The seismic swarm PS20070404.2 was recorded 129 km south-southwest of Isangel, Vanuatu. It began at 20:26 on 3 April 2007 and ended at 16:38 on 4 April 2007, lasting 20 hours and 12 minutes and producing six earthquakes. The sequence opened with a magnitude 6.2 event at 20:26 on 3 April at 12 km depth. On 4 April, a magnitude 6.2 shock occurred at 11:00 at 13 km depth, followed two minutes later by a magnitude 6.5 event at 10 km depth. Three additional events of magnitudes 4.9, 5.0 and 5.0 were recorded at 10 km depth between 13:05 and 16:38. Vanuatu occupies the central New Hebrides Arc, where the Australian Plate subducts eastward beneath the Pacific Plate at rates exceeding 10 cm per year. This convergent margin generates frequent intermediate-depth and shallow crustal seismicity. The April 2007 swarm occurred within the shallow portion of the Wadati-Benioff zone, consistent with stress release along the plate interface or within the overriding plate. Since 1 January 2000, only two prior swarms have been identified in the immediate area: one in 2003 and one in 2006. The 2007 swarm followed a magnitude 7.1 earthquake on 25 March 2007 located 52 km from the swarm centroid, suggesting possible aftershock triggering or continued adjustment of regional stress. Additional strong events within 100 km include magnitude 7.3 shocks on 9 April 2008, a magnitude 7.0 event on 13 January 2011, a magnitude 7.0 shock on 3 September 2011, a magnitude 7.0 earthquake on 19 November 2017, and a magnitude 7.1 event on 7 December 2023. Such swarms typically reflect transient increases in pore-fluid pressure or aseismic slip along the subduction thrust. Their low aftershock productivity relative to mainshock-aftershock sequences distinguishes them from classic foreshock-mainshock patterns, yet they remain valuable indicators of elevated seismic hazard in this rapidly deforming arc.
SeismoSight internal swarm catalogue PS20070404.2.
USGS Earthquake Catalog (events 2000–2023).
Bird, P. (2003). An updated digital model of plate boundaries. Geochemistry, Geophysics, Geosystems.