What Is It?
Scientists have learned that the Earth's crust is fractured
into a series of "plates" that have been moving very slowly over
the Earth's surface for millions of years.
Two of these moving plates meet in western California; the
boundary between them is the San Andreas fault. The Pacific
Plate (on the west) moves northwestward relative to the North
American Plate (on the east), causing earthquakes along the
fault. The San Andreas is the "master" fault of an intricate
fault network that cuts through rocks of the California coastal
region. The entire San Andreas fault system is more than 800
miles long and extends to depths of at least 10 miles within the
Earth. In detail, 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. Almost
any road cut in the zone shows a myriad of small fractures, fault
gouge (pulverized rock), and a few solid pieces of rock.
A simplified map of the Earth's crustal plates
Full size image - 34k.