 Earthquakes occurring at a depth of less than are classified as "shallow-focus" earthquakes, while those with a focal-depth between are commonly termed "mid-focus" or "intermediate-depth" earthquakes.In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from ).These seismically active areas of subduction are known as Wadati–Benioff zones.