Seismic Swarm South of the Kermadec Islands: Analysis of the June 2020 Event

A seismic swarm was recorded south of the Kermadec Islands beginning at 12:49 on 18 June 2020 and concluding at 17:10 on 19 June 2020. Over this 28-hour period, nine earthquakes occurred, with the initial event registering magnitude 7.4 at a depth of 10 km. Subsequent events ranged from magnitude 4.0 to 5.5, all at the same shallow depth of 10 km.

This swarm unfolded in the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate. The region experiences frequent seismic activity due to rapid plate convergence rates exceeding 6 cm per year. Shallow depths like those observed here are characteristic of the seismogenic zone near the trench, where brittle failure predominates.

The sequence included the following events in chronological order: the magnitude 7.4 mainshock at 12:49 on 18 June, followed by magnitude 5.0 and 5.5 events within three hours. Additional magnitude 5.5, 4.0, 5.3, and 5.2 shocks occurred later that day. On 19 June, magnitude 5.4 and 5.1 events marked the final activity. The largest event, magnitude 7.4, was located 56 km from the swarm center and aligns with documented strong earthquakes in the area since 2000.

Historical records indicate seven prior swarms in the region since 1 January 2000, occurring in 2001 (one swarm), 2004 (two swarms), 2005 (one swarm), 2015 (one swarm), 2016 (one swarm), and 2019 (one swarm). These episodes reflect episodic strain release along the subduction interface, often without a single dominant mainshock.

The Kermadec subduction zone forms part of the Pacific Ring of Fire and hosts both interplate thrust earthquakes and intraplate normal-faulting events. Shallow swarms at 10 km depth, as seen in this case, typically arise from fluid migration or stress triggering within the overriding plate and megathrust. Updated monitoring by global networks confirms ongoing activity patterns consistent with long-term subduction dynamics.

Such swarms contribute to understanding seismic hazard in remote oceanic settings, where events can generate local tsunamis despite limited population exposure. The 2020 sequence underscores the persistent seismicity of this tectonically active margin.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical events since 2000).
Global CMT Project (subduction zone parameters).