Seismic Swarm PS20160713.1 in the Kermadec Islands Region

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone in the southwestern Pacific Ocean, where the Pacific Plate converges with and subducts beneath the Australian Plate at rates of 5–7 cm per year. This tectonic setting produces one of Earth’s most seismically active environments, characterized by frequent shallow to intermediate-depth earthquakes along the plate interface and within the overriding plate. The Kermadec Trench reaches depths exceeding 10 km, and the associated volcanic arc includes several submarine and island volcanoes that reflect ongoing subduction-related magmatism.

Between 11:56 UTC on 13 July 2016 and 02:24 UTC on 14 July 2016, a seismic swarm comprising ten earthquakes was recorded in the Kermadec Islands region. All events occurred at shallow depths of 10–12 km. Magnitudes ranged from 4.8 to a maximum of 6.3, with the largest shock taking place at 12:11 UTC on 13 July. The sequence began with a magnitude 5.9 event, followed rapidly by additional shocks of 4.8, 5.9, 5.1, 5.8, 5.1, 5.0, and two final events of 5.0 and 5.1 on 14 July. The entire swarm lasted 14 hours and 27 minutes.

Such swarm activity is uncommon in the catalog since 2000, with only six swarms previously identified: two in 2005, two in 2006, one in 2011, and one in 2012. A notable independent event, a magnitude 7.0 earthquake on 18 February 2009, occurred 63 km from the 2016 swarm centroid, underscoring the region’s capacity for both isolated large shocks and clustered moderate-magnitude sequences.

Analysis of the 2016 swarm indicates a brief period of elevated stress release along a limited segment of the subduction interface. The tight clustering of depths and the absence of a clear mainshock-aftershock decay pattern are consistent with swarm behavior driven by fluid migration or localized aseismic slip rather than a single large rupture. No damage or tsunami was reported from these events, reflecting both their moderate magnitudes and offshore location.

Continued monitoring of the Kermadec subduction zone remains essential for understanding the interplay between swarm activity and the potential for larger megathrust earthquakes along the plate boundary.

References
SeismoSight internal swarm classification database
USGS Earthquake Hazards Program regional tectonic summaries