Kermadec Islands Seismic Swarm of June 2023

A seismic swarm designated PS20230613.1 occurred in the Kermadec Islands region between 17:00 on 13 June 2023 and 16:53 on 14 June 2023. Over this 23-hour-53-minute period, seven earthquakes were recorded, with magnitudes ranging from 4.4 to 5.5 and focal depths between 18 and 62 km. The events clustered tightly in both time and space, characteristic of swarm activity rather than a typical mainshock-aftershock sequence.

The sequence began with a magnitude 5.2 event at 50 km depth, followed within minutes by a magnitude 5.4 shock at 43 km. Subsequent events included two additional magnitude 5.2 quakes, a magnitude 4.4 event, a magnitude 5.3 shock, and a final magnitude 5.5 earthquake at a relatively shallow 18 km depth. All events were located within the Kermadec arc, an area known for persistent moderate seismicity driven by plate-boundary processes.

The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate converges with and subducts beneath the Australian Plate at rates exceeding 6 cm per year. This tectonic setting produces the Kermadec Trench, one of the deepest oceanic trenches on Earth, and generates frequent earthquakes across a wide range of depths. Intermediate-depth events commonly occur within the subducting slab, while shallower activity reflects both slab deformation and upper-plate faulting.

Seismic swarms have been documented in the region since at least 2000, with 31 swarms recorded through mid-2023. Notable prior episodes occurred in 2021 (10 swarms) and earlier years including 2014, 2016, and 2019. The June 2023 swarm fits this established pattern of episodic clustered activity without a dominant mainshock.

Strong earthquakes have also punctuated the instrumental record. Since 2000, four events exceeding magnitude 7.0 have struck within approximately 100 km of the swarm centroid, including the magnitude 8.1 earthquake of 4 March 2021, the magnitude 7.6 and 7.4 events of July and October 2011, and the magnitude 7.0 shock of September 2008. These larger ruptures highlight the capacity of the subduction interface and adjacent structures to release substantial seismic energy.

The June 2023 swarm produced no reported damage or tsunami, consistent with its moderate magnitudes and offshore location. Continued monitoring by regional and global networks remains essential for tracking evolving activity along this highly active plate boundary.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
GNS Science New Zealand seismic reports
International Seismological Centre (ISC) Bulletin