Over time large strains can accumulate in rocks next to the fault zone. Finally, stresses become so large at the locked fault zones rupture and slip. This rapid slippage releases the strain energy which was accumulated over a long period of time in a process called elastic rebound. As the fault zone slips seismic waves are released. Body waves travel from the earthquake focus in all directions whereas surface waves travel around the earth rather than through it.
Surface waves are slower than body waves.
Regardless of its wavelength each type of body waves travels at one velocity. Compressional waves - body waves characterized by vibrations parallel to the propagation direction of the wave. This wave travels at 5 to 7 km per second near the surface of the earth and can pass through the liquid core of the earth. Shear waves - body waves characterized by a series of sidewise or shearing movements. It shear waves which can not pass through a liquid such as the outer core of the earth.