Seismic Swarm PS20051208.1: Kermadec Islands, New Zealand

The Kermadec Islands form part of an active volcanic arc situated along the Kermadec-Tonga subduction zone, where the Pacific Plate subducts westward beneath the Australian Plate. This tectonic setting produces frequent seismic activity, including both isolated large-magnitude events and episodic earthquake swarms. Depths of seismicity in the region commonly range from shallow crustal levels near 10 km to intermediate depths exceeding 100 km along the subducting slab. Swarm PS20051208.1 occurred between 23:32 UTC on 7 December 2005 and 05:32 UTC on 8 December 2005, lasting five hours and fifty-nine minutes. Five earthquakes were recorded during this interval, with the following parameters: a magnitude 6.4 event at 21 km depth at 23:32:51 on 7 December; a magnitude 5.0 event at 10 km depth at 23:39:46; a magnitude 5.1 event at 10 km depth at 23:41:15; a magnitude 4.9 event at 22 km depth at 05:00:15 on 8 December; and a magnitude 5.4 event at 10 km depth at 05:32:08. The sequence exhibited a tight temporal clustering without a clear dominant mainshock-aftershock pattern, consistent with swarm behavior observed in subduction environments. Since 1 January 2000, only two swarms have been documented in the Kermadec Islands, with the earlier episode occurring in 2003. The 2005 swarm was centered in a region that has also hosted several strong earthquakes in the same period. Notable events include the magnitude 7.1 earthquake of 24 April 2023 located 35 km from the swarm center, the magnitude 8.1 and 7.4 earthquakes of 4 March 2021 at 41 km and 53 km respectively, the magnitude 7.3 event of 15 June 2019 at 88 km, and the magnitude 7.0 earthquake of 29 September 2008 at 37 km. These larger ruptures reflect the high strain accumulation and release characteristic of the subduction interface. Earthquake swarms in this setting often arise from fluid migration, aseismic slip, or localized stress perturbations along the plate boundary. The shallow focal depths recorded in PS20051208.1 suggest activity within or immediately above the seismogenic portion of the megathrust. Continued monitoring of such sequences contributes to understanding precursory patterns that may precede larger events in the Kermadec arc.

References

USGS Earthquake Catalog GeoNet New Zealand Seismic Database SeismoSight internal swarm classification records