Seismic Swarm PS20060524.1: Analysis of Activity Near Isangel, Vanuatu

A seismic swarm designated PS20060524.1 occurred 103 km SSW of Isangel, Vanuatu, on 24 May 2006. The sequence began at 09:10 and concluded at 21:32, encompassing five earthquakes within a span of 12 hours and 22 minutes. These events clustered tightly in both time and location, characteristic of swarm behavior in subduction-zone settings.

The individual earthquakes registered as follows: at 09:10:26 a magnitude 5.5 event at 37 km depth; two minutes later at 09:10:28 a magnitude 5.1 event at 80 km depth; at 09:45:34 a magnitude 5.5 event at 40 km depth; at 16:47:40 a magnitude 5.5 event at 10 km depth; and at 21:32:53 a magnitude 5.3 event at 49 km depth. Depths ranged from shallow crustal levels to intermediate values, reflecting the complex faulting within the subducting slab.

Vanuatu lies along the convergent boundary where the Indo-Australian plate subducts beneath the Pacific plate, forming part of the Pacific Ring of Fire. This tectonic regime produces frequent seismic activity and volcanic arcs across the archipelago. The region south of Isangel, on Tanna Island, experiences elevated seismicity due to slab bending and dehydration processes at depth. Historical records document recurrent moderate-to-large events, underscoring the persistent strain accumulation along the megathrust and associated intraslab faults.

Since 2000, several strong earthquakes have struck within tens of kilometers of the 2006 swarm center. A magnitude 7.1 event occurred 118 km south of Isangel on 7 December 2023, approximately 41 km from the swarm epicenters. On 3 September 2011 a magnitude 7.0 quake struck 133 km SSE of Isangel, 82 km distant. A magnitude 7.0 event took place 118 km NNE of Tadine, New Caledonia, on 13 January 2011, 56 km from the swarm. Two magnitude 7.3 events occurred near the Loyalty Islands on 9 April 2008, 34 km and 40 km from the 2006 center respectively. An additional magnitude 7.1 shock was recorded 119 km south of Isangel on 25 March 2007, 45 km away.

Swarm sequences such as PS20060524.1 typically arise from fluid migration or stress triggering within the overriding or subducting plate, rather than from a single mainshock-aftershock cascade. The 2006 events exhibited similar magnitudes without a dominant shock, consistent with distributed deformation along pre-existing fractures in this highly active margin.

References

SeismoSight internal catalog for swarm classification PS20060524.1.
USGS Earthquake Catalog for regional strong-event parameters.
Global CMT Project for focal mechanism context in the Vanuatu subduction zone.