Seismic Swarm PS20230523.1 Southeast of the Loyalty Islands

The region southeast of the Loyalty Islands lies within a tectonically complex zone of the southwestern Pacific Ocean, where the Australian and Pacific plates interact along the New Hebrides subduction system. This setting produces frequent seismic events as the Pacific plate subducts westward beneath the Australian plate, generating both shallow crustal earthquakes and deeper Wadati-Benioff zone activity. The Loyalty Ridge itself represents an uplifted forearc feature influenced by this convergence, with the broader area exhibiting elevated seismicity rates consistent with ongoing plate boundary deformation.

SeismoSight recorded swarm PS20230523.1 beginning at 21:45 on 22 May 2023 and concluding at 09:26 on 23 May 2023. Over the 11-hour 40-minute duration, six earthquakes were detected. The sequence initiated with two magnitude 5.0 events at depths of 10 km, followed by a magnitude 5.1 event at 23 km depth and a magnitude 4.3 event at 10 km. The largest event reached magnitude 6.1 at 10 km depth, after which a final magnitude 5.0 shock occurred at 10 km. These events clustered tightly in both space and time, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

Such swarms have occurred regularly in the region. Since 1 January 2000, 21 swarms have been documented southeast of the Loyalty Islands, distributed across multiple years including 2001, 2003, 2004, 2005, 2008, 2009, 2011, 2015, 2018, 2019, 2021, 2022, and 2023. This pattern reflects persistent stress accumulation and release along the plate interface and associated crustal faults.

The May 2023 swarm followed closely after two major earthquakes in the same vicinity. A magnitude 7.7 event struck on 19 May 2023 approximately 39 km from the swarm centroid, and a magnitude 7.1 event occurred on 20 May 2023 only 19 km away. An earlier magnitude 7.0 earthquake on 31 March 2022 was located 39 km from the same area. These larger events likely altered local stress conditions, potentially triggering the subsequent swarm through static or dynamic stress transfer.

Seismic swarms in subduction-related settings can arise from fluid migration, afterslip, or minor magmatic processes, though the precise mechanism for PS20230523.1 remains under investigation by regional networks. Continued monitoring is warranted given the area's history of both swarm activity and great earthquakes.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification database