Seismic Swarm PS20110820.1: Analysis of Activity South of Port-Vila, Vanuatu

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire, where the Indo-Australian plate subducts beneath the Pacific plate along the New Hebrides Trench. This subduction drives frequent seismicity, with the region experiencing both isolated large-magnitude events and episodic earthquake swarms. The capital, Port-Vila, lies near the convergent margin, making southern offshore zones particularly prone to clustered activity.

The swarm designated PS20110820.1 began at 09:50 on 20 August 2011 and concluded at 02:52 on 22 August 2011. Centered 99 km south of Port-Vila, it produced 18 earthquakes over 41 hours and 2 minutes. The sequence featured two mainshocks exceeding magnitude 7.0, accompanied by multiple aftershocks in the 5.0–5.6 range, with focal depths predominantly between 28 km and 47 km.

The initial event reached magnitude 5.3 at 35 km depth. Roughly seven hours later, a magnitude 7.2 earthquake struck at 32 km depth, followed within minutes by events of 5.3, 5.4, and 6.5. A second major shock of magnitude 7.1 occurred at 28 km depth approximately 30 minutes after the 6.5 event. Subsequent activity included twelve additional events through 22 August, with magnitudes ranging from 5.0 to 5.6 and depths mostly near 35 km. One deeper event reached 80 km.

This swarm fits within Vanuatu’s documented pattern of seismic clustering. Since 2000, twelve swarms have occurred in the broader region, with notable concentrations in 2009 and 2010. Strong earthquakes since the start of 2000 include multiple events above magnitude 7.0, such as the magnitude 7.3 on 17 December 2024 located 77 km from the swarm center, the two 2011 mainshocks themselves, and earlier events in 2010 and 2002. These occurrences underscore the persistent strain accumulation and release along the subduction interface.

Swarm sequences in this setting often reflect fluid migration or stress transfer within the overriding plate and megathrust zone. Depths in the 28–47 km range align with typical interface and intraslab seismicity in southern Vanuatu. The 2011 swarm’s proximity to later strong events highlights the area’s continued potential for significant rupture.

SeismoSight internal records.
USGS Earthquake Catalog (historical magnitudes and locations).
Global CMT Project (focal mechanisms and depths).