Seismic Swarm PS20010228.1: Analysis of Activity Southeast of the Loyalty Islands

The seismic swarm PS20010228.1 occurred in a tectonically active zone 222 km ESE of Tadine, New Caledonia. The sequence began at 12:30 on 28 February 2001 and concluded at 03:59 on 1 March 2001, lasting 15 hours and 29 minutes. During this period, seven earthquakes were recorded, all at a focal depth of 10 km. The events ranged in magnitude from 5.1 to 6.7, with the largest shock measuring 6.7 at 12:30:14 on 28 February. Subsequent events included magnitudes of 5.4, 6.3, 5.6, 5.4, 5.1, and a final 5.8 on 1 March.

This swarm reflects clustered seismic energy release typical of the region’s shallow crustal dynamics. The consistent 10 km depth across all events suggests activation along a near-surface fault segment within the overriding plate. Such swarms often occur without a single dominant mainshock-aftershock decay pattern, instead indicating distributed stress adjustment along pre-existing structures.

The broader geological setting lies along the southeastern margin of the Loyalty Islands, where the Australian and Pacific plates interact through a combination of subduction and strike-slip motion. The Loyalty Ridge, an uplifted oceanic feature, forms part of the complex boundary zone east of New Caledonia’s Norfolk Ridge. This configuration produces frequent shallow seismicity as the Pacific plate subducts westward beneath the Australian plate, generating both thrust and transform faulting. Historical records show the area has hosted multiple magnitude-7+ events since 2000, underscoring persistent tectonic loading.

Notable strong earthquakes near the swarm centroid include the magnitude 7.1 event of 20 May 2023 located 92 km away, the magnitude 7.0 of 31 March 2022 situated 38 km distant, and the magnitude 7.5 of 5 December 2018 positioned 91 km from the center. Additional large shocks comprise the magnitude 7.1 of 29 August 2018 (28 km), the magnitude 7.1 of 3 January 2004 (57 km), and the magnitude 7.3 of 27 December 2003 (56 km). These events demonstrate that the 2001 swarm took place within a corridor of recurrent high-magnitude activity.

The 2001 swarm’s temporal clustering and uniform shallow depths provide insight into short-term strain release along secondary faults accommodating regional plate motion. Continued monitoring of similar sequences aids in characterizing background seismicity rates and potential precursory patterns ahead of larger ruptures in this segment of the southwest Pacific.

References
SeismoSight internal swarm classification PS20010228.1
USGS earthquake catalog records for events since 2000