Seismic Swarm PS20050811.1: Analysis of Activity East of Tadine, New Caledonia

On 11 August 2005, a seismic swarm designated PS20050811.1 was recorded approximately 229 km east of Tadine in the Loyalty Islands region of New Caledonia. The sequence began at 07:02 and concluded at 22:44 local time, encompassing a total duration of 15 hours and 42 minutes. During this interval, eight earthquakes were registered, with magnitudes ranging from 5.0 to 6.2 and focal depths predominantly at 10 km, except for one event at 33 km.

The swarm initiated with a magnitude 5.2 event at 07:02:51, followed by two magnitude 5.0 shocks at 08:46:38 and 09:05:39. The largest event, magnitude 6.2, occurred at 09:08:46, accompanied immediately by a magnitude 5.5 aftershock at 09:08:49. Subsequent activity included a magnitude 5.6 at 11:50:51 and a magnitude 5.5 at 11:50:54 (33 km depth). The sequence ended with a magnitude 5.0 event at 22:44:58.

This activity occurred within a tectonically dynamic zone characterized by the convergence of the Australian and Pacific plates. The Loyalty Islands lie along the Loyalty Ridge, where subduction-related processes and back-arc extension contribute to elevated seismicity. The region experiences frequent moderate to large earthquakes due to its position near the New Hebrides Trench system and associated fault networks.

Historical records since 2000 indicate four prior swarms in the vicinity: one each in 2001, 2003, 2004, and 2005. Stronger events have also punctuated the area's seismic history, including a magnitude 7.1 earthquake southeast of the Loyalty Islands on 20 May 2023 (98 km from the swarm center), a magnitude 7.0 on 31 March 2022 (49 km away), a magnitude 7.5 on 5 December 2018 (71 km distant), a magnitude 7.1 on 29 August 2018 (37 km away), a magnitude 7.1 on 3 January 2004 (31 km distant), and a magnitude 7.3 on 27 December 2003 (42 km from the center).

Such swarms provide insight into episodic stress release along local fault structures, often without a single dominant mainshock. The 2005 sequence aligns with patterns of clustered moderate-magnitude activity typical of this plate boundary setting.

References

SeismoSight internal classification data for swarm PS20050811.1 and historical statistics.