San Andreas Fault Trail

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San Andreas Fault Trail – A Self-Guided Earthquake Tour

The San Andreas Fault Trail

Los Trancos Open Space Preserve is an ideal spot

to learn about earthquake geology because the San

Andreas Fault, one of the world’s longest and most

active faults, splits the preserve. The San Andreas

Fault Trail was built for visitors, with the hope 

that the more one knows about their geologic 

environment, the better one will be able to protect

themselves and their property against injury or 

damage from future earthquakes.  The San Andreas

Fault trail was established in 1977, with volunteer

assistance from geology professor Tim Hall, Nick

Hall, and Foothill College geology students.

To understand how earthquakes happen, one

must first look at how a fault, such as the San

Andreas, is formed. The earth’s interior is separated

into three distinct layers: core, mantle, and crust.

The earth’s outer surface, or crust, is a jigsaw puz-

zle of 60-mile-thick plates of solid rock floating on

a hot, plastic-like layer in the earth’s upper mantle.

The continents sit on 12 major plates that continue

to slowly drift, collide, and reshape the face of our

globe. The grinding of one plate against another

forms a fault zone made up of crushed rock. 

On the preserve, the San Andreas fault zone is 

a 0.3-mile-wide belt where the Pacific plate is

grinding northwestward against the American plate. 

Friction prevents the two plates from sliding

smoothly past each other, and causes tremendous

energy to build up in the rocks deep within the

earth. Ultimately, the stress will overcome the 

friction and the rocks will snap into new positions.

This ”snap“ releases the accumulated energy in 

the form of a major earthquake, such as the San

Francisco earthquake in 1906, or the Loma 

Prieta earthquake in 1989. Since the American 

and Pacific plates continue to try to move each

year, as they have for millions of years, major

earthquakes in the Bay Area are inevitable. The 

San Andreas Fault moves an average of 3/4 inch 

per year on the peninsula.

Station 1

The trail starts on the left (western) side of the

parking lot. Go through the fence and up the hill 

to Station No. 1.

Using the sketch below, try to identify Loma

Prieta, a mountain 23 miles to the southeast near

the site of the 1989 earthquake. Loma Prieta is

located on the American plate, underlying most 

of North America and half of the Atlantic Ocean.

Visitors are now standing on the eastern edge of 

the Pacific plate. The San Andreas Fault, located

along Stevens Creek Canyon, is the boundary

between the Pacific and American plates.

Look at the large boulders at one’s feet. This 

particular type of rock, which is called a con-

glomerate, a sedimentary rock made of round 

pebbles, came from a unique source - the flanks 

of Loma Prieta, 23 miles away! About 2 million

years ago, water carried these boulders from Loma

Prieta westward across the San Andreas Fault and

deposited them on the Pacific plate. Since then,

successive slips along the San Andreas Fault have

slowly shifted the Pacific plate in a northwesterly

direction, bringing the boulders to their present

position. There must have been about 12,000

major earthquakes in the time since these boulders

tumbled downstream to bring them here.

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SAN ANDREAS FAULT TRAIL

MAIN FAULT BREAK 1906 EARTHQUAKE

INTERPRETIVE STATION

CREEK

TRAIL

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tNumbered posts mark 

the interpretive stations 

as described in this

brochure.

tPosts with yellow bands

mark the main fault break

from the 1906 earthquake.

Posts with white bands 

mark the minor fault breaks.

A

n

dre

as

Fau

lt

➔

AMERICAN

PLATE

PACIFIC 

PLATE

➔

San

BLACK MOUNTAIN

MT.  UMUNHUM    LOMA PRIETA

STEVENS 

CREEK CANYON

FAULT ZONE

➬

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San Andreas Fault Trail – A Self-Guided Earthquake Tour

By looking at the landscape, visitors can find 

clues that they are in a fault zone. One clue is 

the shallow, but significant depression below, 

just north of the parking lot. In the winter, this

depression, called a sag pond, catches water and

becomes a marsh. Aside from human-made stock

ponds, there are only two types of places in the

coastal range where ponds are found: fault zones

and landslide areas. This is because over time

erosion occurs, filling in depressions and ponds. 

So, when one sees a pond in this area one knows 

it must have been created fairly recently. In this

case, by the 1906 earthquake. Sag ponds like this

one indicate that the earth’s crust was stretched,

allowing parts of the hillside to sink. Notice the 

line of posts with white stripes on the other side 

of the sag pond. They mark a minor fault within 

the San Andreas fault zone that moved slightly 

in 1906.

Station 2

Visitors are standing in an unusual place along 

the San Andreas Fault. Although this area is in the

fault zone, it is still elevated. The headwaters for

Stevens Creek, which flows to the south, and Los

Trancos Creek and Corte Madera Creek, which

flow to the north, begin on this ridge. Typically,

nature exploits weaknesses, such as the crushed 

or broken rocks found in the fault zone. These

areas erode more quickly and are lower in the

landscape - remember the Stevens Creek Canyon

that marks the fault to the south. On the other

hand, areas that are made of harder, less erodible

material stand up high in the landscape.

From here, one can see the wide valley that

delineates the fault to the north. Notice Crystal

Springs Reservoir and San Andreas Lake, which

are part of this well-defined valley. These reservoirs

were built in the late 1800s to store water for San

Francisco. Little did the engineers realize that the

natural depressions that made this valley the 

perfect location to store water were created by 

an active fault!

