Earthquake

the often violent movement of the earth along a fault or fault zone due to tectonic upheaval caused by the sudden release of stress that has built up over some time. The focus is the point at which the earthquake originates within the earth and earthquakes are classified by depth of focus: shallow -- less than 70 km; intermediate -- 70-300 km; deep -- more than 300 km. Attempts to predict earthquakes rely upon measurement of stress increases, but can so far be related only to an increasing risk of activity.

Taken from Dictionary of Science