Seismic Activity in the Tonga Subduction Zone: Analysis of the November 2009 Swarm

The Tonga region forms part of the Pacific Ring of Fire and lies above the Tonga Trench, where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 15 cm per year. This tectonic setting produces frequent intermediate-depth and shallow earthquakes, along with volcanic activity on the overriding plate. The area near ‘Ohonua on the island of ‘Eua experiences recurrent seismicity due to slab bending, dehydration, and frictional locking along the megathrust interface.

On 24 November 2009, a seismic swarm designated PS20091124.1 occurred approximately 120 km east-northeast of ‘Ohonua. The sequence began at 12:47 UTC and concluded at 17:21 UTC, lasting four hours and thirty-three minutes. Five earthquakes were recorded during this interval. The two largest events, both magnitude 6.8, occurred within two seconds of each other at depths of 18 km and 60 km. Subsequent events included a magnitude 5.6 at 13:04, a magnitude 5.1 at 13:32, and a magnitude 4.8 at 17:21, all at shallow depths near 10 km. Such rapid succession of moderate-to-large events within a confined volume suggests localized stress transfer along the subducting slab or within the overriding crust.

Historical records indicate only two swarms have been identified in the region since 2000. The earlier swarm occurred in 2006, coinciding with a period of heightened activity that included the magnitude 8.0 Tonga earthquake of 3 May 2006, centered 74 km from the 2009 swarm location, and an associated magnitude 7.8 event 97 km away. These large ruptures likely altered the regional stress field, influencing subsequent swarm behavior.

A magnitude 7.0 earthquake on 30 March 2025, located 61 km south-southeast of Pangai and 60 km from the swarm center, further illustrates the persistent seismic productivity of the Tonga subduction zone. Such events underscore the capacity for both isolated large ruptures and clustered moderate sequences within the same tectonic framework.

Seismic swarms in subduction zones often reflect fluid migration, aseismic slip, or aftershock triggering rather than a single mainshock-aftershock sequence. In the Tonga setting, intermediate-depth events may arise from phase changes within the slab, while shallow events align with crustal faulting above the plate interface. Continued monitoring remains essential for distinguishing background seismicity from precursory patterns ahead of future large earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Tectonic framework descriptions from peer-reviewed studies on Tonga subduction dynamics