Seismic Swarm PS20110429.1 in the Vanuatu Region

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire, where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 10 cm per year. This subduction drives frequent moderate-to-large earthquakes and occasional seismic swarms throughout the island arc. The region’s seismicity reflects both interplate thrust faulting and intraslab events within the downgoing slab, with hypocenters commonly ranging from shallow crustal depths to more than 100 km.

On 29 April 2011 a swarm designated PS20110429.1 began at 07:21 UTC and concluded at 13:08 UTC, spanning 5 hours and 47 minutes. During this interval six earthquakes of magnitude 5.0–5.1 were recorded. The sequence initiated with an event of magnitude 5.0 at 10 km depth, followed within 15 minutes by two closely spaced shocks of magnitudes 5.1 and 5.0 at 10 km and 60 km, respectively. Subsequent events at 09:38, 09:47 and 13:08 UTC each registered magnitude 5.0 at depths of 10 km, 23 km and 10 km. All six events clustered near the same epicentral area, consistent with a short-lived episode of strain release along the subduction interface or within the upper portion of the subducting slab.

Seismic swarms of this character have occurred repeatedly in Vanuatu. Since 1 January 2000 the region has experienced 13 documented swarms. The years 2002, 2004, 2005, 2006 and 2007 each recorded one swarm; 2008, 2009, 2010 and 2011 each recorded two. These clusters typically last from a few hours to several days and consist predominantly of events in the magnitude 4.5–5.5 range, rarely producing damage but serving as indicators of ongoing plate-boundary deformation.

Four and a half months earlier, on 25 December 2010, a magnitude 7.3 earthquake struck 141 km west of Isangel, approximately 45 km from the later swarm centroid. That large thrust event released substantial strain along the plate interface and may have altered local stress conditions, potentially contributing to the April 2011 swarm through afterslip or viscoelastic relaxation.

Collectively, the 2011 swarm and the preceding great earthquake underscore the persistent seismic hazard along the Vanuatu subduction zone. Continued monitoring of swarm activity provides valuable constraints on fault behavior and helps refine probabilistic assessments of future large events in this rapidly deforming arc.

References USGS Earthquake Catalog (events 2000–2011) SeismoSight internal swarm classification PS20110429.1