Seismic Swarm PS20160406.1: Analysis of Vanuatu Earthquake Sequence

Vanuatu occupies a tectonically active segment of the Pacific Ring of Fire, where the Australian Plate subducts beneath the Pacific Plate along the New Hebrides Trench. This convergent margin produces frequent shallow to intermediate-depth earthquakes and occasional volcanic activity across the archipelago. The region experiences elevated seismicity due to rapid plate convergence rates exceeding 10 centimeters per year, resulting in complex fault interactions and periodic earthquake swarms.

The swarm designated PS20160406.1 began at 06:54 on 6 April 2016 and concluded at 03:32 on 7 April 2016, spanning 20 hours and 38 minutes. Six earthquakes were recorded during this interval. The sequence opened with a magnitude 5.8 event at 35 km depth, followed two minutes later by the largest shock of magnitude 6.7 at 24 km depth. Subsequent events included a magnitude 5.9 at 35 km, a magnitude 5.1 at 35 km, and a magnitude 4.9 at 35 km, all on 6 April. The final event, a magnitude 6.7 at 27 km depth, occurred on 7 April. Depths remained predominantly in the upper crust, consistent with activity along the subduction interface and overlying crustal faults.

Such swarms reflect episodic stress release without a single dominant mainshock-aftershock pattern. The close temporal spacing and comparable magnitudes of the two 6.7 events suggest triggered slip on adjacent fault segments. Depths between 24 and 35 km place the activity near the plate interface, where fluid migration or aseismic slip may facilitate swarm behavior.

Since 2000, nine swarms have occurred in the same general area of Vanuatu. These took place in 2000, 2006, 2008, 2009 (three separate episodes), 2010 (two episodes), and 2011. The recurrence indicates that localized stress conditions periodically favor swarm-type sequences rather than isolated large events.

A magnitude 7.2 earthquake struck approximately 36 km from the swarm center on 27 May 2010. That event, located 100 km west-northwest of Sola, highlights the potential for larger ruptures within the same tectonic corridor. The proximity of the 2010 mainshock to the 2016 swarm center underscores ongoing strain accumulation along this portion of the subduction zone.

Collectively, the 2016 swarm fits within Vanuatu’s established pattern of clustered seismicity driven by plate boundary dynamics. Continued monitoring of depth distributions and magnitude progressions in future swarms can refine understanding of stress transfer mechanisms in this high-hazard setting.