Seismic Swarm PS20120128.1: Kermadec Islands Activity in Context

On 28 January 2012, a seismic swarm designated PS20120128.1 was recorded in the Kermadec Islands region of New Zealand. The sequence began at 17:42 UTC and concluded at 20:00 UTC, lasting approximately 2 hours and 17 minutes. During this period, ten earthquakes were registered, with magnitudes ranging from 5.0 to 5.9 and focal depths predominantly between 10 and 25 km.

The events unfolded rapidly, beginning with a magnitude 5.9 earthquake at 21 km depth, followed by nine additional shocks. Most subsequent events clustered at 10 km depth, with two at 18 km and 25 km. Magnitudes included five events of 5.0–5.2, three of 5.3–5.4, and the initial 5.9. This tight temporal clustering without a dominant mainshock-aftershock pattern is characteristic of swarm behavior in subduction settings.

The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate at rates exceeding 6 cm per year. This tectonic environment produces frequent intermediate-depth and shallow seismicity, as well as arc volcanism. The swarm location sits near the trench axis, where bending-related faulting and fluid migration along the plate interface can trigger episodic earthquake clusters.

Since 2000, ten swarms have occurred in the same general area, with notable concentrations in 2003 (two events), 2006 (three), and 2008 (three). The 2011 swarm preceded a magnitude 7.6 earthquake by several months, illustrating how swarm activity can coincide with periods of elevated regional stress. The 2012 swarm itself remained moderate in scale, with no events exceeding magnitude 6.0.

Larger earthquakes have repeatedly struck within tens of kilometers of the swarm centroid. Notable examples include the magnitude 8.1 event of 4 March 2021 (23 km distant), the magnitude 7.4 aftershock on the same day (71 km), and the magnitude 7.1 shock of 24 April 2023 (82 km). Earlier strong events comprise the magnitude 7.6 of July 2011 (75 km) and the magnitude 7.0 of September 2008 (58 km). These occurrences underscore the persistent seismic productivity of the northern Kermadec segment.

Swarm sequences such as PS20120128.1 provide insight into short-term stress redistribution along the subduction interface. The predominance of 10 km depths suggests activation of the upper plate or shallow megathrust, while the rapid onset and decay reflect transient triggering mechanisms rather than long-term afterslip. Continued monitoring of similar clusters aids in refining probabilistic forecasts for larger ruptures in this high-hazard corridor.

References

USGS Earthquake Catalog
GNS Science New Zealand Seismicity Reports
Global CMT Catalog for focal mechanisms in the Tonga-Kermadec arc