Once the failure has occurred, movement may occur along a zone of weakness called a fault. The fault may be quite long and measure as long as 625 miles. One of the most famous faults in California is the San Andreas Fault. The San Andreas Fault is a series of cracks in the earth's crust extending about 600 miles from the Gulf of California northwest along the California coast. The 1906 San Francisco earthquake was caused by the movement of the San Andreas Fault.
Many earthquakes occur each year in the United States.
On average there are more than 800,000 earthquake, but most are small and not felt by humans. A severe earthquake, with a magnitude of greater than 8.0, can be expected every eight to ten years. Although these earthquakes are still capable of destruction, a significant number of smaller earthquakes occur each year. The vast majority of earthquakes are located within narrow zones that correspond to the boundaries of the rocky crust of the earth or the lithospheric plates. These plates are blocks of the earth's crust which are continuously moving relative to each other.