San Anderas indd



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The San Andreas Fault

Point Reyes

National Seashore

Protection for your Cultural and Natural Heritage



The world-famous San Andreas Fault separates the Point Reyes peninsula from the main land of 

California.  The fault is much more than a simple boundary.  It has created the landscape of this area 

with its long bays and lagoons and the narrow Olema Valley.  It looms and promises to change the 

landscape again with the next big earthquake.  Will the “Big One” be today, next week, or five years from 

now?  It’s a hard question to answer because there is so much that is still not known about earthquakes 

and seismic activity.  However, it is known that the earthquakes we experience are cause by the shifting 

and movements of the vast plates that cover the earth…some of which we live on.

Think Big, Think Plate Tectonics

To understand the San Andreas Fault, we must leave the Olema Valley to examine 

the larger global theory Plate Tectonics.  Geologists believe that the outer layer 

of the earth is segmented into about a dozen rigid plates.  These plates which are 

up to 62 miles thick, float on a fluid layer of molten earth.  Currents within this 

layer slowly move and shift the surface plates that we live on.  This movement 

causes plates to collide in convergent plate boundaries, to separate in divergent 

plate boundaries or slide past one another in transform plate boundaries.  

Where these plates meet, boundaries are the site where mountains are 

created, volcanoes erupt, and earthquakes occur. 

The San Andreas Fault is a transform plate boundary that separates the two 

largest plates in the world, the Pacific and North American plates.   The 

oceanic Pacific plate, which includes Point Reyes, is slowly grinding to the 

northwest at about two inches per year in relation to the North American plate.  This movement is responsible for the 

topography of the area as well as the frequent earthquakes of varying magnitude.

Hop a Ride on the Pacific Plate

Visitors to Point Reyes National Seashore will enter the San Andreas Fault zone and cross from the North American 

to the Pacific plate. Even though the Point Reyes peninsula is currently attached to the mainland, in many ways you 

have set foot onto a land in motion.  The peninsula is thought to have traveled 280 miles over the last 30 million 

years from its origins near Los Angeles.   

The Pacific plate and the Point Reyes Pen-

insula is traveling northwest at two inches 

a year in relation to the continental main-

interp4\mydocuments\publications\handouts\currenthandouts\TheSanAndreas\TheSanAndreasFault




When exploring the peninsula, you are exploring a land apart. It is easy to recognize that Point Reyes is unlike 

the surrounding California mainland.  There are distinct differences in terrain and vegetation.  Its geologic 

history is the key.  With its origins further south, the parent rocks and soils of the Point Reyes Peninsula are 

different from the soils and substrate of the continental mainland. This difference in soils affects the plants that 

are able to grow here. Here at Point Reyes, Douglas firs and the ancient bishop pine dominate the Inverness 

Ridge while elsewhere along the California coast, you’ll find coast redwoods thriving. 

The geology affects more than just its plants and topography.  Indirectly, it influences all the species of animals 

that are dependent on habitat created by the peninsula’s geologic migration.  It even impacts us…sometimes in 

devastating ways.

Not Soon Forgotten

During the brief presence of humans along the fault, the San Andreas 

has played a significant role in human history.  Perhaps the most 

famous earthquake in the western world occurred the San Francisco 

Bay Area on April 18, 1906.  When the earthquake struck, the Point 

Reyes Peninsula jolted northwest about 20 feet (6 meters).  In nearby 

San Francisco, fires resulting from the earthquake crippled the city 

causing millions of dollars of damage and hundreds of deaths.

On October 17,1989, San Francisco Bay Area residents were again 

reminded of the power of the San Andreas and its adjacent faults.  

The Loma Prieta Earthquake, whose epicenter was in the Santa Cruz 

Mountains, caused the collapse of a portion of the Bay Bridge, a 

freeway overpass in Oakland and many other structures.  

Experience the Fault

The San Andreas Fault serves as a reminder to us of the incredible power of the earth upon which we live for 

a very brief moment in time.  Today, a stroll along the Earthquake Trail in Bear Valley offers the visitor an 

account of the geology behind earthquakes and specifically the 1906 earthquake.  The paved .6 mile Earthquake 

Trail is located across the parking lot from the Bear Valley Visitor Center, just to the left of the restrooms. 

The summit of Mount Wittenberg at 1407 ft is an excellent elevated observation point of the San Andreas 

Fault zone.  This vantage point provides a view of the Olema Valley and Tomales Bay.  Mt. Wittenberg is a four-

mile round trip hike from the Bear Valley Visitor Center.  Start on the Bear Valley Trail and turn right on the Mt. 

Wittenberg Trail.  Follow that trail to the summit.  Pick up a trail map from the Visitor Center before you set 

out.


Learn more about the San Andreas and earthquakes around the world on the Internet at 

http://quake.wr.usgs.gov.

A surface rupture was visible in the Olema area 

after the 1906 earthquake after the peninsula 

jolted 20 feet to the northwest. 

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