Seismic Swarm South of the Kermadec Islands: March 2004

A notable seismic swarm occurred south of the Kermadec Islands between 22:07 on 9 March 2004 and 02:16 on 11 March 2004. Over 28 hours and 8 minutes, 11 earthquakes were recorded. This event, classified internally as PS20040309.1, forms part of the limited swarm activity documented in the region since 2000.

The sequence began with a magnitude 5.8 event at 22:07 on 9 March at a depth of 10 km. Subsequent shocks included a magnitude 6.2 event at 22:56 on the same day at 18 km depth, followed by several magnitude 5.0–5.1 events at 10 km depth through the night. Activity continued on 10 March with events of magnitudes 5.7, 5.4, and 5.1, all at 10 km depth. The swarm concluded with three magnitude 5.4 events on 11 March, the final one at 02:16.

This swarm represents one of only two such episodes recorded in the area since 1 January 2000. The earlier swarm took place in 2001, with one additional swarm occurring in 2004. Such clusters illustrate episodic seismic behavior along the plate boundary rather than isolated mainshock-aftershock sequences.

The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate subducts westward beneath the Australian Plate. This tectonic setting produces frequent earthquakes at shallow to intermediate depths, driven by the convergence of the two plates at rates exceeding 5 cm per year. The trench reaches depths greater than 10 km, creating one of the most active seismic corridors in the southwest Pacific. Historical records show that the broader Kermadec region experiences regular moderate to large earthquakes, reflecting ongoing subduction processes that have shaped the volcanic arc over millions of years.

Swarm events in subduction zones like this often arise from fluid migration or localized stress changes along the plate interface. The 2004 swarm’s concentration of events at depths of 10–18 km aligns with typical shallow crustal responses in the overriding plate. All listed events remained below magnitude 7.0, consistent with the swarm’s moderate energy release.

References
SeismoSight internal swarm classification records.
USGS Earthquake Catalog for Kermadec region tectonic framework.
Global subduction zone summaries from the National Earthquake Information Center.