Seismic Hazards in Southern Vanuatu: The 2011 M7.0 Event and Regional Context

Vanuatu lies along the tectonically active New Hebrides subduction zone, where convergence between the Australian and Pacific plates produces frequent large earthquakes. The island arc experiences both shallow crustal events and deeper intraslab seismicity, with hypocenters commonly exceeding 100 km depth due to the steep subduction angle. Southern Vanuatu, near Tanna Island and the town of Isangel, sits directly above this Wadati-Benioff zone, resulting in elevated seismic hazard. On 3 September 2011 at 22:55 UTC, a magnitude 7.0 earthquake struck 133 km SSE of Isangel at a focal depth of 185.1 km. The event originated within the subducting slab and was felt across Tanna and neighboring islands, though its depth limited surface damage. No significant tsunami was generated, consistent with the intermediate-depth source mechanism. Since 2000, four additional magnitude 7.0 or greater earthquakes have occurred within 100 km of the 2011 hypocenter, underscoring the persistent activity of the region. These include a magnitude 7.1 event on 25 March 2007 located 119 km S of Isangel, another magnitude 7.1 on 7 December 2023 situated 118 km S of Isangel, a magnitude 7.0 on 14 September 2022 centered 209 km SSE of Isangel, and the 2011 mainshock itself. The spatial clustering of these events reflects the geometry of the subducting plate and repeated stress accumulation along the interface and within the slab. Geological records indicate that Vanuatu has experienced great earthquakes (M8+) in the past, though the southern segment tends to produce more frequent moderate-to-large intraslab events rather than megathrust ruptures. Ongoing plate convergence at rates of approximately 10 cm per year sustains the seismic cycle. Monitoring by regional networks continues to track aftershock sequences and background seismicity to refine hazard assessments for the island chain.

References

USGS Earthquake Catalog (event parameters and locations) Global CMT Project (focal mechanisms for regional events)