The topographic texture of western California is controlled by
the San Andreas fault system, the tectonic expression of the Pacific
plate sliding northwestward along the western margin of the North
American plate. Hundreds of miles long and up to a mile wide,
the San Andreas Fault Zone has been active since its original
development in the Tertiary.
About 10 percent of the present plate motion is compressional,
which means horizontal forces are shortening and wrinkling the
crust along the fault zone. This movement has created the parallel
coastal northwest-southeast mountain ranges such as the Coastal
Ranges along California's central coast. Comparatively quiet during
the period between the 1906 San Francisco earthquake and the 1989
Loma Prieta event, the fault is again showing activity.
