Seismic Swarm Activity Near Port-Vila, Vanuatu in January 2002

Vanuatu occupies a highly active tectonic setting within the Pacific Ring of Fire. The islands sit above the subduction zone where the Indo-Australian Plate descends beneath the Pacific Plate, producing frequent earthquakes and volcanic activity. This convergent margin generates both shallow crustal events and deeper intraslab seismicity, with the region experiencing some of the highest rates of seismic energy release globally.

On 6 January 2002, a seismic swarm was recorded 31 km northwest of Port-Vila. The sequence began at 07:41 local time and concluded by 17:09, spanning 9 hours and 28 minutes. Five earthquakes were detected during this interval, all with focal depths of 10 km. The events registered magnitudes of 5.0, 5.4, 5.6, 4.8, and 5.4 respectively. Such swarms represent clusters of seismicity without a single dominant mainshock, often linked to fluid migration or stress adjustments along fault networks in subduction environments.

This swarm occurred four days after a magnitude 7.2 earthquake on 2 January 2002, located 23 km from the swarm center. The proximity suggests the larger event may have altered local stress conditions, triggering the subsequent cluster. Historical records since 2000 indicate this was the sole swarm identified in the immediate vicinity through the present. In contrast, the broader area has hosted multiple strong earthquakes, including magnitude 7.3 events in August 2010 only 5–9 km from the swarm epicenter, a pair of magnitude 7.1–7.2 shocks in August 2011 located 91–96 km to the south, and a magnitude 7.3 event on 17 December 2024 situated 21 km from the 2002 swarm center.

These larger events underscore the persistent seismic hazard along Vanuatu’s western margin. The 2010 doublet and the recent 2024 shock both highlight the potential for magnitude 7+ ruptures within tens of kilometers of the capital, Port-Vila. Shallow focal depths around 10 km, as observed in the 2002 swarm, increase the likelihood of strong ground shaking in populated areas.

Long-term monitoring by regional and global networks continues to refine understanding of swarm behavior in this setting. While individual swarms remain infrequent in the catalog since 2000, they provide valuable data on short-term stress transfer following major subduction-zone earthquakes.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geoscience Australia Earthquake Database