Seismic Swarm PS20210216.1 Near Port-Vila, Vanuatu: Geological Context and Event Analysis

Vanuatu occupies a highly active segment of the Pacific Ring of Fire along the New Hebrides subduction zone. Here the Australian plate converges with and subducts beneath the Pacific plate at rates exceeding 10 cm per year, generating frequent earthquakes and arc volcanism. The capital, Port-Vila, sits on the island of Efate within this tectonic setting, where intermediate-depth and shallow crustal events are common.

Seismic swarm PS20210216.1 was recorded 71 km west of Port-Vila. The sequence began at 10:34 on 15 February 2021 and concluded at 12:21 on 17 February 2021, spanning 49 hours and 47 minutes. Fourteen earthquakes were registered during this interval, with magnitudes ranging from 5.0 to 6.2 and focal depths between 9 km and 14 km. The largest event, magnitude 6.2, occurred at 00:49 on 16 February at 13 km depth and was followed within minutes by several magnitude 5+ aftershocks, indicating rapid stress redistribution along a shallow fault segment.

Subsequent activity included additional magnitude 5.0–5.5 events distributed over the following 36 hours, consistent with swarm behavior rather than a classic mainshock-aftershock sequence. Depths remained shallow throughout, suggesting failure within the overriding plate or at the plate interface.

Since 1 January 2000, seventeen swarms have been documented in the same region. These occurred in 2002 (two events), 2004, 2005, 2009 (three), 2010, 2011 (four), 2012, 2015, 2016, 2018, and 2020. The recurrence points to episodic aseismic slip or fluid migration that periodically triggers clustered seismicity without producing a single dominant rupture.

Several strong earthquakes have also struck nearby since 2000. Notable examples include a magnitude 7.3 event 24 km WNW of Port-Vila on 17 December 2024 (37 km from the swarm center), a magnitude 7.1 event 125 km west of Port-Vila on 2 February 2012 (75 km from the swarm center), and multiple magnitude 7.2–7.3 shocks in 2010 and 2002 within 50 km of the 2021 swarm locus. These larger events demonstrate the capacity of the subduction interface to release substantial energy while the intervening periods are punctuated by swarm-type activity.

The 2021 swarm fits within Vanuatu’s established pattern of moderate, clustered seismicity that releases strain without escalating to great earthquakes. Continued monitoring remains essential given the proximity to populated areas and the potential for both seismic and tsunami hazards.

References:
SeismoSight internal swarm classification PS20210216.1
USGS Earthquake Catalog (historical events)
Global CMT catalog (moment-tensor solutions)