san andreas fault trail a self-guided earthquake tour

7/28/2019 San Andreas Fault Trail a Self-Guided Earthquake Tour

1/3

San Andreas Fault Trail A Self-Guided Earthquake Tour

The San Andreas Fault TrailLos Trancos Open Space Preserve is an ideal spot

to learn about earthquake geology because the SanAndreas Fault, one of the worlds longest and mostactive faults, splits the preserve. The San AndreasFault Trail was built for visitors, with the hopethat the more one knows about their geologicenvironment, the better one will be able to protectthemselves and their property against injury ordamage from future earthquakes. The San AndreasFault trail was established in 1977, with volunteerassistance from geology professor Tim Hall, NickHall, and Foothill College geology students.

To understand how earthquakes happen, onemust first look at how a fault, such as the SanAndreas, is formed. The earths interior is separatedinto three distinct layers: core, mantle, and crust.The earths outer surface, or crust, is a jigsaw puz-

zle of 60-mile-thick plates of solid rock floating ona hot, plastic-like layer in the earths upper mantle.The continents sit on 12 major plates that continueto slowly drift, collide, and reshape the face of ourglobe. The grinding of one plate against anotherforms a fault zone made up of crushed rock.On the preserve, the San Andreas fault zone isa 0.3-mile-wide belt where the Pacific plate isgrinding northwestward against the American plate.

Friction prevents the two plates from slidingsmoothly past each other, and causes tremendousenergy to build up in the rocks deep within theearth. Ultimately, the stress will overcome the

friction and the rocks will snap into new positions.This snap releases the accumulated energy inthe form of a major earthquake, such as the SanFrancisco earthquake in 1906, or the LomaPrieta earthquake in 1989. Since the Americanand Pacific plates continue to try to move eachyear, as they have for millions of years, majorearthquakes in the Bay Area are inevitable. TheSan Andreas Fault moves an average of 3/4 inchper year on the peninsula.

Station 1The trail starts on the left (western) side of the

parking lot. Go through the fence and up the hillto Station No. 1.

Using the sketch below, try to identify LomaPrieta, a mountain 23 miles to the southeast nearthe site of the 1989 earthquake. Loma Prieta islocated on the American plate, underlying mostof North America and half of the Atlantic Ocean.Visitors are now standing on the eastern edge ofthe Pacific plate. The San Andreas Fault, locatedalong Stevens Creek Canyon, is the boundarybetween the Pacific and American plates.

Look at the large boulders at ones feet. This

particular type of rock, which is called a con-glomerate, a sedimentary rock made of roundpebbles, came from a unique source - the flanksof Loma Prieta, 23 miles away! About 2 millionyears ago, water carried these boulders from LomaPrieta westward across the San Andreas Fault anddeposited them on the Pacific plate. Since then,successive slips along the San Andreas Fault haveslowly shifted the Pacific plate in a northwesterlydirection, bringing the boulders to their presentposition. There must have been about 12,000major earthquakes in the time since these boulderstumbled downstream to bring them here.

7 7

1

2

3

456

7

89

SAN ANDREAS FAULT TRAIL

MAIN FAULT BREAK 1906 EARTHQUAKE

INTERPRETIVE STATION

CREEK

TRAIL

7

tNumbered posts markthe interpretive stationsas described in thisbrochure.

tPosts with yellow bandsmark the main fault breakfrom the 1906 earthquake.Posts with white bandsmark the minor fault breaks.

And

reasFau

lt

AMERICAN

PLATE

PACIFIC

PLATE

San

BLACK MOUNTAIN

MT. UMUNHUM LOMA PRIETA

STEVENS

CREEK CANYON

FAULT ZONE

1

continued1


2/3


By looking at the landscape, visitors can findclues that they are in a fault zone. One clue isthe shallow, but significant depression below,just north of the parking lot. In the winter, thisdepression, called a sag pond, catches water andbecomes a marsh. Aside from human-made stock

ponds, there are only two types of places in thecoastal range where ponds are found: fault zonesand landslide areas. This is because over timeerosion occurs, filling in depressions and ponds.So, when one sees a pond in this area one knowsit must have been created fairly recently. In thiscase, by the 1906 earthquake. Sag ponds like thisone indicate that the earths crust was stretched,allowing parts of the hillside to sink. Notice theline of posts with white stripes on the other sideof the sag pond. They mark a minor fault withinthe San Andreas fault zone that moved slightlyin 1906.

Station 2

Visitors are standing in an unusual place alongthe San Andreas Fault. Although this area is in thefault zone, it is still elevated. The headwaters forStevens Creek, which flows to the south, and LosTrancos Creek and Corte Madera Creek, whichflow to the north, begin on this ridge. Typically,nature exploits weaknesses, such as the crushedor broken rocks found in the fault zone. Theseareas erode more quickly and are lower in thelandscape - remember the Stevens Creek Canyonthat marks the fault to the south. On the otherhand, areas that are made of harder, less erodiblematerial stand up high in the landscape.

From here, one can see the wide valley thatdelineates the fault to the north. Notice CrystalSprings Reservoir and San Andreas Lake, whichare part of this well-defined valley. These reservoirswere built in the late 1800s to store water for SanFrancisco. Little did the engineers realize that thenatural depressions that made this valley theperfect location to store water were created byan active fault!

