Seismic Swarm PS20150907.1 South of the Kermadec Islands
A seismic swarm designated PS20150907.1 was recorded south of the Kermadec Islands from 09:13 on 7 September 2015 to 11:37 on 8 September 2015. Over this 26-hour period, nine earthquakes occurred, with magnitudes ranging from 4.0 to 6.3 and focal depths between 10 km and 29 km.
The sequence began with a magnitude 6.3 event at 17 km depth, followed within minutes by a magnitude 5.0 shock at 29 km. Subsequent events included a magnitude 5.9 at 10 km depth later that afternoon, a magnitude 4.0 and 5.2 pair in the evening at shallow depths, and four additional events on 8 September with magnitudes between 5.0 and 5.8, all at depths of 10–16 km. The final event marked the end of the swarm.
The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent intermediate-depth and shallow earthquakes, as well as volcanic activity associated with the subduction of oceanic lithosphere. The region forms part of the Pacific Ring of Fire and experiences elevated seismicity due to plate boundary stresses and occasional slab dehydration processes.
Since 1 January 2000, four swarms have been documented in the area, occurring in 2001 (one swarm), 2004 (two swarms), and 2005 (one swarm). These episodes reflect episodic release of accumulated strain along the subduction interface and adjacent crustal faults. On 18 June 2020, a magnitude 7.4 earthquake struck 66 km from the 2015 swarm center, underscoring the ongoing potential for larger events in this segment of the plate boundary.
Analysis of swarm characteristics indicates clustered shallow to intermediate-depth activity consistent with stress triggering within the overriding plate and near the subduction interface. Depths predominantly around 10 km suggest involvement of the upper crust, while the initial deeper event at 29 km may relate to slab-related processes. Such patterns align with the broader tectonic framework of rapid subduction and back-arc extension in the Kermadec region.