Tectonic forces create tension and pressure and the stress begins to build up inside the earth.When the stress tends to be more than what the rocks of the earth can bear ,the rocks are broken and displaced from their state of equilibrium.It is known as faulting.The energy accumulated during faulting is released.This release of energy gives rise to mighty waves.These waves originate from a point called Focus in the interior of the earth and then spread out in all directions.On the surface whatever comes into their contact begins to vibrate.The chief cause of earthquakes felt often in California in USA is often the San Andreas Fault found there.