National Geographic : 1973 Jan
the Golden State... Shelter Cove THRUSTING ASHORE from the Gulf of California, the San Andreas Fault system reaches almost to Oregon before vanishing beneath the Pacific. In addition to the master fault, there are hundreds of branch fractures. Major ones are shown here; all can cause quakes. SAN FRANCI Between Cholame and San Juan Bautista, the sides fail to lock completely. Instead, they ease gradually past each other San Juan Ba in a movement known as "creep." STAFFARTISTLLOYDK. TOWNSEND Montere COMPILEDBY GEORGEW. BEATTY Sa SOURCE OF QUAKES lies deep in bedrock; most tremors along the San Andreas begin about five miles down. Seismometer networks locate the focus, the point of fracture. From it, seismologists plot the epicenter-the surface point directly above. Violent quakes rupture the surface, offsetting streams and splitting hills to form scarps. ... and threaten San Fancisco Three major faults slice the San Francisco Bay region, home of 4,700,000 Californians. The San Andreas shifted 20 feet in 1906. Both the Hayward and Calaveras Faults undergo creep, possibly relieving crustal stresses. The map opposite follows a new detailed study spearheaded by the Geological Survey. Bedrock areas best withstand shaking. Unconsolidated soils show fair stability. Mud and fill areas amplify shaking and in places flow like liquid when vibrated. Unstable bedrock could produce earth slides.