Seismic Swarm PS20080409.1: Analysis of April 2008 Activity Near Vanuatu

A seismic swarm designated PS20080409.1 was recorded 93 km south-southwest of Isangel, Vanuatu, from 14:20 on 8 April 2008 to 04:17 on 11 April 2008. Over 61 hours and 57 minutes, the sequence produced 21 earthquakes. The events clustered in a tectonically active segment of the New Hebrides subduction zone, highlighting the region's persistent seismic hazard.

The swarm began with moderate events on 8 April, including magnitudes 5.2 and 5.0 at depths of 10 km and 35 km. Activity intensified on 9 April with a rapid succession of larger shocks. Notable events included a pair of magnitude 6.4 and 6.3 earthquakes within seconds at 11:13, followed by an additional magnitude 6.3 at 11:23. The sequence peaked with two magnitude 7.3 events at 12:46, occurring at 33 km and 80 km depth. Subsequent shocks ranged from magnitude 5.0 to 6.3 through 10 April, concluding with a magnitude 5.2 event on 11 April. Depths predominantly fell between 10 km and 80 km, consistent with both crustal and intermediate-depth seismicity along the subduction interface.

Vanuatu lies along the Pacific Ring of Fire at the convergent boundary where the Australian plate subducts eastward beneath the Pacific plate. This New Hebrides trench system generates frequent earthquakes through megathrust slip, intraslab deformation, and volcanic-arc processes. The Loyalty Islands region, immediately south of the swarm epicenters, experiences elevated strain accumulation due to the oblique subduction angle and the presence of the Loyalty Ridge, which influences local stress distribution.

Historical records since 2000 document five prior swarms in the immediate vicinity, occurring in 2006 (one swarm) and 2007 (four swarms). Strong earthquakes in the same period include a magnitude 7.3 event on 9 April 2008 located 27 km from the swarm center, a second magnitude 7.3 shock 34 km away on the same day, and additional magnitude 7+ events in 2007, 2011, and 2023, all within 63 km of the 2008 swarm centroid. These patterns underscore recurring clusters of large-magnitude activity along this segment of the subduction zone.

The 2008 swarm exemplifies how moderate-to-large events can occur in tight temporal and spatial succession, driven by stress transfer along the plate interface and within the downgoing slab. Such sequences contribute to the long-term seismic energy release that characterizes the Vanuatu arc.

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