Seismic Swarm in the Kermadec Islands: The September 2008 Event and Regional Context

The Kermadec Islands region forms part of New Zealand’s northern offshore territory and lies along the Kermadec Trench, one of the world’s deepest subduction zones. Here the Pacific Plate descends beneath the Australian Plate at rates exceeding 6 cm per year, generating frequent earthquakes and volcanic activity. The trench extends more than 1,000 km northeast from New Zealand’s East Cape and reaches depths greater than 10,000 m. Intermediate-depth seismicity, typically between 30 and 70 km, is characteristic of the Wadati-Benioff zone associated with this subduction interface.

On 29 September 2008 a seismic swarm occurred near the central Kermadec Islands. The sequence began at 15:19 UTC with a magnitude 7.0 earthquake at 36 km depth and concluded at 23:36 UTC with a magnitude 4.3 event at 35 km depth. Within the eight-hour window, five earthquakes were recorded, with magnitudes 7.0, 5.3, 5.0, 5.0 and 4.3, all nucleating at nearly identical depths between 35 and 36 km. The tight clustering in time and space, together with the rapid decay in magnitude, is consistent with swarm behaviour rather than a classic mainshock-aftershock sequence.

Since 1 January 2000 the same source region has hosted five additional swarms (two in 2003, one in 2005 and two in 2006). The 2008 swarm therefore represents the sixth documented swarm episode in the modern instrumental record. Stronger individual events have also occurred nearby, including a magnitude 7.0 earthquake on the same day 27 km from the swarm centroid, a magnitude 8.1 event on 4 March 2021 located 52 km away, and a magnitude 7.1 shock on 24 April 2023 situated only 12 km from the 2008 swarm centre. These large events underscore the persistent seismic productivity of the subduction interface.

The 2008 swarm’s hypocentral depths coincide with the inferred locked-to-sliding transition on the plate interface, where episodic slow slip and fluid migration can trigger clustered seismicity. Although the events did not produce a tsunami, their proximity to the trench axis illustrates the region’s capacity for both moderate swarms and great earthquakes. Continued monitoring by GeoNet and international networks remains essential for characterising swarm recurrence and assessing implications for regional seismic hazard.

References
USGS Earthquake Catalog (events 2000–2023)
GeoNet New Zealand National Seismic Network
Global CMT Catalog
Kermadec Trench tectonic summaries, GNS Science