Seismic Swarm PS20031002.1 in the Kermadec Islands, New Zealand

The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate subducts beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent seismic activity and volcanic arcs across the region, extending from New Zealand’s North Island northward toward Tonga. The zone ranks among the most seismically active on Earth, with a history of both moderate swarms and great earthquakes capable of generating tsunamis.

Between 01:05 UTC on 2 October 2003 and 00:53 UTC on 3 October 2003, a swarm of six earthquakes occurred near the Kermadec Islands. All events were recorded at a focal depth of 33 km. The sequence began with two magnitude-5.3 shocks at 01:05 and 01:54, followed by magnitude-5.0 events at 05:41 and 06:03, a magnitude-4.6 shock at 06:37, and a final magnitude-5.3 event at 00:53 on 3 October. The entire swarm lasted 23 hours and 47 minutes.

Since 1 January 2000, only one swarm has been documented in the immediate area: the October 2003 sequence itself. No additional swarms have been recorded through the present.

Larger earthquakes have occurred within 90 km of the 2003 swarm center. These include a magnitude-7.1 event on 24 April 2023, the magnitude-8.1 mainshock and magnitude-7.4 aftershock of 4 March 2021, a magnitude-7.3 earthquake on 15 June 2019, and a magnitude-7.0 shock on 29 September 2008. Such events underscore the persistent strain accumulation along the subduction interface.

The 2003 swarm, while modest in individual magnitudes, illustrates the clustered seismicity that can precede or accompany larger ruptures in this subduction environment. Continued monitoring remains essential given the region’s capacity for great earthquakes.

References
SeismoSight internal swarm classification PS20031002.1
USGS Earthquake Catalog (events since 2000)
Global CMT Catalog (focal mechanisms and depths)