Station 3

This picture was taken shortly after the 1906earthquake. To get the same perspective, walk outto the end of the posts and look back in the directionfrom which one came. In the photograph, there aretwo cracks running along what is now a fairly flat

road-like feature in the landscape. This is where themain active fault slipped during the 1906 earthquake.The cracks in the photograph are open. This meansthat there was not only lateral movement duringthe 1906 earthquake, but that the sides of the faultmoved farther apart than they were before thequake. Since then, these cracks have filled in.Every time the ground ruptures, the sides of thefault will move farther apart and then fill in. Thisis why a flat area that resembles a roadbed can beseen. This feature is called a bench and is typicallyfound along faults. Historically, these benches wereoften used as roadbeds.

Station 4This is how a fence built across the fault

might have looked immediately after the 1906earthquake. The fence has been reconstructed fromcentury-old materials found on the preserve. An

original section of this fence is still standing on thedownhill side of the trail toward Los Trancos Creek.During the 1906 quake, faulting may have movedthe far (right hand) segment as much as 3 feet in anorthwesterly direction.

In 1906, about 400 miles of the San AndreasFault slipped, from San Juan Bautista to CapeMendocino. Starting off the Golden Gate, the faultunzipped itself at over 5,000 miles per hour, taking20 seconds to reach this point. However, the northernpart of the fault slipped much more than the southernpart of the fault. The section north of the GoldenGate shifted about 15 to 16 feet, the sectionbetween San Francisco and Portola Valley shifted 7to 9 feet, and from this area southward it only shifted2 to 3 feet. The 1906 earthquake may not havereleased all the energy stored in the southern part ofthe fault. Smaller quakes, similar to the Loma Prieta,may have released the energy. In any case, energyis continuously being stored under our feet and weshould be prepared for the results of its release.

Station 5Notice the willows and abundant brush to the

left of the trail. These plants indicate that there ismore water here than in other areas. Springs areoften found along faults. As faults move, the rockin the fault is ground into a fine clay called faultgouge. The clay binds the crushed rock togetherand forms a barrier to the downhill flow of under-ground water. When underground water hits thisclay barrier, the water builds up on the uphill sideof the fault and eventually bubbles out of theground, forming springs or wet spots.

continued2


3/3


When the ground becomes saturated withwater, landslides are prone to occur, especiallyif the ground is shaken by a strong earthquake.Visitors can see that the area below the willows is arecovering landslide area. It is covered with poisonoak, 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 sagpond at ones feet and the road-like bench uponwhich the fault markers are standing. The sag pondis filling in with sediment, and if one is hereduring the dry months, it probably looks like ashallow depression. The trail along which one justwalked is on a slight ridge, higher than the sagpond. Rarely does one find ridges like this onerunning parallel to the contours, which are linesthat connect two points that are at the same elevation.Usually these ridges follow the drainage down thehill. This is called a pressure ridge and is anotherfault feature.

Station 7

From this point, one can see two oak trees infront of them, one on either side of the trail. Noticehow the main trunks on these trees are growingparallel to the ground, with large limbs growing

vertically not a typical growth pattern.The main trunks of these trees were cored, aprocess where a small plug of wood is removed tocount the tree rings. The rings show that the maintrunks date back to 1899. The vertical limbs arenewer; they date back to 1908. These trees werequite possibly thrown to the ground by the shakingproduced by the 1906 earthquake. Even so, therewas enough root contact with the ground after thequake to allow the trees to survive. The branchesstarted to sprout from the side and grow upwardstoward the sunlight. The unusual shape of thesetrees is probably due to the 1906 quake.

Station 8

Walking to the next station, notice the small valleythat runs parallel to the contours. This landscapefeature is another clue that something unusual isgoing on under ones feet. Normally, a valley likethis is formed by draining water and generallyheads downhill perpendicular to the contours. Thisanomaly tells one that either landsliding or faultinghas occurred here. The posts mark a minor fault.

Station 9

Look around. Can any unusual, fault-relatedlandscape features be seen? Notice the road-likebench in front and to the left. Visitors are back atthe main active fault. Imagine the amount of energythat is currently locked beneath ones feet!

Earthquake Preparedness

People can prepare for an earthquake by doing anumber of things. Assemble an earthquake disastersupply kit. Inspect ones home for items that couldfall or break in an earthquake and secure them.Identify an out-of-state contact person for familymembers to contact if an earthquake occurs.

During an earthquake, protect ones self. Ifindoors, get under a table or desk and hold onuntil the shaking stops. Be alert for glass and otherfalling objects. If outdoors, stay away from buildingsand trees that could fall. If in a car, stop the caraway from trees and underpasses, and stay in it.Be aware that there will be aftershocks. Listen tothe radio for emergency information.

The above earthquake preparedness informationwas provided by the American Red Cross. To obtainmore information about earthquake preparedness,contact the local American Red Cross.

This is the end of the interpretive stations alongthe Fault Trail. To complete the loop, bear left at the

next junction and return to the parking lot. If visitorsare interested in exploring additional earthquakefeatures, Monte Bello Open Space Preserve, acrossPage Mill Road, has excellent examples of sagponds and active landsliding. The District alsooffers docent-led hikes. Please call the administrativeoffice at 650-691-1200 to request the schedule fordocent-led tours.

3

LANDSLIDE

LOS TRANCOS CREEK

TRAIL

SPRING

CLAY SEAMSALONG FAULT

Station 5

san andreas fault trail a self-guided earthquake tour

Documents