Station 3

This picture was taken shortly after the 1906

earthquake. To get the same perspective, walk out

to the end of the posts and look back in the direction

from which one came. In the photograph, there are

two cracks running along what is now a fairly flat

road-like feature in the landscape. This is where the

main active fault slipped during the 1906 earthquake.

The cracks in the photograph are open. This means

that there was not only lateral movement during 

the 1906 earthquake, but that the sides of the fault

moved farther apart than they were before the

quake. Since then, these cracks have filled in. 

Every time the ground ruptures, the sides of the 

fault will move farther apart and then fill in. This 

is why a flat area that resembles a roadbed can be

seen. This feature is called a bench and is typically

found along faults. Historically, these benches were

often used as roadbeds.

Station 4

This is how a fence built across the fault 

might have looked immediately after the 1906

earthquake. The fence has been reconstructed from

century-old materials found on the preserve. An

original section of this fence is still standing on the

downhill side of the trail toward Los Trancos Creek.

During the 1906 quake, faulting may have moved

the far (right hand) segment as much as 3 feet in a

northwesterly direction.  

In 1906, about 400 miles of the San Andreas

Fault slipped, from San Juan Bautista to Cape

Mendocino. Starting off the Golden Gate, the fault

unzipped itself at over 5,000 miles per hour, taking

20 seconds to reach this point. However, the northern

part of the fault slipped much more than the southern

part of the fault. The section north of the Golden

Gate shifted about 15 to 16 feet, the section

between San Francisco and Portola Valley shifted 7

to 9 feet, and from this area southward it only shifted

2 to 3 feet. The 1906 earthquake may not have

released all the energy stored in the southern part of

the fault. Smaller quakes, similar to the Loma Prieta,

may have released the energy. In any case, energy

is continuously being stored under our feet and we

should be prepared for the results of its release.

Station 5

Notice the willows and abundant brush to the

left of the trail. These plants indicate that there is

more water here than in other areas. Springs are

often found along faults. As faults move, the rock 

in the fault is ground into a fine clay called fault

gouge. The clay binds the crushed rock together

and forms a barrier to the downhill flow of under-

ground water. When underground water hits this

clay barrier, the water builds up on the uphill side

of the fault and eventually bubbles out of the

ground, forming springs or wet spots.

continued

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San Andreas Fault Trail – A Self-Guided Earthquake Tour

When the ground becomes saturated with 

water, landslides are prone to occur, especially 

if the ground is shaken by a strong earthquake.

Visitors can see that the area below the willows is a

recovering landslide area. It is covered with poison

oak, which often indicates a history of landsliding.

Station 6

Having just crossed over the main fault again,

one can see the prominent fault features - the sag

pond at one’s feet and the road-like bench upon

which the fault markers are standing. The sag pond

is filling in with sediment, and if one is here 

during the dry months, it probably looks like a 

shallow depression. The trail along which one just

walked is on a slight ridge, higher than the sag

pond. Rarely does one find ridges like this one 

running parallel to the contours, which are lines

that connect two points that are at the same elevation.

Usually these ridges follow the drainage down the

hill. This is called a pressure ridge and is another

fault feature.

Station 7

From this point, one can see two oak trees in

front of them, one on either side of the trail. Notice

how the main trunks on these trees are growing

parallel to the ground, with large limbs growing 

vertically — not a typical growth pattern. 

The main trunks of these trees were cored, a

process where a small plug of wood is removed to

count the tree rings. The rings show that the main

trunks date back to 1899. The vertical limbs are

newer; they date back to 1908. These trees were

quite possibly thrown to the ground by the shaking

produced by the 1906 earthquake. Even so, there

was enough root contact with the ground after the

quake to allow the trees to survive. The branches

started to sprout from the side and grow upwards

toward the sunlight. The unusual shape of these

trees is probably due to the 1906 quake.

Station 8

Walking to the next station, notice the small valley

that runs parallel to the contours. This landscape

feature is another clue that something unusual is

going on under one’s feet. Normally, a valley like

this is formed by draining water and generally

heads downhill perpendicular to the contours. This

anomaly tells one that either landsliding or faulting

has occurred here. The posts mark a minor fault.

Station 9

Look around. Can any unusual, fault-related 

landscape features be seen? Notice the road-like

bench in front and to the left. Visitors are back at

the main active fault. Imagine the amount of energy

that is currently locked beneath one’s feet! 

Earthquake Preparedness

People can prepare for an earthquake by doing a

number of things. Assemble an earthquake disaster

supply kit. Inspect one’s home for items that could

fall or break in an earthquake and secure them.

Identify an out-of-state contact person for family

members to contact if an earthquake occurs. 

During an earthquake, protect one’s self. If

indoors, get under a table or desk and hold on 

until the shaking stops. Be alert for glass and other

falling objects. If outdoors, stay away from buildings

and trees that could fall. If in a car, stop the car

away from trees and underpasses, and stay in it. 

Be aware that there will be aftershocks. Listen to 

the radio for emergency information.

The above earthquake preparedness information

was provided by the American Red Cross. To obtain

more information about earthquake preparedness,

contact the local American Red Cross.

This is the end of the interpretive stations along

the Fault Trail. To complete the loop, bear left at the

next junction and return to the parking lot. If visitors

are interested in exploring additional earthquake

features, Monte Bello Open Space Preserve, across

Page Mill Road, has excellent examples of sag

ponds and active landsliding. The District also

offers docent-led hikes. Please call the administrative

office at 650-691-1200 to request the schedule for

docent-led tours.

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LANDSLIDE

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LOS TRANCOS CREEK

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TRAIL

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SPRING

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CLAY SEAMS

ALONG FAULT

Station 5