An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath earth"s surface. Seismic waves are the waves of energy produced by an earthquake and cause ground to tremble and shake. Breaks between two rock masses is called a fault. Classic explanation for why earthquakes takeplace is called elastic rebound theory which involves sudden release of progressively stored elastic strain energy in rocks, causing movement along a fault. The deep seated internal forces (tectonic forces) act on a mass of rock over many decades. Rocks on surface are brittle, rocks deep in the earth are ductile. Volcanic eruptions and mineral transformations may also cause earthquakes, not just fault movements. Focus or hypocentre is the point of the earthquake where the seismic waves first originate. The epicenter is the point on the earth"s surface directly above the focus.