Seismic Swarm PS20050925.1: Insights into Vanuatu's Tectonic Activity

A seismic swarm designated PS20050925.1 occurred 63 km west-northwest of Port-Vila, Vanuatu, from 12:55 on 25 September 2005 to 01:34 on 26 September 2005. In 12 hours and 38 minutes, eight earthquakes were recorded, with magnitudes ranging from 5.1 to 6.1 and focal depths between 10 and 35 km. The sequence began with a magnitude 6.1 event at 30 km depth, followed by several magnitude 5.5–5.6 shocks at similar depths later that evening. Additional events included a pair of magnitude 5.5 and 5.6 tremors near 23:00, a magnitude 5.1 shock at 27 km depth, and a final magnitude 5.6 event at 35 km depth just after midnight.

Vanuatu sits within the tectonically active New Hebrides subduction zone, where the Australian Plate converges with and subducts beneath the Pacific Plate at rates exceeding 10 cm per year. This setting produces frequent moderate-to-large earthquakes and positions the archipelago along the Pacific Ring of Fire. The 2005 swarm aligns with typical patterns of clustered seismicity in this region, where stress transfer along the plate interface can trigger multiple events over short periods without a single dominant mainshock.

Since 2000, three swarms have been documented near Port-Vila, including two in 2002 and one in 2004. The 2005 sequence represents the third such episode in the early 21st century. Stronger individual earthquakes have also struck nearby, including a magnitude 7.2 event 50 km west-northwest of Port-Vila in January 2002, magnitude 7.3 shocks in August 2010 approximately 45–53 km from the swarm center, a magnitude 7.1 event 125 km west of the capital in February 2012, and a magnitude 7.3 earthquake 24 km west-northwest of Port-Vila in December 2024. These occurrences underscore the persistent seismic hazard within 55 km of the 2005 swarm location.

The combination of shallow-to-intermediate depth events in the swarm and the proximity of subsequent large earthquakes illustrates ongoing strain accumulation and release along the subduction interface. Continued monitoring remains essential for understanding precursory patterns in this high-hazard zone.

References
United States Geological Survey Earthquake Catalog
Global Centroid Moment Tensor Project
SeismoSight internal swarm classification records