The San Andreas Fault is a geologic fault zone between two tectonic plates that runs from San Francisco south to San Diego in California. It is an area of frequent earthquakes caused by the plates sliding past each other. It is so called because it runs along the San Andreas Valley.
The San Andreas fault was brought dramatically to world attention in 1906 when sudden displacement along the fault produced the great San Francisco earthquake and fire. This earthquake was but one of many that have resulted throughout its life of about 15-20 million years.
Body.
The entire San Andreas fault system is more than 800 miles long and extends to depths of at least 10 miles within the Earth. The fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. Many smaller faults branch from and join the San Andreas fault zone. .
The Pacific plate is moving northwest in relation to the North American plate, and it is believed that the total displacement along the fault since its formation more than 30 million years ago has been about 350 mi. Movement along the fault causes earthquakes; several thousand occur annually.
The basic science is pretty straightforward. The earth lurches from time to time because its outer shell is broken into huge, solid plates floating on a layer of molten rock that has the consistency of Silly Putty. These tectonic plates are constantly jostling each other, like rafts crowded into a small pond, and its along the boundaries where they meet that most quakes are born. The two plates that form California's infamous San Andreas Fault, the Pacific and the North American plates, are the largest on Earth. And they are moving inexorably in opposite directions.
Conclusion.
There are records of earthquakes on the southern San Andreas fault going back thousands of years and can estimate an average repeat time for certain segments of the fault. We understand that the Los Angeles Basin is being broadly compressed from north to south due to a ben in the San Andreas, and that the compression is causing the San Gabriel and Santa Monica Mountains to be pushed upward on systems of faults at their bases.
