Seismic Swarm PS20150219.1: Insights into Vanuatu Tectonics

Vanuatu occupies a highly active segment of the Pacific Ring of Fire, where the Pacific Plate subducts westward beneath the Indo-Australian Plate at rates exceeding 10 cm per year. This convergent margin produces frequent shallow earthquakes and volcanic activity along the island arc. The archipelago’s geology reflects ongoing plate collision, with rapid uplift, active volcanoes, and a history of large-magnitude events that have repeatedly reshaped coastal landscapes.

On 19 February 2015, a seismic swarm designated PS20150219.1 was recorded 73 km east-southeast of Lakatoro. The sequence began at 13:18 UTC and concluded by 19:56 UTC, encompassing five events within six hours and thirty-eight minutes. The initial shock measured magnitude 6.4 at 10 km depth, followed rapidly by a magnitude 5.2 event at identical depth. Subsequent activity included a magnitude 5.4 shock at 14:32 UTC, a magnitude 4.7 event at 19:54 UTC, and a final magnitude 5.1 earthquake at 19:56 UTC that originated slightly deeper at 24 km. All events clustered tightly in both space and time, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

Such swarms arise when fluid migration or aseismic slip transiently increases stress on nearby faults within the subduction interface or overlying crust. In Vanuatu’s tectonic setting, these episodes commonly occur at shallow depths where the plate boundary experiences episodic unlocking. The 2015 swarm’s concentration at 10 km depth aligns with the typical seismogenic zone for this region.

Since 2000, ten swarms have been documented in the same vicinity. Earlier episodes occurred in 2002 (two swarms), 2004, 2005, 2008, 2009 (three swarms), 2010, and 2012. This recurrence underscores persistent strain accumulation along the subduction zone. A notable larger event, magnitude 7.0, struck 3 km northwest of Norsup on 28 April 2016, approximately 85 km from the 2015 swarm center, further illustrating the area’s capacity for energetic rupture.

Collectively, these observations highlight Vanuatu’s position as one of Earth’s most seismically productive arcs. Continued monitoring of swarm patterns provides valuable constraints on subduction dynamics and helps refine regional hazard assessments.

References: SeismoSight internal classification records for swarm PS20150219.1 and historical statistics since 2000. USGS Earthquake Catalog for regional tectonic framework and event verification.