clipped from: earthobservatory.nasa.gov   
NASA Radar Provides 3-D View of San Andreas Fault
Much of the length of the fault is lined by a distinct trough.
The black line curving along the east side of the fault is I-280, and California highway 92 runs from the top right across to the center left of the image, heading towards Half Moon Bay to the west. San Mateo and Burlingame are parts of the large urbanized area in various tones of pink and green, and Foster City is the area of curved streets extending out into the San Francisco Bay at the top right.

Earth’s crust is a collection of fractured plates that shift across the planet’s surface over millions of years. Two of the major plates meet at the coast of California; the Pacific Plate, under the ocean, is drifting slowly northwest, grinding along the margin of the North American Plate, to the east. Where they meet, the intense pressure of the collision creates an a 800-mile-long fracture zone called the San Andreas Fault.