depth under the island arc: these quakes define the Wadati-Benioff zones. The volcanic arc forms on the overriding plate over the point where the subducting plate reaches a depth of roughly 120 km and is a zone of volcanic activity between 50 and 200 km in width.

The shape of a volcanic arc is typically convex towards the subducting plate. This is a consequence of the spherical geometry of the Earth. The subducting plate behaves like a flexible thin spherical shell, and such a shell be bent