Earthquake seismic waves. Artwork of a section through an earthquake showing seismic waves (red) radiating out from the focus (or hypocentre, red dot). The epicentre is the point on the surface above this. The earthquake is caused by the lateral movement of two tectonic plates at a strike-slip fault. The yellow arrows show the movement of each plate relative to the other. The plates are prevented from moving by the friction between them until the pressure builds to a level at which it overcomes the friction, and the energy is suddenly released as destructive seismic waves. The San Andreas fault in California, USA, is a strike-slip fault like this one.