Kermadec Islands Earthquake Swarm PS20230424.1: Seismic Activity and Regional Context

The Kermadec Islands region, located northeast of New Zealand along the Tonga-Kermadec subduction zone, represents one of Earth's most active tectonic boundaries. Here, the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 6 cm per year, generating frequent earthquakes, volcanic arcs, and occasional seismic swarms. This environment has produced multiple magnitude 7+ events since 2000, reflecting the zone's capacity for both clustered microseismicity and large megathrust ruptures.

Swarm PS20230424.1 began at 03:47 on 23 April 2023 and concluded at 16:26 on 24 April 2023, spanning 36 hours and 39 minutes. During this interval, seven earthquakes were recorded, with magnitudes ranging from 5.0 to 7.1 and focal depths between 35 and 46 km. The sequence opened with a magnitude 5.1 event at 44 km depth, followed hours later by a magnitude 5.0 at 45 km. The largest shock, magnitude 7.1, occurred at 00:41 on 24 April at 46 km depth, approximately 30 km from the swarm centroid. Subsequent events included a magnitude 5.5 at 35 km, two magnitude 5.2 and 5.0 shocks at 35 km, and a final magnitude 5.0 at 35 km. Depths shallowed slightly after the mainshock, consistent with aftershock migration along the plate interface.

Such swarms are not uncommon in the Kermadec arc. Since 1 January 2000, 28 swarms have been documented, with notable clusters in 2021 (nine events) and earlier periods including 2014 and 2008 (three each). These episodes typically reflect fluid migration or stress triggering within the subduction interface rather than precursory activity leading to a single great earthquake.

The 24 April 2023 magnitude 7.1 event aligns with a pattern of strong shocks in the vicinity. Comparable events since 2000 include the magnitude 8.1 and 7.4 earthquakes of 4 March 2021 (37–46 km from the swarm center), the magnitude 7.3 of 15 June 2019 (91 km distant), and the magnitude 7.0 of 29 September 2008 (29 km distant). Depths of these events cluster between 30 and 50 km, indicating rupture along the deeper portion of the seismogenic zone.

Geological monitoring of the Kermadec-Tonga system underscores its role in accommodating Pacific-Australian convergence. Historical records show that while most swarms remain below magnitude 6, they can coincide with or follow larger ruptures, highlighting the region's persistent seismic hazard. Ongoing subduction continues to load the interface, sustaining both swarm activity and the potential for future great earthquakes.

References
SeismoSight internal swarm classification PS20230424.1 (2023 data)
USGS Earthquake Catalog (Kermadec region events 2000–2023)