Seismic Swarm PS20250531.1 in the Kermadec Islands Region

A seismic swarm designated PS20250531.1 occurred in the Kermadec Islands region, beginning at 14:28 on 31 May 2025 and concluding at 22:17 on 1 June 2025. Over this 31-hour and 48-minute period, six earthquakes were recorded. The sequence included events of magnitude 6.1 at a depth of 10 km on 31 May at 14:28:49, followed by two magnitude 6.0 events at depths of 5 km and 10 km on 31 May at 21:21:09 and 21:21:11. Additional activity comprised a magnitude 6.2 event at 10 km depth on 31 May at 22:26:22, a magnitude 4.8 event at 10 km on 31 May at 22:30:59, and a magnitude 5.2 event at 10 km on 1 June at 22:17:07.

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate subducts beneath the Australian Plate at rates exceeding 10 cm per year. This tectonic setting generates frequent seismic activity, including both shallow crustal events and deeper intraslab earthquakes. The area is characterized by a chain of volcanic islands and seamounts resulting from this subduction, with historical records indicating elevated seismicity along the trench. Depths in the reported swarm, predominantly around 10 km, align with typical shallow thrust faulting in the overriding plate or near the plate interface.

Such swarms represent clusters of earthquakes occurring in rapid succession without a single dominant mainshock, often linked to fluid migration or stress redistribution along faults. In subduction zones like the Kermadec, these episodes can reflect adjustments in the locked portion of the megathrust or activation of secondary structures. The 2025 swarm's magnitudes, ranging from 4.8 to 6.2, and short duration suggest localized energy release rather than widespread rupture.

Since 1 January 2000, eight seismic swarms have been documented in the region. Prior occurrences took place in 2006 (two swarms), 2016 (one swarm), 2021 (two swarms), 2022 (one swarm), 2023 (one swarm), and 2024 (one swarm). These events underscore the recurrent nature of clustered seismicity in this arc, potentially tied to episodic slip or volcanic processes.

Monitoring of the Kermadec region remains essential given its potential for great earthquakes exceeding magnitude 8.0, which could generate tsunamis affecting distant coastlines. The 2025 swarm adds to the observational record of moderate-magnitude sequences that help refine models of subduction zone behavior.

References: SeismoSight internal classification data for swarm PS20250531.1 and historical statistics. Geological Survey of New Zealand (GNS Science) reports on Kermadec-Tonga arc tectonics.