M 8.1; 2021 Kermadec Islands, New Zealand Earthquake; (4 Mar 2021) (55km from the swarm center)
M 7.4; Kermadec Islands region; (21 Oct 2011) (74km from the swarm center)
M 7.6; Kermadec Islands region; (6 Jul 2011) (44km from the swarm center)
M 7.0; Kermadec Islands, New Zealand; (29 Sep 2008) (92km from the swarm center)
Seismic Swarm PS20210427.1 in the Kermadec Islands Region
A seismic swarm designated PS20210427.1 occurred in the Kermadec Islands region between 13:42 on 26 April 2021 and 04:24 on 28 April 2021. Over 38 hours and 42 minutes, the sequence comprised 10 earthquakes, with magnitudes ranging from 4.4 to 5.9 and focal depths predominantly near 10 km, except for one event at 56 km and another at 13 km.
The sequence began with a magnitude 5.2 event at 56 km depth. Subsequent activity included a magnitude 5.7 shock at 10 km depth, followed closely by a magnitude 5.2 event at similar depth. Additional events on 27 April reached magnitudes of 4.4, 5.0, 5.9, 5.1, and 5.3, all at shallow depths around 10–13 km. The swarm concluded with two magnitude 5.1 and 5.0 events on 28 April at 10 km depth.
The Kermadec Islands lie along 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 intermediate-depth and shallow seismicity, as well as arc volcanism. The region forms part of the Pacific Ring of Fire and has hosted multiple magnitude 7+ earthquakes since 2000, including events of M8.1 in March 2021 (55 km from the swarm center), M7.6 and M7.4 in July and October 2011 (44 km and 74 km distant), and an M7.0 in March 2023 (98 km distant).
Since 2000, the Kermadec Islands region has recorded 26 swarms, with notable concentrations in 2021 (7 swarms), 2014 (3), 2006 (3), and 2008 (3). These swarms typically reflect fluid migration or stress redistribution along the subduction interface rather than foreshock-mainshock-aftershock sequences driven by a single large rupture.
The April 2021 swarm fits the pattern of moderate-magnitude, shallow activity common in this subduction environment, where clusters of events occur without producing a dominant mainshock exceeding magnitude 6.0. No damage or tsunami was reported from this particular sequence.
References
SeismoSight internal swarm classification database.
USGS Earthquake Catalog (events since 2000).
Global CMT catalog for regional tectonics.