Seismic Swarm PS20160430.1: Analysis of Activity Near Lakatoro, Vanuatu

Vanuatu lies within the tectonically active Pacific Ring of Fire, where the Indo-Australian Plate subducts beneath the Pacific Plate along the New Hebrides Trench. This convergence produces frequent earthquakes and volcanic activity across the archipelago, with Malekula Island and surrounding areas experiencing elevated seismicity due to the complex plate boundary geometry. The region has a well-documented history of both isolated large-magnitude events and clustered swarm sequences.

Seismic swarm PS20160430.1 was recorded 25 km south-southeast of Lakatoro. The sequence began at 22:09 on 29 April 2016 and concluded at 20:30 on 30 April 2016, spanning 22 hours and 21 minutes. Five earthquakes were detected during this interval, with magnitudes ranging from 5.0 to 5.8 and focal depths between 2 km and 35 km. The largest event, magnitude 5.8, occurred at a shallow depth of 2 km on 30 April at 08:35:42 UTC. Subsequent events included a magnitude 5.2 shock at 6 km depth later that day.

Such swarms reflect episodic stress release along local fault systems without a single dominant mainshock-aftershock pattern. In the broader Vanuatu context, shallow crustal events like those in this swarm often occur in zones of forearc deformation, while deeper events align with the subducting slab interface.

Historical records since 2000 indicate 11 prior swarms in the same general area, occurring in 2002 (two events), 2004, 2005, 2008, 2009 (three events), 2010, 2012, and 2015. This pattern underscores the recurrent nature of clustered seismicity near the central Vanuatu islands. A magnitude 7.0 earthquake struck 3 km northwest of Norsup on 28 April 2016, approximately 29 km from the swarm centroid, followed days later by the PS20160430.1 sequence. An earlier magnitude 7.2 event occurred 56 km east of Luganville in August 2007, 85 km from the 2016 swarm location.

These observations illustrate the persistent seismic hazard in Vanuatu driven by ongoing plate convergence. Continued monitoring remains essential for understanding swarm dynamics and their relationship to larger tectonic processes.

References

USGS Earthquake Catalog
Global CMT Project
Vanuatu Meteorology and Geo-Hazards Department reports