Seismic Swarm West of Port-Vila, Vanuatu: January 2002

Vanuatu lies along the convergent boundary between the Australian and Pacific plates, where the latter subducts beneath the former at rates exceeding 10 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional seismic swarms within the overriding plate and along the subduction interface. The capital, Port-Vila, on Efate Island, sits above a region of active faulting influenced by both the main subduction zone and local back-arc structures.

Between 17:22 on 2 January and 07:13 on 4 January 2002, a swarm of nine earthquakes occurred 34 km west of Port-Vila. The sequence began with a magnitude 7.2 event at 21 km depth, followed within minutes by several magnitude 5+ aftershocks clustered near 10 km depth. A magnitude 6.6 earthquake occurred roughly 17 hours later, accompanied by additional events of magnitude 5.1–5.5. All but the initial shock were located at depths of 10 km, indicating activity within the shallow crust.

The 2002 swarm fits a recurring pattern of clustered seismicity near Efate. Since 2000, multiple magnitude 7+ earthquakes have struck within 100 km of the swarm centroid, including events in 2010 (M 7.3), 2011 (M 7.1 and M 7.2), 2012 (M 7.1), and most recently a magnitude 7.3 on 17 December 2024 located 24 km west-northwest of Port-Vila. These larger shocks demonstrate that the area remains capable of generating strong ground motion despite the modest size of most swarm events.

Seismic swarms in subduction zones can reflect fluid migration, stress transfer from a mainshock, or slow slip episodes. In the present case the sequence opened with a sizable mainshock and transitioned into a short-lived aftershock series rather than a prolonged swarm of small events, illustrating how even brief clusters contribute to the overall seismic energy release along the plate boundary.

• SeismoSight internal swarm catalogue (PS20020102.1)
• USGS Earthquake Catalog (events since 2000)