ocks or aftershocks. Foreshocks can occur weeks or months in advance of a main temblor; whereas aftershocks succeed major earthquakes. Aftershocks are much more common than foreshocks. In fact, aftershocks are almost predictable. Practically every major earthquake is followed by an aftershock series of varying strength. These shocks are directly related to the strength of the main event, and may continue for extended periods of time (Iacopi 10-25).
Following movement along a fault, shock waves emanate outward from an epicenter. These complex waves are of the following three general types: (1) P waves; (2) S waves; and (3) L waves. The P wave consists of a longitudinal sound wave. These waves create a "push-pull" effect as they move through the Earth (Iacopi 10-25). Propagating at a rate of approximately 3.5 miles per second, P waves are typically the first to reach the surface. In contrast, S waves, or shear waves, are slower than the P waves. They travel at about 2 miles per second. The S waves generally move at right angles to their direction of motion. Perhaps S wave motion is best illustrated by the snap of a whip. Although the wave moves along the length of the whip, the actual motion is at right angles. Lastly, the L, or long waves, are the slowest of the three. These surface waves are typically only distinguishable at great distances from an earthquake's epicenter. Because of their long-period, L waves can be responsible for such phenomena as the swaying of tall buildings (Iacopi 10-25).
Perhaps the most significant fault in California is the San Andreas. This fault--which lies approximately 60 miles east of downtown Los Angeles--demarcates the border between the Pacific and North American tectonic plates. In Southern California, geologic activity along the San Andreas is rather complicated. This is because near Los Angeles the fault turns abruptly to the west before resuming its northwesterly course....