Seismic Swarm Activity in the Kermadec Islands Region: Analysis of Event PS20200118.1

The Kermadec Islands region forms part of the extensive Kermadec-Tonga subduction zone in the southwestern Pacific Ocean. Here the Pacific Plate converges with and subducts beneath the Australian Plate at rates exceeding 6 cm per year, generating one of the world’s most seismically active environments. This tectonic setting produces frequent earthquakes across a wide range of magnitudes and depths, along with associated volcanic arcs.

On 18 January 2020, a short-lived earthquake swarm designated PS20200118.1 was recorded in the Kermadec Islands region. The sequence began at 02:07 UTC and concluded at 02:25 UTC, lasting 17 minutes and comprising five events. The individual earthquakes registered magnitudes of 5.3, 5.0, 4.7, 5.2, and 5.1, with focal depths predominantly at 10 km and one event at 2 km. Such rapid clustering of moderate-magnitude shocks is characteristic of swarm behavior in subduction-related fracture zones, where fluid migration or stress transfer along the plate interface can trigger multiple failures without a single dominant mainshock.

Swarm activity has occurred repeatedly in this region since 2000. A total of 14 swarms have been identified, distributed across the following years: two in 2003, one in 2005, three in 2006, two in 2008, one in 2011, one in 2012, two in 2014, and two in 2016. These episodes illustrate the persistent, episodic nature of seismicity driven by ongoing plate convergence.

Two notable strong earthquakes have also occurred nearby since 2000. A magnitude 7.4 event struck on 21 October 2011, located 27 km from the 2020 swarm center, followed by a magnitude 7.6 shock on 6 July 2011, 35 km from the same reference point. Both events underscore the capacity of the subduction zone to release significant energy while also highlighting the spatial proximity of large and moderate activity within the same tectonic corridor.

Overall, the January 2020 swarm fits within the established pattern of clustered seismicity that defines the Kermadec region. Continued monitoring remains essential for understanding stress evolution along this active plate boundary.

References:
SeismoSight internal swarm classification database
USGS Earthquake Catalog (regional tectonics summary)
Global Seismographic Network historical records