Most earthquakes are shallow.
They are concentrated no deeper than 20-25 kilometers down. However, a few extremely deep quakes rumble at depths of about 600 kilometers. Geophysicists are really not certain what causes the very deep quakes, because at 640 kilometers rocks are so hot that they flow rather than snap under geological stresses. The more common, shallow earthquakes are generally created when rocks snap and fracture. Phase changes often involve volume changes that, if sudden, might generate seismic waves.