Information and resources about the San Andreas Fault The San Andreas fault's nick name is "SAF". San Andreas Fault Information Map of California showing the SAF, Natural Landmark plaque and the Field Guide to the San Andreas Fault. The San Andreas Fault is the most famous fault in the world. Its notoriety comes partly from the disastrous 1906 San Francisco earthquake , but rather more importantly because it passes through California, a highly-populated state that is frequently in the news. And with many research institutions dedicated to studying such an accessible fault, the SAF has become a household name. But what is a fault? And what is the SAF? A fault is a planar crack in a rock along which slippage has taken place . Most faults are small - even microscopic - and are not important. Some faults are many miles long. Faults can be classified according to which of the three directions of space the rocks on either side move. When the motion is predominantly vertical, they are called dip slip faults . Dip slip faults with dips less than 45 degrees are called thrust faults . If the motion is mostly horizontal and parallel to the fault plane, the fault is called a strike slip (or transform) fault.

The SAF is a right lateral transform fault. This means that if two people face each other across the fault and it moves, each person will see the other person move to the right. If the rocks move horizontally apart or together, they are called divergent or convergent , respectively. Convergent faults raise pressure ridges and mountain ranges. Divergent faults create gaps or sags. When plate boundaries are convergent there is always a subduction zone. When divergent, they usually open valleys on land and oceanic ridges like the Mid Atlantic Ridge. At plate boundaries, the fault plane is seldom vertical, i.e. a dip of 90 degrees. Indeed, almost none of the SAF's fault plane is vertical. The San Andreas Fault is a place where two tectonic plates touch, the North American and Pacific Plates. The plates are rigid (or almost rigid) slabs of rock that comprise the crust and upper mantle of the Earth. The SAF is about 700 miles long as the crow flies and about 800 miles long when its curves are measured. It is roughly ten miles deep, and reaches from the Salton Sea in Imperial county to Cape Mendocino in Humboldt county.

The plates are continually moving but where the touch each other, they get stuck. As the rest of the plates moves, the stuck parts deform like compressing a spring so they build up stress in the rocks along the fault. When the rock breaks or slips, the suddenly plates move, causing an earthquake. The entire process is called elastic rebound . As they break and scrape by one another, they produce seismic waves that travel through the ground and shake the surface. We know this shaking as earthquakes. While we think of plates as rigid, they can stretch a little, like pizza crust. That is why we can have an earthquake on the SAF in northern California but not on the SAF in southern California. There are many major faults in California that are parallel to the SAF, like the Hayward Fault . This results in a curious effect: while the Pacific Plate is moving northwest relative to the North American Plate at an average rate of about 2.5 inches per year (about as fast as your fingernails grow), the average slip rate along the SAF is less, about 1.5 inches per year. The difference is being taken up by slip along the other parallel faults. This can be demonstrated by a deck of cards .

People often think of the SAF as a line in the ground, and that by hopping across it, they can go from one plate to the other. This is not true. Although the most recent break in the ground that defines the trace of the SAF is a narrow line, the actual plate boundary should be viewed as a fault zone. The width of a fault zone can be less than a mile wide (Olema Trough) or many miles across (Salton Trough). California is famous for its earthquakes on the SAF (1906 San Francisco Earthquake, 1857 Fort Tejon earthquake), but the state's quakes are not particularly large. The largest earthquakes in the world occur near subduction zones, where one plate is sliding under the other (Alaska, Chile, Indonesia, etc.) Only in northwestern California north of Cape Mendocino is there a subduction zone, the Cascadia megathrust, and it is not directly related to the SAF. Here the small Juan de Fuca plate is subducting under the North American Plate. Given enough time, the Cascadia megathrust will produce an earthquake much larger than any quake on the SAF. It has happened before.

The SAF is about 28 million years old . It 'began' when the East Pacific Rise, the boundary between the Farallon and Pacific Plates, subducted under the North American Plate near what is now Los Angeles. Spreading northwest and southeast, The SAF gradually grew, and it still growing. The grinding plates and earthquakes are gradually warping and reshaping Califonia. In a few million years, California will look very different than it does today. Consider this: California is about 1000 miles long. That's 1000 miles x 5280 ft/mile x 12 inches/ft = 63,360,000 inches At the measured slip rate of about 2.5 inches per year, it will take 63,360,000 inches/ 2.5 inches/year = 25.3 million years for the state to be sheared and twisted beyond all recognition. With similar arithmetic, we can see that 25 million years ago, California shared not the faintest resemblance with the modern paradise we know today.

