Seismic Swarm PS20071014.1 Near New Caledonia: Geological Context and Event Analysis

The seismic swarm designated PS20071014.1 occurred in the waters east of New Caledonia’s Loyalty Islands. It began at 17:45 on 13 October 2007 and concluded at 12:28 on 14 October 2007, spanning 18 hours and 42 minutes. Five earthquakes were recorded during this period, with the largest reaching magnitude 6.1. The swarm’s epicentral area lay approximately 154 km east-northeast of Tadine on Maré Island.

The five events unfolded as follows. The initial shock measured magnitude 6.1 at a depth of 37 km. Roughly ten hours later, two closely spaced events occurred at 03:48 on 14 October: a magnitude 5.2 at 10 km depth and a magnitude 5.3 at 80 km depth. A magnitude 5.0 event followed at 07:28 at 35 km depth, and the swarm ended with a magnitude 4.8 shock at 44 km depth.

This portion of the southwest Pacific sits at the boundary between the Australian and Pacific plates. Convergence along the New Hebrides subduction zone generates frequent moderate-to-large earthquakes and occasional swarms. The Loyalty Islands and nearby Norfolk Ridge lie on the overriding plate, where stresses from slab subduction and possible slab tearing produce both shallow and intermediate-depth seismicity.

Since 2000, nine earthquake swarms have been identified in the same general area. Earlier swarms occurred in 2001, 2003, 2004 (two episodes), 2005, 2006, and three additional episodes in 2007. The region has also produced several magnitude-7 or larger earthquakes within roughly 85 km of the 2007 swarm center, including events on 25 March 2007 (M 7.1), 3 September 2011 (M 7.0), 19 November 2017 (M 7.0), 5 December 2018 (M 7.5), and 7 December 2023 (M 7.1). These larger shocks illustrate the persistent seismic hazard associated with the plate boundary.

Earthquake swarms in subduction settings often reflect fluid migration or localized stress adjustments along the plate interface or within the subducting slab. Depths recorded during the 2007 swarm range from 10 km to 80 km, consistent with activity both above and within the downgoing Australian plate. No damage or tsunami was reported from these moderate-magnitude events, although the proximity to inhabited islands underscores the importance of continued monitoring.

Ongoing plate convergence in the New Hebrides system will sustain seismic activity. Historical patterns indicate that swarms and larger thrust or intraslab events remain characteristic of the region east of New Caledonia.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geoscience Australia Seismology (ga.gov.au)