Seismic Swarm Activity Near Vanuatu: December 2010 Analysis

An earthquake swarm designated PS20101225.1 was recorded 150 km west-northwest of Isangel, Vanuatu, beginning at 13:16 on 25 December 2010 and concluding at 09:46 on 29 December 2010. Over 92 hours and 29 minutes, the sequence produced 65 earthquakes. The event initiated with a magnitude 7.3 shock at 16 km depth, followed by a dense series of aftershocks ranging from magnitude 4.6 to 6.4, with focal depths predominantly between 9 km and 80 km.

The swarm exhibited typical characteristics of clustered seismicity in a subduction environment. Early activity on 25 December featured multiple events above magnitude 5.0 within the first few hours, including paired shocks at similar times but differing depths, such as the 5.3 at 10 km and 5.4 at 60 km at 13:59. Subsequent days showed sustained moderate-magnitude releases, peaking again with a magnitude 6.0 event on 26 December at 13 km depth and a magnitude 6.4 shock on 29 December at 16 km depth that marked the final significant release.

Vanuatu lies along the New Hebrides subduction zone, where the Indo-Australian plate converges with the Pacific plate at rates exceeding 10 cm per year. This tectonic setting produces frequent shallow to intermediate-depth earthquakes and occasional deeper activity associated with slab dehydration. The December 2010 swarm occurred within this active margin, consistent with regional patterns of episodic seismic swarms driven by stress transfer along the plate interface and upper-plate faults.

Historical records maintained by SeismoSight indicate seven prior swarms in the vicinity since 1 January 2000, occurring in 2002, 2006, 2007, 2008 (three separate episodes), and 2009. These clusters demonstrate recurring swarm behavior in the area. Notable strong earthquakes since 2000 include a magnitude 7.3 event 141 km west of Isangel on 25 December 2010, located 28 km from the swarm center, as well as two magnitude 7.3 shocks near the Loyalty Islands and 71 km southwest of Isangel on 9 April 2008, positioned 95 km and 93 km from the swarm center respectively.

Such sequences highlight the persistent seismic hazard in Vanuatu, where rapid plate convergence sustains elevated strain accumulation and release. Monitoring of swarm parameters, including temporal distribution and depth variation, aids in distinguishing these events from isolated mainshock-aftershock sequences and supports improved characterization of regional tectonics.

References
SeismoSight internal swarm classification database (PS20101225.1 parameters and historical statistics).
USGS Earthquake Catalog (strong earthquake locations and magnitudes since 2000).