Seismic Swarm PS20060418.1: Analysis of Activity Northwest of Sola, Vanuatu

Seismic swarm PS20060418.1 occurred in the waters northwest of Sola, Vanuatu, between 23:49 on 17 April 2006 and 20:15 on 18 April 2006. Over this 20-hour 25-minute period, five earthquakes were recorded at a location 202 km northwest of Sola. The sequence began with a magnitude 6.1 event at 16 km depth, followed minutes later by a magnitude 5.8 shock at 74 km depth. Activity resumed the next day with two magnitude 5.3 events at 33 km and 57 km depths, concluding with a magnitude 4.5 event at 40 km depth.

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire. The archipelago sits above the subduction zone where the Pacific Plate descends beneath the Australian Plate at rates exceeding 10 cm per year. This convergence produces frequent intermediate-depth and shallow seismicity, as well as arc volcanism. The swarm’s epicentral area lies along the northern extension of the New Hebrides Trench, where the plate interface exhibits complex geometry and variable coupling.

Swarm events of this type are common in subduction fore-arc and back-arc settings. They often reflect fluid migration or transient stress changes rather than progressive mainshock-aftershock sequences. The 2006 swarm’s mixed focal depths, spanning the upper 74 km, are consistent with deformation distributed across both the subducting slab and the overlying plate. No larger mainshock preceded or followed the cluster, aligning with the characteristic pattern of seismic swarms.

Historical records maintained since 2000 indicate only two prior swarms in the immediate region: one in 2000 and another in 2004. This low frequency underscores that clustered activity remains episodic despite the region’s persistently high background seismicity. In October 2009, a notable sequence of great earthquakes occurred within tens of kilometres of the 2006 swarm centre, including events of magnitude 7.4, 7.8, and 7.7. These larger ruptures highlight the potential for the same fault segments to host both moderate swarms and great earthquakes over decadal timescales.

The 2006 swarm provides a useful reference for understanding short-term clustering in northern Vanuatu. Continued monitoring by regional networks improves the resolution of such sequences and supports refined assessments of seismic hazard along the Vanuatu subduction zone.

References
USGS Earthquake Catalog
Global CMT Catalog
SeismoSight internal swarm classification database