Seismic Swarm in the Kermadec Islands Region, March 2021

A seismic swarm designated PS20210316.1 occurred in the Kermadec Islands region between 19:28 on 15 March 2021 and 13:13 on 16 March 2021. Over 17 hours and 45 minutes, five earthquakes were recorded with magnitudes ranging from 4.4 to 5.3 and focal depths between 10 and 14 km. The sequence began with a magnitude 5.3 event at 19:28 on 15 March, followed by magnitude 5.2 and 5.1 shocks later that evening, a magnitude 5.0 event early on 16 March, and concluded with a magnitude 4.4 earthquake at 13:13. All events clustered at shallow depths, consistent with typical swarm behavior in subduction-related crust. The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate converges with and subducts beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces one of the world’s highest concentrations of intermediate-depth and shallow seismicity. The trench reaches depths greater than 10 km and is flanked by active volcanic arcs, including the Kermadec Ridge. Earthquake swarms in this region often reflect fluid migration, stress transfer along the plate interface, or minor slip on subsidiary faults within the overriding plate. Since 2000, 23 swarms have been documented in the Kermadec Islands region, with notable clusters in 2006, 2008, 2014, and 2021. The March 2021 swarm followed the magnitude 8.1 mainshock of 4 March 2021 by eleven days and was located approximately 55 km from its epicenter. Earlier large events include the magnitude 7.4 and 7.6 earthquakes of October and July 2011, situated 73 km and 50 km from the swarm center, respectively, and the magnitude 7.0 event of September 2008 located 90 km away. These episodes illustrate the persistent seismic productivity of the subduction interface and its capacity to generate both isolated large ruptures and prolonged swarm sequences. Swarm activity of this type provides valuable data on short-term stress evolution within the subduction system. Continued monitoring by regional seismic networks remains essential for understanding precursory patterns that may precede future great earthquakes along the Kermadec Trench. References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
GeoNet New Zealand (geonet.org.nz)