). These larger events underscore the potential for significant seismic release along the same structural corridor.

Seismic swarms in subduction settings often result from fluid migration, afterslip, or episodic slow-slip phenomena that trigger multiple moderate events without a single dominant mainshock. The 2005 sequence fits this pattern, with two events exceeding magnitude 6.0 occurring within seconds of each other, followed by smaller aftershocks that decayed rapidly. Such activity provides insight into the stress regime and may indicate transient changes in pore pressure along the plate boundary.

Ongoing monitoring by regional networks remains essential for understanding recurrence patterns in this part of the Pacific Ring of Fire. The combination of shallow swarm events and occasional great earthquakes highlights the need for continued assessment of seismic hazard for New Caledonia and surrounding islands.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (global events since 2000)
Geological framework derived from published studies on Norfolk Ridge and Vanuatu subduction zone tectonics