The archipelago, which was formerly known as New Hebrides, lies atop a complex and active plate boundary in the southwestern Pacific Ocean.
[1] The primary tectonic feature of the 1,200 km (750 mi) island chain is the New Hebrides Trench, the convergent boundary between the Australian and Pacific plates.
Along the Wadati–Benioff zone, earthquake activity has been observed as shallow, intermediate, and deep-focus events at depths of up to 700 km (430 mi).
[2] While much of the island arc experiences intermediate-depth earthquakes along a Wadati–Benioff zone that dips steeply at 70°, the area adjacent to the d'Entrecasteaux Ridge does not.
The uncertainty, which also affects the Tonga arc, is due to the influence of spreading at the North Fiji Basin.