Meanwhile, the Farrallon plate continues to scrape along the bottom of the North American plate, probably destined to sink into the mantle, melt and be recycled. All states and all continents have faults but California has more than most. What makes the SAF so important is that it is a plate boundary and that it is on land. Most plate boundaries are in the ocean and so they are hard to study. The San Andreas Fault The San Andreas Fault by Sandra S. Schulz and Robert E. Wallace The presence of the San Andreas fault was brought dramatically to world attention on April 18, 1906, when sudden displacement along the fault produced the great San Francisco earthquake and fire. This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about 15-20 million years. What Is It? Scientists have learned that the Earth's crust is fractured into a series of "plates" that have been moving very slowly over the Earth's surface for millions of years. Two of these moving plates meet in western California; the boundary between them is the San Andreas fault.

The Pacific Plate (on the west) moves northwestward relative to the North American Plate (on the east), causing earthquakes along the fault. The San Andreas is the "master" fault of an intricate fault network that cuts through rocks of the California coastal region. The entire San Andreas fault system is more than 800 miles long and extends to depths of at least 10 miles within the Earth. In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. Many smaller faults branch from and join the San Andreas fault zone. Almost any road cut in the zone shows a myriad of small fractures, fault gouge (pulverized rock), and a few solid pieces of rock. Where Is It? The San Andreas fault forms a continuous narrow break in the Earth's crust that extends from northern California southward to Cajon Pass near San Bernardino. Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates. In this stretch of the fault zone, the name "San Andreas" generally is applied to the northeastern most branch. What Surface Features Characterize It?

Over much of its length, a linear trough reveals the presence of the San Andreas fault; from the air, the linear arrangement of lakes, bays, and valleys in this trough is striking. Viewed from the ground, however, the features are more subtle. For example, many people driving near Crystal Springs Reservoir, near San Francisco, or along Tomales Bay, or through Cajon or Tejon Passes may not realize that they are within the San Andreas fault zone. On the ground, the fault can be recognized by carefully inspecting the landscape. The fault zone is marked by distinctive landforms that include long straight escarpments, narrow ridges, and small undrained ponds formed by the settling of small blocks within the zone. Many stream channels characteristically jog sharply to the right where they cross the fault. What Kind of Movement Has Occurred Along the Fault? Blocks on opposite sides of the San Andreas fault move horizontally. If a person stood on one side of the fault and looked across it, the block on the opposite side would appear to have moved to the right. Geologists refer to this type fault displacement as right-lateral strike-slip.

During the 1906 earthquake in the San Francisco region, roads, fences, and rows of trees and bushes that crossed the fault were offset several yards, and the road across the head of Tomales Bay was offset almost 21 feet, the maximum offset recorded. In each case, the ground west of the fault moved relatively northward. Sudden offset that initiates a great earthquake occurs on only one section of the fault at a time. Total offset accumulates through time in an uneven fashion, primarily by movement on first one, and then another section of the fault. The sections that produce great earthquakes remain "locked" and quiet over a hundred or more years while strain builds up; then, in great lurches, the strain is released, producing great earthquakes. Other stretches of the fault, however, apparently accommodate movement more by constant creep than by sudden offsets that generate great earthquakes. In historical times, these creeping sections have not generated earthquakes of the magnitude seen on the "locked" sections. Geologists believe that the total accumulated displacement from earthquakes and creep is at least 350 miles along the San Andreas fault since it came into being about 15-20 million years ago.

Studies of a segment of the fault between Tejon Pass and the Salton Sea revealed geologically similar terranes on opposite sides of the fault now separated by 150 miles, and some crustal blocks may have moved through more than 20 degrees of latitude. Although it is difficult to imagine this great amount of shifting of the Earth's crust, the rate represented by these ancient offsets is consistent with the rate measured in historical time. Surveying shows a drift at the rate of as much as 2 inches per year. What Is an Earthquake? The crustal plates of the Earth are being deformed by stresses from deep within the Earth. The ground first bends, then, upon reaching a certain limit, breaks and "snaps" to a new position. In the process of breaking or "faulting," vibrations are set up that are the earthquakes. Some of the vibrations are of very low frequency, with many seconds between waves, whereas other vibrations are of high enough frequency to be in the audible range. The vibrations are of two basic types, compression waves and transverse or shear waves. Since the compression waves travel faster through the Earth, they arrive first at a distant point; they are known as primary or "P" waves.

The transverse waves arriving later are referred to as shear or "S" waves. In an earthquake, people may note first a sharp thud, or blast-like shock, that marks the arrival of the P wave. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S wave. What Do Earthquake "Magnitude" and "Intensity" Mean? Magnitude is a measure of the size of an earthquake. The Richter Scale, named after Charles F. Richter of the California Institute of Technology, is the best known scale for the measuring of magnitude (M) of earthquakes. The scale is logarithmic; a recording of 7, for example, signifies a disturbance with ground motion 10 times as large as a recording of 6. The energy released by an earthquake of M 7, however, is approximately 30 times that released by an earthquake of M 6; an earthquake of M 8 releases 900 times (30x30) the energy of an earthquake of M 6. An earthquake of magnitude 2 is the smallest earthquake normally felt by humans. Earthquakes with a Richter value of 5 or higher are potentially damaging.

