The San Andreas Fault System

The California coast as we know it has been and continues to be created by the collision of two immense tectonic plates, the North American continental plate and the Pacific oceanic plate. The head-on collision of the two plates about 150 million years ago created the Sierra Nevada Range. Approximately 20 million years ago, the Pacific plate stopped moving east and began sliding northwest along the North American plate’s edge. The meeting of the plates became a sideswipe collision, replacing the Sierra-building head-on crash. The San Andreas Fault System formed to accommodate this movement.

The San Andreas fault and the smaller affiliated faults comprising the system have, more than any other factor, created the California coast we know today. The San Andreas stretches 750 miles northwest from Mexico’s Gulf of California to Cape Mendocino near Eureka. The northern half of the San Andreas fault runs on or near the coast. South of Monterey the Fault Zone runs from 30 to 90 miles inland. California is slowly being torn apart along the San Andreas fault. The parts of California west of the fault, including San Diego, Los Angeles, Monterey, Point Reyes

and Point Arena, are moving north at a rate of one to two inches a year. In effect these movements constantly re-create the California coast as we know it. Fortunately the shift is slow enough to allow us time to enjoy the spectacular beauty of this leading edge of the North American continent. Most of the movement along the San Andreas fault is slow and constant, not discernable without scientific instruments. Only after pressure builds as the sliding motion gets caught in a particular area do we have earthquakes. Then the movement becomes violent, jumping as much as 20 feet in seconds. California’s best hope is that movements along the fault continue at a steady, incremental pace, avoiding the resistance that causes earthquakes.

The San Andreas fault first comes ashore at Alder Creek north of Point Arena, the Coastal Trail crossing it near the start of this section. The Fault Zone follows the canyons of the Garcia and Gualala Rivers southeast before angling back offshore south of Fort Ross. It returns to land briefly near Bodega Bay then again through western Marin County before running offshore again from Bolinas to Pacifica where the Fault Zone comes ashore to stay.