The difference between the epicenter and the focus of an earthquake lies in the location of their origins. The focus is the exact point inside the crust of the earth where the quake begins, while the epicenter is the point on the earth's surface directly above the focus.
At the focus, the rocks shift and the existing fault line in the earth ruptures, causing the earthquake. Seismic waves, in the form of either body waves or surface waves, travel outward in all directions, causing the shaking and rolling sensations felt above the ground. The focus can occur at convergent plate boundaries where two separate plates have a head-on collision, or it can be triggered by two plates sliding past one another, which occurs at transform boundaries. Since the epicenter is the closest point on the earth's surface to the focus below, the most violent shaking and damage typically occurs at this point. The focus is often a shallow one near the surface of the earth, which typically releases the most energy. Deep-focus earthquakes are buried very deep inside the layers of the earth's crust. When the epicenter of a quake lies beneath an ocean, the disturbance sometimes triggers a dangerous tsunami which potentially inflicts extensive damage many miles away.