What is a hypocenter?
The hypocenter is the point within the earth where the earthquake started, or nucleated. Faults are two dimensional structures within the crust of the Earth. The point at which the fault first begins to rupture is the hypocenter. Rupturing will continue to radiate along the fault plane outward from the hypocenter. The epicenter is the point on the surface of the earth directly above the underground hypocenter. The hypocenter does not need to be in the center of the fault rupture. In the case of the the 2004 Parkfield quake, the hypocenter was at the south end of the section of fault the ruptured. Rupture started at the hypocenter and propogated almost entirely to the north, with very little propogation to the south. For large earthquakes where the fault ruptures for hundreds or even a thousand or more kilomenters and the hypocenter/epicenter is at one end of the rupture, the hypocenter/epicenter ploted on a map may be far from the location of greatest shaking and damage.
