Seismic Swarm PS20051028.1 South of the Kermadec Islands

A minor seismic swarm designated PS20051028.1 occurred south of the Kermadec Islands between 07:25 on 27 October 2005 and 03:26 on 28 October 2005. Within roughly 20 hours, five earthquakes were recorded, with magnitudes ranging from 4.7 to 5.1 and focal depths between 2 km and 50 km.

The sequence began with a magnitude 5.0 event at 10 km depth, followed by two magnitude 5.1 events at 50 km depth later the same day. The swarm concluded with a magnitude 4.7 shock at only 2 km depth and a final magnitude 5.0 event at 36 km depth. Such clustered activity is characteristic of the region’s subduction-related seismicity, where stress adjustments along the plate interface can trigger multiple moderate events in short succession.

The Kermadec Islands lie along the Kermadec-Tonga subduction zone, where the Pacific plate converges with and descends beneath the Australian plate at rates exceeding 6 cm per year. This tectonic setting produces one of the world’s most active seismic belts, with frequent earthquakes at depths from shallow crustal levels to more than 600 km. The October 2005 swarm was located within the shallow to intermediate portion of this Benioff zone, consistent with typical rupture patterns observed along the trench.

Since 1 January 2000, only three swarms have been documented in the immediate area, including one event in 2001 and two in 2004. The 2005 swarm therefore represents the third recorded episode in the 21st century up to that date. On 18 June 2020, a magnitude 7.4 earthquake struck approximately 93 km from the 2005 swarm centroid, underscoring the continued potential for larger events within the same tectonic corridor.

Seismic swarms in subduction zones often reflect fluid migration or episodic slow-slip phenomena that modulate stress without producing a single dominant mainshock. The 2005 sequence lacked any notably larger event and instead displayed a tight temporal and spatial grouping, aligning with this pattern. Continued monitoring by regional networks remains essential for distinguishing background seismicity from possible precursors to great earthquakes along the plate boundary.

References:
SeismoSight internal swarm catalogue (PS20051028.1 parameters).
USGS Earthquake Catalog (M 7.4 event of 18 June 2020).
Global subduction-zone compilations (Bird, 2003; Syracuse & Abers, 2006).