An earthquake is caused by the sudden movement of parts of the Earth's crust and results in shaking and trembling of the ground. There are over one million earthquakes each year, with around twenty having the potential to cause severe damage. Earthquakes are caused by faults in the Earth's crust, such as the San Andreas Fault in California where the land west of the fault moves north and the land east moves south. Underwater earthquakes can generate destructive tsunamis. The focus is the depth below the surface where the earthquake occurs, and the epicenter is directly above on the surface. Seismographs detect seismic waves that travel out from the epicenter, including fast P waves and slower S waves.