Seismic Swarm Activity in the Kermadec Islands Region: March 2021 Event

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent seismic activity, including shallow crustal events, intermediate-depth earthquakes, and occasional volcanic unrest along the island arc. The zone extends northward from New Zealand’s North Island and is recognized for its high seismic productivity within the Pacific Ring of Fire.

Between 19:38 UTC on 4 March 2021 and 09:35 UTC on 7 March 2021, a seismic swarm designated PS20210305.1 occurred in this region. Over 61 hours and 57 minutes, 28 earthquakes were recorded. Event depths were predominantly 10 km, with two slightly deeper events at 15 km and 17 km. Magnitudes ranged from 4.5 to 6.2, featuring several events of magnitude 5.8 or greater. The strongest shocks reached magnitude 6.2 on 6 March.

The swarm exhibited a typical pattern of clustered activity without a single dominant mainshock, with multiple events of comparable size occurring in close succession. Notable clusters included three magnitude 5+ events within the first 12 hours and a sequence of five magnitude 5+ shocks between 07:02 and 10:34 UTC on 6 March. Such swarms reflect stress redistribution along the subduction interface or within the overriding plate rather than a classic foreshock-mainshock-aftershock sequence.

Since 1 January 2000, eleven seismic swarms have been identified in the Kermadec Islands region. Prior episodes occurred in 2005 (1 swarm), 2006 (3 swarms), 2008 (1), 2011 (1), 2012 (1), 2014 (1), 2016 (1), 2020 (1), and 2021 (1, including the event described here). This recurrence indicates persistent tectonic loading along the subduction zone, where episodic swarm activity alternates with periods of relative quiescence.

Seismic swarms in subduction settings like the Kermadec arc can precede larger ruptures or occur independently as expressions of aseismic slip. Continued monitoring remains essential given the region’s capacity to generate great earthquakes and associated tsunami hazards.

References

USGS Earthquake Catalog
GNS Science New Zealand Seismic Reports
SeismoSight internal swarm classification data