Seismic Swarm PS20081005.1: Analysis of Kermadec Islands Activity

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 6 cm per year. This tectonic setting produces frequent seismic swarms and occasional great earthquakes, with the subducting slab generating intermediate-depth events alongside shallow crustal activity. The islands themselves represent the emergent portions of a volcanic arc built on this convergent margin, characterized by steep bathymetry and active submarine volcanism.

Swarm PS20081005.1 was recorded between 14:50 on 4 October 2008 and 02:01 on 6 October 2008, spanning 35 hours and 10 minutes. Seven earthquakes were detected during this interval, with the following sequence: a magnitude 5.3 event at 36 km depth at 14:50:30 on 4 October, followed immediately by a magnitude 5.4 event at 40 km depth; a magnitude 6.1 event at 10 km depth at 09:12:36 on 5 October; a magnitude 4.4 event at 35 km depth at 10:11:52; a magnitude 5.4 event at 35 km depth at 10:44:59; a magnitude 5.2 event at 35 km depth at 14:09:49; and a magnitude 5.1 event at 39 km depth at 02:01:05 on 6 October. Depths clustered predominantly in the 35–40 km range, except for the shallowest and largest shock, consistent with typical patterns of swarm migration along the plate interface or within the overriding crust.

Since 1 January 2000, six swarms have occurred in the Kermadec Islands area according to internal classification records. These include two swarms in 2003, one in 2005, two in 2006, and the present swarm in 2008. Such episodic clusters reflect the region’s high background seismicity driven by ongoing subduction.

Major earthquakes since 2000 further illustrate the zone’s potential. A magnitude 7.1 event struck 93 km from the swarm center on 24 April 2023. A magnitude 7.0 earthquake occurred 97 km away on 16 March 2023. The magnitude 8.1 2021 Kermadec Islands earthquake was located 91 km distant on 4 March 2021. A magnitude 7.3 shock took place 93 km away on 15 June 2019. These events underscore the capacity for both moderate and great earthquakes within a short distance of swarm epicenters.

Seismic swarms in subduction zones like the Kermadec arc often precede or accompany slow slip events, fluid migration, or minor volcanic unrest. The 2008 swarm’s rapid succession of events, dominated by moderate magnitudes and intermediate depths, aligns with stress transfer along the plate boundary rather than a single mainshock-aftershock sequence. Continued monitoring remains essential given the proximity of recent large ruptures.

References
SeismoSight internal swarm classification database (PS20081005.1 parameters and historical swarm counts).
USGS earthquake catalog (strong earthquake locations and magnitudes since 2000).