The mystery of too-deep earthquakes

Look at the depth distribution of earthquakes on Earth (Fig. 1):

Fig. 1: Depths of earthquakes on Earth. Shallow earthquakes (0-60 km) are in red, intermediate-depth earthquakes (60-300 km) in purple and deep earthquakes (>300 km) in blue. Data from the International Seismological Centre.

In general, earthquakes are located at the boundaries between tectonic plates. Shallow earthquakes (< 60 km) happen at all plate boundary types, but intermediate (60-300 km) and deep (> 300 km) earthquakes mainly occur in subduction zones, where one plate moves beneath another. Because these earthquakes are located either within the subducting plate or between the two plates, they get deeper and deeper the further they are from the surface trace of the plate boundary. Because the plate located west of South-America moves towards the east and is subducted under South-America (Fig. 2), the earthquakes on the west coast of South-America get deeper from west to east (Figs 1, 2). Continue reading