Earthquakes are caused by sudden energy releases in the earth’s crust which create seismic waves that make the ground shake. Earthquakes are usually caused by slippage on a fault due to friction between moving tectonic plates but can also be caused by volcanic eruptions or man made explosions. Millions of earthquakes occur each year, though only a small fraction is strong enough to be felt. Even fewer cause damage. Earthquakes occur at depths of 435 miles to just under the earth’s surface, and the strength of shaking weakens with increasing distance from the earthquake’s source.