Seismic Swarm PS20091014.1: Geological Context and Event Analysis Near Sola, Vanuatu

The earthquake swarm designated PS20091014.1 occurred approximately 167 km northwest of Sola in Vanuatu’s Banks Islands region. Registered between 20:48 on 13 October 2009 and 03:19 on 14 October 2009, the sequence comprised five events within a 6-hour-30-minute window. Magnitudes ranged from 5.0 to 5.4, with focal depths varying between 24 km and 113 km. The individual events unfolded as follows: a 5.2-magnitude shock at 42 km depth at 20:48:11 UTC on 13 October, followed minutes later by a 5.4-magnitude event at 113 km depth; subsequent activity included two 5.4- and 5.0-magnitude quakes at shallower depths of 24 km and 35 km around 00:55 and 03:18 on 14 October, concluding with a final 5.0-magnitude shock at 80 km depth.

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire, where the Indo-Australian Plate subducts westward beneath the Pacific Plate along the New Hebrides Trench. This convergent margin drives frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. The Sola area lies near the central portion of the Vanuatu arc, where convergence rates exceed 10 cm per year, producing a high density of moderate-to-large earthquakes and occasional swarm activity. Historical records since 2000 document seven prior swarms in the vicinity, occurring in 2000, 2004, 2006, 2007, and three instances in 2009, underscoring the region’s recurrent clustered seismicity.

This particular swarm followed closely after a series of major earthquakes in early October 2009. Four events exceeding magnitude 7.0 struck within 21–35 km of the swarm centroid on 7 October, including magnitudes 7.4, 7.8 (two events), and 7.7. An additional magnitude 7.2 earthquake occurred in May 2010 roughly 99 km from the same center. Such large ruptures can alter local stress fields, potentially triggering subsequent moderate sequences through static stress transfer or fluid migration along the subduction interface.

The depth distribution within the swarm—spanning both crustal and upper-mantle levels—reflects the complex geometry of the subducting slab beneath Vanuatu. Shallower events likely originated within the overriding plate or near the plate interface, while deeper shocks sampled intraslab deformation. No damage or tsunami reports were associated with this swarm, consistent with its moderate magnitudes and offshore location.

SeismoSight internal classification
USGS Earthquake Catalog (historical magnitudes and locations)
Global CMT Project (focal mechanisms and depths)
Pacific Tsunami Warning Center regional bulletins