Some of the world's largest recorded earthquakes--on January 31, 1906, off the coast of Colombia and Ecuador, and on March 2, 1933, off the east coast of Honshu, Japan--had magnitudes of 8.9 on this scale, which is open ended. As the Richter scale does not adequately differentiate between the largest earthquakes, a new "moment magnitude" scale is being used by seismologists to provide a better measure. On the moment magnitude scale, the San Francisco earthquake is estimated at magnitude 7.7 compared to an estimated Richter magnitude of 8.3. Intensity is a measure of the strength of shaking experienced in an earthquake. The Modified Mercalli Scale represents the local effect or damage caused by an earthquake; the "intensity" reported at different points generally decreases away from the earthquake epicenter. The intensity range, from I - XII, is expressed in Roman numerals. For example, an earthquake of intensity II barely would be felt by people favorably situated, while intensity X would produce heavy damage, especially to unreinforced masonry. Local geologic conditions strongly influence the intensity of an earthquake.

Commonly, sites on soft ground or alluvium have intensities 2 to 3 units higher than sites on bedrock. Earthquakes Along the Fault Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth's crust. The largest historical earthquakes that occurred along the San Andreas fault were those in 1857 and 1906. The earthquake of January 9, 1857, in southern California apparently was about the same magnitude as the San Francisco earthquake of 1906. According to newspaper accounts, ground movement in both cases was roughly the same type. An account of the 1857 earthquake describes a sheep corral cut by the fault that was changed from a circle to an "S"-shape--movement clearly representative of right-lateral strike-slip. Studies of offset stream channels indicate that as much as 29 feet of movement occurred in 1857. The San Francisco earthquake and fire of April 18, 1906, took about 700 lives and caused millions of dollars worth of damage in California from Eureka southward to Salinas and beyond.

The earthquake was felt as far away as Oregon and central Nevada. The 1906 earthquake, which has been estimated at a magnitude 8.3 on the Richter Scale, caused intensities as high as XI on the Modified Mercalli Scale. Surface offsets occurred along a 250- mile length of the fault from San Juan Bautista north past Point Arena and offshore to Cape Mendocino. On May 18, 1940, an earthquake of magnitude 7.1 occurred along a previously unrecognized fault in the Imperial Valley. Similar movement on the Imperial fault occurred during an earthquake in November 1979. The greatest surface displacement was 17 feet of right-lateral strike-slip in the 1940 earthquake. Clearly, this fault is part of the San Andreas system. Other earthquakes of probable magnitudes of 7 or larger occurred on the Hayward fault in 1836 and 1868 and on the San Andreas fault in 1838. When Could the Next Large Earthquake Occur Along the San Andreas Fault? Along the Earth's plate boundaries, such as the San Andreas fault, segments exist where no large earthquakes have occurred for long intervals of time.

Scientists term these segments "seismic gaps" and, in general, have been successful in forecasting the time when some of the seismic gaps will produce large earthquakes. Geologic studies show that over the past 1,400 to 1,500 years large earthquakes have occurred at about 150-year intervals on the southern San Andreas fault. As the last large earthquake on the southern San Andreas occurred in 1857, that section of the fault is considered a likely location for an earthquake within the next few decades. The San Francisco Bay area has a slightly lower potential for a great earthquake, as less than 100 years have passed since the great 1906 earthquake; however, moderate-sized, potentially damaging earthquakes could occur in this area at any time. A great earthquake very possibly will not occur unannounced. Such an earthquake may be preceded by an increase in seismicity for several years, possibly including several foreshocks of about magnitude 5 along the fault. Before the next large earthquake, seismologists also expect to record changes in the Earth's surface, such as a shortening of survey lines across the fault, changes in elevation, and effects on strainmeters in wells.

A key area for research on methods of earthquake prediction is the section of the San Andreas fault near Parkfield in central California, where a moderate-size earthquake has occurred on the average of every 20-22 years for about the last 100 years. Since the last sizeable earthquake occurred in 1966, Parkfield has a high probability for a magnitude 5-6 earthquake before the end of this century and possibly one may occur within a few years of 1988. The U.S. Geological Survey has placed an array of instruments in the Parkfield area and is carefully studying the data being collected, attempting to learn what changes might precede an earthquake of about that size. What Can Be Done About the Faults and Earthquakes? Even though people cannot stop earthquakes from happening, they can learn to live with the problems caused by earthquakes. Three major lines of defense against earthquake hazards are being developed. Buildings in earthquake-prone areas should be designed and constructed to resist earthquake shaking. Building codes that require attention to earthquake shaking have been improving in recent decades and constitute a first line of defense. In some cities, programs are underway to strengthen or tear down older buildings most likely to collapse during earthquakes.

