When an earthquake fault ruptures, the rupture does not affect a large area simultaneously. Instead, the rupture introduces a localized disturbance in the area surrounding the fault, and then this distur bance propagates in the form of spherical waves to other points throughout the earth. In this way, an earthquake disturbance is always carried to great distances, often thousands of kilometers away from the fault where it originates. An analogy that is often used to explain this phenomenon is that of a small pebble dropped into a large deposit of water. After the pebble is dropped, one can see that waves are generated and propagated in all directions, whereas if a small fl oating object is placed on the water surface, the object will oscillate about its original position.