Seismic Swarm PS20160619.1 Near Vanuatu: Characteristics and Tectonic Context

A seismic swarm designated PS20160619.1 occurred approximately 107 km south-southwest of Isangel, Vanuatu, in the southwestern Pacific Ocean. The sequence began at 20:38 on 18 June 2016 and concluded at 03:50 on 20 June 2016, spanning 31 hours and 12 minutes. During this interval, eight earthquakes were recorded, with magnitudes ranging from 4.3 to 6.3 and focal depths between 10 km and 60 km.

The sequence initiated with a magnitude 5.1 event at 20:38 on 18 June at a depth of 10 km. The largest events followed on 19 June at 09:47, comprising two magnitude 6.3 earthquakes at depths of 10 km and 13 km. Subsequent activity included a magnitude 4.3 at 10:02, two magnitude 5.1 events at 10:04 and 10:31, a magnitude 5.0 at 10:31, and a magnitude 6.0 event at 03:50 on 20 June at 15 km depth. These events clustered tightly in both time and space, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Vanuatu lies along the New Hebrides subduction zone, where the Australian plate converges with the Pacific plate at rates exceeding 10 cm per year. This tectonic setting produces frequent shallow to intermediate-depth seismicity and generates the volcanic arc of the Vanuatu islands. The swarm location falls within a region of elevated background seismicity near the transition between the Vanuatu and Loyalty Islands segments of the arc.

Historical records since 2000 indicate 16 prior swarms in the immediate vicinity, with notable concentrations in 2011 (five swarms), 2007 (four swarms), and 2008 (three swarms). Additional swarms occurred in 2006, 2010, and 2015. The area has also experienced multiple magnitude 7 or greater earthquakes since 2000, including a magnitude 7.3 event on 9 April 2008 located 22 km from the 2016 swarm center, another magnitude 7.3 on the same date 29 km away, a magnitude 7.1 on 25 March 2007 74 km distant, a magnitude 7.0 on 13 January 2011 45 km away, and a magnitude 7.1 on 7 December 2023 70 km from the swarm epicenter.

Such recurrent swarms and large thrust events reflect ongoing plate-boundary deformation and episodic strain release along the subduction interface. The 2016 swarm, with its rapid succession of moderate events at shallow depths, aligns with patterns observed in this highly active margin.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog for regional tectonic and historical event data