Earthquakes rarely take place at the surface of the Earth but at some depth within it. Though focal depths are usually shallow, earthquakes have been detected as deep as 720 km. Based on the depth of the focus earthquakes can be classified as:
Shallow: when the depth of origin is less than 60 km.
Intermediate: when the depth of origin is between 60 and 300 km.
Deep: when the depth of origin is greater then 300 km.