Seismic Swarm Activity in the Kermadec Islands: The June 2019 Event

The Kermadec Islands region, located northeast of New Zealand along the Pacific Ring of Fire, experienced a notable seismic swarm from 22:55 on 15 June 2019 to 07:01 on 19 June 2019. Over approximately 80 hours, 21 earthquakes were recorded, with magnitudes ranging from 5.0 to 7.3 and depths between 10 and 46 km. This swarm highlights the persistent tectonic stresses in one of the world's most active subduction zones.

The sequence began with a magnitude 7.3 event at 46 km depth, followed by numerous aftershocks and triggered quakes. Key events included a magnitude 6.3 at 21 km depth on 16 June, a magnitude 6.1 at 12 km on 17 June, and a magnitude 6.4 at 30 km on 19 June, marking the swarm's conclusion. Intermediate events clustered around magnitudes 5.0 to 5.9, predominantly at shallow to intermediate depths of 10–43 km, reflecting typical patterns of stress release along the plate interface.

Geologically, the Kermadec Islands form part of the Kermadec-Tonga arc, where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 6 cm per year. This oblique subduction drives frequent seismicity, including both interplate thrust events and intraslab earthquakes. The arc features active volcanism and a deep trench exceeding 10 km, contributing to high seismic hazard. The 2019 swarm occurred near the central Kermadec segment, consistent with historical clustering in zones of elevated coupling.

Since 2000, 17 swarms have been documented in the region, occurring in clusters during 2003, 2004, 2005, 2006, 2008, 2011, 2012, 2014, 2016, and 2019. These episodes typically involve elevated rates of moderate-magnitude events without a single dominant mainshock, distinguishing them from isolated large earthquakes. In addition, two strong events exceeding magnitude 7 have struck nearby since 2000: a magnitude 7.3 on 15 June 2019 (38 km from the swarm center) and a magnitude 7.1 on 24 April 2023 (93 km from the center).

Such activity underscores the Kermadec region's capacity for both swarm-type sequences and great earthquakes, informed by ongoing monitoring from regional seismic networks. Continued observation aids in refining models of subduction dynamics and associated risks.

References

• GeoNet New Zealand seismic catalog (prompt data utilized for swarm parameters)
• USGS Earthquake Hazards Program records for regional events since 2000