M 7.1; 125 km W of Port-Vila, Vanuatu; (2 Feb 2012) (71km from the swarm center)
M 7.3; Vanuatu; (10 Aug 2010) (29km from the swarm center)
M 7.3; 33 km NW of Port-Vila, Vanuatu; (10 Aug 2010) (36km from the swarm center)
M 7.2; 50 km WNW of Port-Vila, Vanuatu; (2 Jan 2002) (16km from the swarm center)
Seismic Swarm Activity West of Port-Vila, Vanuatu in June 2009
Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire along the New Hebrides subduction zone, where the Australian Plate converges with the Pacific Plate at rates exceeding 10 cm per year. This setting produces frequent moderate-to-large earthquakes and occasional seismic swarms driven by slab dehydration, crustal faulting, and volcanic processes beneath the central islands.
On 12 June 2009 a swarm comprising seven earthquakes occurred 55 km west-northwest of Port-Vila. The sequence began at 08:47 UTC and concluded at 14:32 UTC, spanning five hours and forty-five minutes. Events ranged in magnitude from 5.0 to 6.0 and originated at depths of 10–60 km, consistent with activity both in the overriding plate and within the subducting slab.
The temporal distribution showed two distinct clusters. The first cluster initiated with a magnitude-5.3 event at 10 km depth followed 8 seconds later by a magnitude-5.0 shock at 60 km. A second pair occurred at 09:24 UTC (magnitudes 5.4 and 5.0). The largest events, both magnitude 6.0, struck at 09:44 UTC at 15 km and 60 km depth. The sequence closed with a magnitude-5.3 event at 14:32 UTC and 10 km depth.
Since 1 January 2000, six swarms have been recorded in the same region, with prior episodes in 2002 (two swarms), 2004 (one), 2005 (one), and a second swarm in 2009. This recurrence indicates episodic release of strain along the plate interface and adjacent crustal faults.
Multiple strong earthquakes have also occurred nearby since 2000. The closest was a magnitude-7.2 event on 2 January 2002, only 16 km from the 2009 swarm center. Additional magnitude-7.3 shocks took place on 10 August 2010 (29 km and 36 km distant), a magnitude-7.1 event on 2 February 2012 (71 km distant), and a magnitude-7.3 earthquake on 17 December 2024 (42 km distant). These larger events underscore the persistent seismic hazard along this portion of the subduction zone.
Seismic swarms such as the 2009 sequence provide insight into short-term stress migration and fluid involvement within the subduction system, complementing the long-term record of great earthquakes that characterize Vanuatu’s tectonic environment.
References
USGS Earthquake Catalog (2000–2024)
Global CMT Catalog
Pacific Island Seismicity Database