Seismic Swarm PS20110109.1: Analysis of Earthquake Activity Near Isangel, Vanuatu

On 9 January 2011, a seismic swarm designated PS20110109.1 occurred approximately 106 km west-northwest of Isangel, Vanuatu. The sequence began at 10:03 local time and concluded at 19:24 the same day, spanning 9 hours and 20 minutes. During this interval, 12 earthquakes were recorded, with magnitudes ranging from 4.9 to 6.5 and focal depths between 10 km and 60 km. The two largest events reached magnitudes 6.5 at 10:03 and 6.1 at 17:21, both at relatively shallow depths of 22 km and 18 km, respectively. Subsequent events clustered around magnitudes 5.2 to 5.7, predominantly at depths of 10 km, with a few occurring deeper at 40–60 km.

This swarm exemplifies clustered seismic activity typical of subduction zone environments, where stress accumulation along plate boundaries releases in rapid succession without a single dominant mainshock. The temporal pattern shows an initial strong event followed by aftershocks that maintained elevated rates for several hours before tapering. Depths indicate activity within the upper crust and near the plate interface, consistent with the tectonic setting of the region.

Vanuatu forms part of the Vanuatu subduction zone, where the Indo-Australian Plate subducts eastward beneath the Pacific Plate at rates exceeding 10 cm per year. This convergence drives frequent seismicity and volcanism across the archipelago. The island arc has experienced recurrent earthquake swarms since at least 2000, with ten documented episodes through 2010, including clusters in 2008 (three swarms) and 2010 (two swarms). Earlier events occurred singly in 2002, 2004, 2006, 2007, and 2009.

A magnitude 7.3 earthquake struck 141 km west of Isangel on 25 December 2010, approximately 36 km from the January 2011 swarm centroid. Such proximity highlights the persistent seismic hazard along this segment of the plate boundary. Historical records indicate that Vanuatu experiences some of the highest rates of great earthquakes globally due to its position within the Pacific Ring of Fire.

The 2011 swarm did not produce reported damage or tsunami, likely owing to its offshore location and moderate magnitudes. Monitoring by regional seismic networks continues to track such sequences to refine understanding of stress transfer and potential foreshock behavior in subduction settings.

References

USGS Earthquake Catalog (historical data since 2000)
Global CMT Catalog (tectonic parameters)
Pacific Tsunami Warning Center regional reports