Seismic Swarm PS20150202.1: Tectonic Insights Near Vanuatu

The seismic swarm PS20150202.1 began at 13:34 on 1 February 2015 and ended at 22:37 on 2 February 2015. Centered 135 km south-southeast of Isangel, Vanuatu, the sequence produced six earthquakes across 33 hours and three minutes. Magnitudes ranged from 4.6 to 5.7, with focal depths between 7 km and 30 km. The events unfolded as follows: a magnitude 5.7 shock at 7 km depth initiated the swarm, followed within minutes by magnitude 5.0 and 5.1 events at 10 km depth. On 2 February, a magnitude 4.6 earthquake occurred at 30 km, succeeded by a magnitude 5.0 event at 10 km, and the sequence closed with a magnitude 5.4 shock at 14 km.

These closely spaced, moderate-magnitude events illustrate typical swarm behavior in subduction environments, where fluid migration and stress transfer along the plate interface can trigger clusters without a single dominant mainshock. The shallow to intermediate depths align with the seismogenic zone above the subducting slab.

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire. Convergence between the Australian and Pacific plates occurs along the New Hebrides subduction zone at rates exceeding 10 cm per year. This rapid subduction generates frequent seismicity, volcanic activity, and occasional tsunamigenic earthquakes. The island arc sits directly above the trench, where the Australian plate descends steeply, producing both shallow crustal events and deeper intraslab earthquakes.

Since 1 January 2000, seventeen swarms have been documented in the broader region. Activity clustered in certain years, with four swarms recorded in 2007 and three in 2008. Earlier episodes occurred in 2001, 2003, 2004, 2005 (two), 2006, 2009 (two), and 2011 (two). Such recurrent swarms reflect persistent strain accumulation and release along the megathrust and associated crustal faults.

Major earthquakes have also struck nearby. A magnitude 7.0 event on 14 September 2022 occurred 209 km south-southeast of Isangel, only 20 km from the 2015 swarm center. Additional magnitude 7+ shocks include a magnitude 7.1 event 238 km east-southeast of Tadine, New Caledonia, in 2018 (84 km from the swarm), a magnitude 7.0 shock 133 km south-southeast of Isangel in 2011 (76 km distant), and a magnitude 7.3 earthquake 201 km east-southeast of Tadine in 2003 (89 km away). These larger ruptures demonstrate the capacity of the subduction interface to release substantial energy while smaller swarms accommodate distributed deformation.

Continued monitoring of swarm patterns contributes to refined hazard assessments for Vanuatu and neighboring island nations. The interplay between swarm sequences and infrequent great earthquakes underscores the complex stress evolution along this rapidly converging plate boundary.

Data from SeismoSight internal classification. Geological framework drawn from USGS and Geoscience Australia subduction zone summaries.