Seismic Swarm PS20110831.1 Near Vanuatu: Geological Context and Event Analysis

A seismic swarm designated PS20110831.1 occurred in the waters southwest of Port-Vila, Vanuatu, registering five earthquakes over 18 hours and 47 minutes. The sequence began at 09:14 on 30 August 2011 and concluded at 04:01 on 31 August 2011, with its epicenter located 142 km south-southwest of Port-Vila. All events originated at a consistent depth of 35 km, reflecting activity within the subducting slab beneath the region.

The individual earthquakes included a magnitude 5.2 event at 09:14:05 on 30 August, followed closely by a 5.3 at 09:16:53 and a 5.1 at 09:26:07. Activity continued later that day with a magnitude 5.0 at 21:19:06, and the swarm ended with a 4.8 event at 04:01:10 on 31 August. These moderate tremors clustered tightly in both time and space, characteristic of swarm behavior rather than a mainshock-aftershock sequence.

Vanuatu lies along the Pacific Ring of Fire at the convergent boundary where the Pacific Plate subducts beneath the Australian Plate. This tectonic setting produces frequent seismic activity, including swarms and occasional large thrust earthquakes. Depths around 35 km align with intermediate-depth seismicity typical of the slab interface in this arc system. The swarm's location places it within a corridor of elevated background seismicity extending south from Efate Island.

Historical records since 2000 indicate 14 such swarms in the broader area. These occurred in 2002 (two events), 2004 (one), 2005 (one), 2009 (three), 2010 (three), and 2011 (four), underscoring the recurrent nature of clustered seismicity in the region. The 2011 swarm followed closely after two significant earthquakes on 20 August 2011: a magnitude 7.1 located 64 km south of Port-Vila and a magnitude 7.2 located 71 km south-southwest of Port-Vila. An additional strong event of magnitude 7.3 struck 24 km west-northwest of Port-Vila on 17 December 2024.

Such swarms provide insight into stress transfer along the subduction interface and may occur in response to nearby larger ruptures or aseismic slip. Depths remaining fixed at 35 km across the sequence suggest a localized patch of the plate boundary undergoing distributed failure. Continued monitoring in this tectonically active zone remains essential for understanding patterns that precede larger events.

References

United States Geological Survey Earthquake Catalog (USGS).
SeismoSight internal swarm classification database.