Seismic Swarm PS20061009.1: Analysis of October 2006 Events Near Tonga

The seismic swarm designated PS20061009.1 occurred in the Tonga region of the southwest Pacific Ocean. It began at 13:50 on 8 October 2006 and concluded at 00:09 on 9 October 2006, spanning 10 hours and 19 minutes. The epicenter was located 236 km south-southwest of ‘Ohonua, Tonga. During this interval, five earthquakes were recorded, all at a focal depth of 10 km.

The sequence commenced with a magnitude 5.8 event at 13:50:24 on 8 October, followed by a 5.3 event at 14:10:16. Subsequent shocks included magnitudes of 5.2 at 14:46:54 and 18:54:55 on the same day. The swarm ended with a magnitude 4.7 earthquake at 00:09:59 on 9 October. These events clustered closely in both time and space, characteristic of swarm behavior where no single mainshock dominates.

Tonga lies along the Tonga-Kermadec subduction zone, where the Pacific Plate subducts westward beneath the Indo-Australian Plate at rates exceeding 15 cm per year. This tectonic setting produces frequent shallow seismicity, volcanic arcs, and occasional deeper events extending to 700 km. The 10 km depths of the swarm events align with typical upper-plate or interface activity in this convergent margin. Historical records indicate persistent seismic productivity, with the region contributing significantly to global earthquake catalogs.

Since 1 January 2000, only two swarms have been documented in the immediate area prior to 2006, occurring in 2001 and 2004. This low frequency underscores the episodic nature of swarm activity amid more continuous background seismicity. A notable larger event, magnitude 7.6, struck on 19 March 2009 approximately 191 km south of ‘Ohonua and 93 km from the swarm center, highlighting the potential for stronger earthquakes in the broader zone.

Such swarms provide insight into stress transfer along the subduction interface and may precede or accompany volcanic unrest, though no direct linkage was established in this case. Monitoring remains essential given Tonga’s position on the Pacific Ring of Fire.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical Tonga region data).
Global CMT Project (tectonic context for Tonga-Kermadec subduction).