Seismic Swarm PS20171031.1: Insights from the New Caledonia Region

The seismic swarm designated PS20171031.1 occurred 122 km east of Tadine in New Caledonia's Loyalty Islands. It began at 00:42 on 31 October 2017 and concluded at 09:06 on 3 November 2017, spanning 80 hours and 24 minutes. During this interval, 21 earthquakes were recorded, with magnitudes ranging from 4.4 to 6.7 and focal depths between 6 km and 24 km.

The sequence opened with a magnitude 6.7 event at 24 km depth. Subsequent activity included multiple events above magnitude 5.0 clustered on 31 October and 1 November, featuring two notable shocks of 6.1 and 6.6. Later events maintained moderate magnitudes near 5.0–5.6 at shallow depths, tapering to a final magnitude 5.1 on 3 November. This pattern reflects typical swarm behavior, with energy release distributed across numerous events rather than dominated by a single mainshock-aftershock series.

New Caledonia occupies a tectonically dynamic position within the southwestern Pacific, where convergence between the Australian and Pacific plates drives subduction along the Vanuatu trench system. The Loyalty Islands, including the area east of Tadine on Maré, sit near the plate boundary, experiencing elevated seismicity from both interplate thrusting and intraplate deformation. Historical records since 2000 document 18 prior swarms in the vicinity, concentrated in years such as 2007 (four events) and 2011 (three events), underscoring recurrent episodic activity.

Strong regional earthquakes since 2000 further illustrate the setting's potential. These include a magnitude 7.5 event 166 km east-southeast of Tadine in December 2018, a magnitude 7.0 shock 85 km east-northeast in November 2017, and earlier magnitude 7.1 and 7.3 events in January and December 2003, all within 100 km of the swarm centroid. Such occurrences align with the broader subduction-related hazard that has shaped the archipelago's geology over millions of years.

Analysis of swarm timing shows peak activity within the first 36 hours, followed by declining frequency. Depths remained predominantly crustal, consistent with shallow thrust and normal faulting expected near the trench. The 2017 swarm's proximity to the November 2017 magnitude 7.0 event suggests possible stress interactions along adjacent fault segments.

This episode contributes to understanding the region's seismic cycle, where swarms may represent either slow slip phenomena or fluid migration along faults. Continued monitoring supports improved hazard assessment for the Loyalty Islands and surrounding areas.

References:
SeismoSight internal swarm catalog PS20171031.1
USGS earthquake archive for New Caledonia region (2000–2018)