Seismic Swarm PS20110927.1 Southeast of the Loyalty Islands

The seismic swarm designated PS20110927.1 occurred southeast of the Loyalty Islands, a region situated in the southwestern Pacific Ocean east of New Caledonia. This area lies within a tectonically active zone influenced by the convergence of the Australian and Pacific plates along the New Hebrides subduction system. The Loyalty Islands themselves form part of an uplifted forearc domain shaped by long-term subduction processes, resulting in frequent moderate to strong earthquakes.

The swarm commenced at 17:42 on 27 September 2011 and concluded at 17:56 on 28 September 2011. Over this 24-hour and 14-minute period, eight earthquakes were recorded. Magnitudes ranged from 5.0 to 5.4, with most events occurring at shallow depths of 10 km. Specific events included two initial shocks of magnitude 5.2 at 17:42 on 27 September, followed by additional events of 5.0 and 5.1 later that evening. On 28 September, the largest event reached magnitude 5.4 at 17:36, with two final shocks of 5.1 recorded at 17:56.

This activity aligns with the region's established pattern of clustered seismicity. Historical records since 2000 indicate five prior swarms in the vicinity: one each in 2001, 2003, and 2008, plus two in 2009. Such swarms typically reflect transient stress adjustments along local fault networks rather than foreshock sequences leading to larger mainshocks.

A notable strong earthquake of magnitude 7.7 struck on 10 February 2021 approximately 41 km from the swarm center, underscoring the persistent seismic hazard in this segment of the plate boundary. Depths of swarm events varied modestly, with several at 10 km and others reaching 23 km, 51 km, and 52 km, consistent with the vertical extent of brittle failure in the overriding plate and subducting slab interface.

Overall, the PS20110927.1 swarm exemplifies the episodic nature of moderate-magnitude activity in this subduction-influenced setting, where repeated clustering has been documented over the past two decades.

References

SeismoSight internal swarm classification records (PS20110927.1 parameters and historical swarm statistics since 2000).