M 7.0; Kermadec Islands region; (16 Mar 2023) (66km from the swarm center)
M 8.1; 2021 Kermadec Islands, New Zealand Earthquake; (4 Mar 2021) (53km from the swarm center)
M 7.6; Kermadec Islands region; (6 Jul 2011) (77km from the swarm center)
M 7.0; Kermadec Islands, New Zealand; (29 Sep 2008) (83km from the swarm center)
Seismic Swarm PS20210304.3 in the Kermadec Islands Region
The Kermadec Islands region lies along the Kermadec-Tonga subduction zone, where the Pacific Plate subducts westward beneath the Australian Plate. This convergent margin produces one of the most seismically active environments on Earth, forming the Kermadec Trench and an associated volcanic arc. Subduction occurs at rates of approximately 5–7 cm per year, generating frequent earthquakes at shallow to intermediate depths as well as occasional great events exceeding magnitude 8.
Seismic swarm PS20210304.3 began at 20:24 on 4 March 2021 and ended at 23:48 on 5 March 2021, spanning 27 hours and 24 minutes. Twenty-one earthquakes were recorded during this interval. Magnitudes ranged from 4.9 to 5.8, with the majority occurring at depths of 10 km and one event at 11 km. The swarm followed the magnitude-8.1 mainshock of 4 March 2021 by several hours and was located 53 km from that event’s epicenter.
The sequence comprised the following events (times in UTC):
4 March 2021 – 20:24:14 (M5.1), 22:32:54 (M5.1), 22:33:45 (M5.8), 23:11:17 (M5.4), 23:28:29 (M4.9), 23:37:09 (M5.6);
5 March 2021 – 00:54:37 (M5.2), 00:58:27 (M5.6), 03:18:46 (M5.1), 07:23:16 (M5.4), 07:23:18 (M5.4), 15:00:46 (M5.0), 15:34:13 (M5.0), 19:34:53 (M5.5), 20:20:43 (M5.3), 20:26:35 (M5.8), 20:45:08 (M5.0), 20:52:41 (M5.1), 21:47:20 (M5.0), 22:34:50 (M5.1), 23:48:28 (M5.0).
Since 1 January 2000, nineteen swarms have been documented in the same region. These occurred in the years 2003 (two), 2005 (one), 2006 (three), 2008 (three), 2011 (one), 2012 (one), 2014 (three), 2016 (one), 2019 (two), 2020 (one), and 2021 (one).
Strong earthquakes (M ≥ 7.0) recorded since 2000 within roughly 100 km of the swarm center include:
M7.1 on 24 April 2023 (92 km distant);
M7.0 on 16 March 2023 (66 km distant);
M8.1 on 4 March 2021 (53 km distant);
M7.6 on 6 July 2011 (77 km distant);
M7.0 on 29 September 2008 (83 km distant).
The repeated occurrence of both swarms and great earthquakes underscores the persistent strain accumulation and release along this rapidly converging plate boundary. Continued monitoring remains essential for understanding short-term clustering and long-term seismic hazard in the Kermadec arc.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
GNS Science New Zealand seismic reports
Global CMT Catalog (globalcmt.org)