E3-2 Depth of an Earthquake
Earthquake depth color scale. Shallow-focus earthquakes (0–70 km) are represented in red. Intermediate-focus earthquakes (70–300 km) are represented in yellow. Deep-focus earthquakes (>300 km) are represented in green.

Most earthquakes occur at fairly shallow depths, within 70 km of the Earth’s surface. Earthquakes that occur fewer than 70 km below the Earth’s surface are classified as shallow-focus earthquakes. Since these earthquakes are closest to the Earth’s surface, they also generally cause the most damage. Intermediate-focus earthquakes occur at depths between 70 and 300 km, while deep-focus earthquakes occur at depths greater than 300 km below the Earth’s surface. Intermediate- and deep-focus earthquakes do not generally cause as much damage as shallow-focus earthquakes because their depth is so great that their seismic energy has usually completely dissipated before it reaches the surface. Earthquakes do not occur below a depth of 700 km because at this depth, the rocks are under such great heat and pressure that the rock deforms by flowing rather than by breaking.

The globe view on the right is an earthquake map of Japan showing distinct areas of shallow-focus (red), intermediate-focus (yellow), and deep-focus (green) earthquakes.

Learning Activity

Is there a link between the depth of earthquakes and plate tectonics?

  1. Describe the location of most shallow-focus, intermediate-focus, and deep-focus earthquakes.

    Shallow-focus earthquakes occur in a band near the western coast of South America. Intermediate-focus earthquakes occur in a band inland from the western coast, to the east of the shallow earthquake band. Deep-focus earthquakes occur in a band even deeper inland, and further east than the shallow- or intermediate-focus earthquakes.
  2. The Nazca plate (on the left) and the South America plate (on the right) are in contact near the western edge of South America. What conclusion can we draw about how these plates must be interacting based on our map of earthquake depths?

    Our map clearly shows that the further east we go from the plate boundaries, then the deeper the earthquakes become. We conclude that the Nazca plate is being subducted (is sliding) under the South American plate.

Is there a connection between surface features on the Earth and the depths of earthquakes?

  1. What is the link between Earth surface features and earthquake depth?

    Shallow-focus earthquakes appear to be associated with mid-oceanic ridges, with mountains in the interior of Europe and Asia, and with oceanic trenches and mountains surrounding the Pacific Ocean. Intermediate-focus quakes are primarily associated with mountains and oceanic trenches surrounding the Pacific Ocean, and to a smaller extent with areas in central Turkey and central Asia. Deep-focus earthquakes are mainly confined to mountain and oceanic trench areas in the Pacific Ocean (i.e. South America, the Mariana Trench off Japan, and the Philippines).