Seismic Swarm PS20160204.1: Kermadec Islands Activity in 2016

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 6 cm per year. This tectonic setting produces frequent seismic activity, including earthquake swarms and occasional great earthquakes exceeding magnitude 8. The islands themselves represent a volcanic arc resulting from this subduction, with associated features such as deep ocean trenches and active submarine volcanism. On 4 February 2016, the SeismoSight-classified swarm PS20160204.1 occurred near the Kermadec Islands. The sequence began at 00:07 and concluded at 08:54, spanning 8 hours and 46 minutes. Five earthquakes were recorded during this interval, with the following parameters: a magnitude 5.2 event at 16 km depth at 00:07:47; a magnitude 5.1 event at 25 km depth at 00:48:14; a magnitude 5.0 event at 10 km depth at 00:58:43; a magnitude 5.0 event at 10 km depth at 08:52:15; and a magnitude 4.6 event at 10 km depth at 08:54:34. Such swarms typically lack a dominant mainshock-aftershock pattern and instead reflect distributed stress release along the subduction interface or within the overriding plate. Historical records indicate 13 swarms in the same region since 1 January 2000. These occurred in the following years with the listed counts: two in 2003, one in 2005, two in 2006, three in 2008, one in 2011, one in 2012, and three in 2014. This pattern underscores the episodic nature of clustered seismicity in the Kermadec segment. Notable larger events since 2000 include a magnitude 7.1 earthquake on 24 April 2023 located 59 km from the swarm center, the magnitude 8.1 mainshock of the 2021 Kermadec Islands sequence on 4 March 2021 at 67 km distance, an associated magnitude 7.4 event on the same day at 83 km distance, a magnitude 7.3 earthquake on 15 June 2019 at 76 km distance, and a magnitude 7.0 event on 29 September 2008 at 69 km distance. These demonstrate the potential for the region to generate damaging earthquakes alongside more frequent swarm activity.

• References
• SeismoSight internal swarm classification database
• USGS Earthquake Catalog (queried for regional events 2000–2023)