M 7.0; Kermadec Islands region; (16 Mar 2023) (97km from the swarm center)
M 8.1; 2021 Kermadec Islands, New Zealand Earthquake; (4 Mar 2021) (32km from the swarm center)
M 7.4; Kermadec Islands, New Zealand; (4 Mar 2021) (84km from the swarm center)
M 7.4; Kermadec Islands region; (21 Oct 2011) (97km from the swarm center)
M 7.6; Kermadec Islands region; (6 Jul 2011) (61km from the swarm center)
M 7.0; Kermadec Islands, New Zealand; (29 Sep 2008) (71km from the swarm center)
Seismic Swarm PS20210304.2 in the Kermadec Islands Region
The Kermadec Islands region lies along 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 earthquakes and volcanic activity across the Pacific Ring of Fire. The islands themselves form part of the Kermadec Ridge, an intra-oceanic arc characterized by submarine volcanoes and deep trenches reaching depths over 10 km.
Swarm PS20210304.2 began at 17:41 on 4 March 2021 and concluded at 21:53 on 8 March 2021. Within approximately 100 hours, 107 earthquakes were recorded. The sequence opened with a magnitude 7.4 event at 43 km depth, followed rapidly by numerous aftershocks. A magnitude 8.1 mainshock occurred at 19:28:33 on 4 March at 28 km depth, triggering further activity that included multiple events above magnitude 6.0. Depths for the majority of events clustered between 9 km and 60 km, with most aftershocks occurring at shallow crustal levels around 10 km.
Analysis of the first 100 events reveals a typical aftershock decay pattern following the largest shocks. Early hours featured high rates of magnitude 5+ events, transitioning to lower-magnitude activity by 6–8 March. Notable later events included a magnitude 6.5 at 24 km depth on 4 March and a magnitude 6.3 at 17 km on 5 March. The swarm’s spatial concentration aligns with the subduction interface and overlying crustal faults.
Since 2000, the region has hosted 19 documented swarms, occurring in 2003 (2), 2005 (1), 2006 (3), 2008 (3), 2011 (1), 2012 (1), 2014 (3), 2016 (2), 2019 (2), and 2020 (1). These swarms reflect episodic stress release along the plate boundary. Strong earthquakes since 2000 include the 2021 magnitude 8.1 event itself (32 km from swarm center), a magnitude 7.4 on the same day (84 km), plus earlier magnitude 7+ events in 2023, 2011, and 2008, all located within 100 km of the swarm centroid.
This activity underscores the persistent seismic hazard of the Kermadec subduction zone. Monitoring continues to improve understanding of rupture dynamics and tsunami potential in this remote oceanic setting.
References
USGS Earthquake Catalog
Global CMT Project
NOAA Tsunami Database