Seismic Swarm PS20091011.1: Activity Near Sola, Vanuatu

A seismic swarm designated PS20091011.1 was recorded approximately 193 km west-northwest of Sola in Vanuatu's Banks Islands. The sequence began at 04:47 on 11 October 2009 and concluded at 09:37 on 12 October 2009, spanning 28 hours and 49 minutes. During this period, seven earthquakes were registered, with magnitudes ranging from 5.0 to 6.2 and focal depths between 15 km and 60 km.

The events unfolded as follows. The initial shock on 11 October at 04:47:51 reached magnitude 5.7 at 44 km depth. Subsequent activity included a magnitude 5.0 event at 16:12:14 (15 km depth), followed by a magnitude 5.2 at 16:56:52 (37 km depth). Two magnitude 5.0 earthquakes occurred within minutes at 18:02:03 (16 km depth) and 18:02:12 (60 km depth). On 12 October, a magnitude 5.2 event was noted at 06:37:03 (35 km depth), culminating in the largest shock of magnitude 6.2 at 09:37:21 (42 km depth).

This swarm occurred within the tectonically active New Hebrides subduction zone, where the Indo-Australian plate converges with the Pacific plate at rates exceeding 10 cm per year. Vanuatu experiences frequent seismicity due to this oblique subduction, which generates both interplate thrust events and intraslab earthquakes. The Banks Islands region, including areas northwest of Sola, lies near the northern extent of the arc, where complex faulting and back-arc spreading contribute to elevated seismic hazard.

Historical records indicate that five swarms have occurred in the region since 1 January 2000, with prior episodes in 2004, 2006, 2007, and two in 2009. The October 2009 sequence followed closely after a series of strong earthquakes earlier that month, including events of magnitude 7.4, 7.8, and 7.7 located between 11 km and 55 km from the swarm centroid. Such clustering is consistent with aftershock sequences and stress redistribution in subduction settings.

Vanuatu's seismic history reflects ongoing plate boundary deformation, with the potential for both moderate swarms and larger mainshock-aftershock sequences. Monitoring by regional networks continues to track activity in this high-hazard zone, supporting improved understanding of subduction dynamics and associated risks to nearby island communities.

References

• United States Geological Survey Earthquake Catalog
• Incorporated Research Institutions for Seismology Event Database
• Geoscience Australia Regional Seismicity Reports