An earthquake swarm designated PS20210306.1 occurred in the Kermadec Islands region, beginning at 07:16 on 6 March 2021 and concluding at 02:07 on 7 March 2021. Over the 18-hour-and-50-minute period, ten earthquakes were recorded. The events ranged in magnitude from 4.7 to 6.1, with focal depths between 8 and 19 km. The sequence included two events above magnitude 6.0, highlighting a period of elevated seismic activity along this tectonically active margin.
The Kermadec Islands form 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 convergent boundary produces frequent shallow to intermediate-depth earthquakes and hosts submarine volcanoes. The swarm’s shallow depths place the activity within the overriding plate or near the plate interface, consistent with stress release in a region of ongoing subduction.
Seismic swarms have occurred periodically in the Kermadec Islands since at least 2000, with twelve documented episodes through 2021. Prior swarms took place in 2005 (two events), 2006 (three), 2011 (one), 2012 (one), 2016 (one), 2020 (one), and three in 2021. These clusters typically involve multiple moderate-magnitude events over hours to days without a single dominant mainshock.
A notable larger earthquake, magnitude 7.4, struck the same region on 21 October 2011, approximately 72 km from the 2021 swarm center. Such events underscore the potential for both swarm activity and isolated strong shocks within this subduction setting.
The 2021 swarm events, listed chronologically, were as follows:
Collectively, these data illustrate a typical swarm pattern of closely spaced, moderate-magnitude earthquakes without clear foreshock-mainshock-aftershock progression.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification PS20210306.1