Seismic Swarm PS20030930.1 in the Kermadec Islands

The Kermadec Islands region of New Zealand lies along the active Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate. This tectonic setting produces frequent seismicity, including earthquake swarms and occasional great events exceeding magnitude 7. The swarm designated PS20030930.1 began at 20:52 on 29 September 2003 and concluded at 13:43 on 1 October 2003, lasting 40 hours and 51 minutes. During this interval, seven earthquakes were recorded near the islands.

The sequence opened with a magnitude 5.1 event at 33 km depth on 29 September at 20:52:37. Activity intensified the following day, featuring a magnitude 6.4 shock at 10 km depth at 14:08:37, followed by a magnitude 6.0 event at 33 km depth at 15:22:31. Additional events included two magnitude 5.1 and 4.8 shocks at shallow 10 km depths around 19:03–19:05, a magnitude 5.6 at 10 km at 21:39:14, and a final magnitude 5.0 at 33 km depth on 1 October at 13:43:54. Depths alternated between shallow crustal levels and intermediate depths near 33 km, consistent with the subduction interface and overlying crust in this arc setting.

The Kermadec arc has hosted multiple strong earthquakes since 2000, underscoring its persistent seismic productivity. Notable events include a magnitude 7.1 on 24 April 2023 located 63 km from the swarm center, the magnitude 8.1 mainshock of 4 March 2021 at 69 km distance, an accompanying magnitude 7.4 on the same date 87 km away, a magnitude 7.3 on 15 June 2019 at 78 km, and a magnitude 7.0 on 29 September 2008 situated 72 km distant. These occurrences reflect ongoing plate convergence and occasional megathrust rupture along the subduction interface.

Seismic swarms in subduction zones such as the Kermadec region often arise from localized stress adjustments, fluid migration, or afterslip following larger regional events. The 2003 swarm’s moderate magnitudes and mixed depths illustrate typical background activity within a highly coupled tectonic environment. Long-term monitoring by regional networks continues to document such episodes, contributing to improved understanding of subduction dynamics.

References
SeismoSight internal swarm classification PS20030930.1.
USGS Earthquake Catalog (events since 2000).
GNS Science regional tectonic summaries for the Kermadec arc.