Seismic Swarm PS20080411.2: A Detailed Examination of the April 2008 Event Near Vanuatu

The seismic swarm designated PS20080411.2 was recorded in the waters 98 km south-southwest of Isangel, Vanuatu. Beginning at 15:54 UTC on 11 April 2008 and concluding at 17:56 UTC the same day, the sequence lasted two hours and two minutes and comprised five earthquakes. All events occurred within the shallow to intermediate depth range typical of the Vanuatu subduction zone.

The individual events unfolded as follows. At 15:54:16 UTC a magnitude 5.4 earthquake struck at 35 km depth. Thirty-one minutes later, at 16:35:57 UTC, a magnitude 5.9 event occurred at the same depth. At 17:24:47 UTC a magnitude 4.9 shock was recorded, again at 35 km. The largest event of the swarm, magnitude 6.1, took place at 17:45:01 UTC at a shallower depth of 11 km. The sequence closed with a magnitude 5.0 earthquake at 17:56:56 UTC and 23 km depth.

Vanuatu lies along the convergent margin where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 10 cm per year. This tectonic setting produces one of the world’s highest rates of seismicity and hosts numerous active volcanoes. The April 2008 swarm was centered near the southern termination of the Vanuatu arc, an area characterized by oblique subduction and frequent moderate-to-large thrust earthquakes.

Since 1 January 2000, six seismic swarms have been identified in the immediate vicinity. One swarm occurred in 2006 and four in 2007; the 2008 swarm was the sole event recorded that year. In addition, five earthquakes of magnitude 7.0 or greater have struck within 70 km of the swarm centroid since 2000. These include a magnitude 7.3 event on 9 April 2008 located 28 km from the swarm center and a second magnitude 7.3 shock the same day situated 35 km away. Further strong events comprise a magnitude 7.1 earthquake on 25 March 2007 (66 km distant), a magnitude 7.0 shock on 13 January 2011 (43 km distant), and a magnitude 7.1 event on 7 December 2023 (61 km distant).

The 11 April 2008 swarm followed closely after the pair of magnitude 7.3 earthquakes on 9 April, suggesting possible stress redistribution within the subduction interface. The shallowest event (11 km) and the overall depth range (11–35 km) are consistent with rupture on or near the plate boundary. Such swarms commonly reflect either aftershock sequences or episodic aseismic slip that triggers multiple moderate events over short time intervals.

Ongoing monitoring by regional and global seismic networks continues to track activity along the Vanuatu arc. The combination of frequent swarms and occasional great earthquakes underscores the persistent seismic hazard facing the island nation and neighboring regions of the southwest Pacific.

References
SeismoSight internal classification records
USGS Earthquake Catalog (historical events)
Global CMT Project (moment-tensor solutions for regional events)