A second line of defense involves the selective use of land to minimize the effects of hazardous ground. High- occupancy or critical structures, for example, should not be placed astride the San Andreas fault or on landslide-prone areas. The third line of defense will be the accurate prediction of earthquakes. When such prediction becomes possible, it will permit timely evacuation of the most hazardous buildings. A major program aimed at learning how to predict earthquakes and to assess and minimize their hazards was initiated following the Earthquake Hazards Reduction Act of 1977 and is being carried out by the U.S. Geological Survey, other Federal Agencies, universities, and private groups. This publication is one of a series of general interest publications prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment. To obtain a catalog of additional titles in the series "General Interest Publications of the U.S. Geological Survey," write: U.S. Geological Survey P.O. Box 25286 Denver, CO 80225 As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural resources.

This includes fostering the wisest use of our land and water resources, protecting our fish and wildlife, preserving the environmental and cultural values of our national parks and historical places, and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major responsibility for American Indian reservation communities and for people who live in Island Territories under U.S. administration. San Andreas Fault Line - Fault Zone Map and Photos Home » Earthquakes » San Andreas Fault The San Andreas Fault By David K. Lynch, PhD, author of SanAndreasFault.org . San Andreas Map: The red line on this map follows the surface trace of the San Andreas Fault across California. Areas to the east (right) of the fault are on the North American tectonic plate. Areas to the west (left) of the fault are part of the Pacific tectonic plate. The arrows show the directions of relative motion along the fault. Map copyright by David Lynch ( click to enlarge ). What is the San Andreas Fault? The San Andreas Fault is the sliding boundary between the Pacific Plate and the North American Plate.

It slices California in two from Cape Mendocino to the Mexican border. San Diego, Los Angeles and Big Sur are on the Pacific Plate. San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. And despite San Francisco’s legendary 1906 earthquake , the San Andreas Fault does not go through the city. But communities like Desert Hot Springs, San Bernardino, Wrightwood, Palmdale, Gorman, Frazier Park, Daly City, Point Reyes Station and Bodega Bay lie squarely on the fault and are sitting ducks. What Type of Fault is the San Andreas? The San Andreas Fault is a transform fault . Imagine placing two slices of pizza on the table and sliding them past one another where they touch along a common straight edge. Bits of pepperoni from one side crumble across the boundary onto the anchovy side. The same thing happens with the fault, and the geology and landforms along the mighty rift are extremely complicated. You Can See a Plate Boundary! Photo of the San Andreas Fault near Gorman, California, showing rocks of the Pacific Plate (gray rocks on the left side of the fault) and the North American Plate (tan rocks on the right side of the fault).

There are very few places on Earth where you can see two plates in contact like this. Photograph copyright by David Lynch. Click to enlarge . How Fast Does It Move? The plates are slowly moving past one another at a couple of inches a year - about the same rate that your fingernails grow. But this is not a steady motion, it is the average motion. For years the plates will be locked with no movement at all as they push against one another. Suddenly the built-up strain breaks the rock along the fault, and the plates slip a few feet all at once. The breaking rock sends out waves in all directions, and it is the waves that we feel as earthquakes. Google Map of the San Andreas Fault: This map displays the approximate trace of the San Andreas Fault as a red line atop of a satellite image in the Google Maps program. Using the controls of the map, you can zoom in on the fault and follow it across California from where it is first noted near the Salton Sea in Southern California to where it turns into the Pacific Ocean north of San Francisco. Is the Fault Visible at the Surface?

In many places like the Carrizo Plain (San Luis Obispo County) and the Olema Trough (Marin County), the fault is easy to see as a series of scarps and pressure ridges. In other places, it is more subtle because the fault hasn’t moved in many years and is covered with alluvium, or overgrown with brush. In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates. The hallmark of the San Andreas Fault is the different rocks on either side of it. Being about 28 million years old, rocks from great distances have been juxtaposed against rocks from very different locations and origins. The Salinian block of granite in central and northern California originated in Southern California, and some even say northern Mexico. Pinnacles National Monument in Monterey County is only half of a volcanic complex, the other part being 200 miles southeast in Los Angeles County and known as the Neenach Volcanics. Offset Drainage: Aerial photo of the San Andreas Fault showing drainage that is offset by movement of the fault. Photograph copyright by David Lynch.

Click to enlarge . Fault Myths There are many myths and legends about the San Andreas Fault, the biggest being that it will one day crack and California will slide into the sea. WRONG! It won’t happen and it can’t happen. Nor is there any thing such as “earthquake weather” or preferred times of day when earthquakes hit. The World's Most Famous Fault The San Andreas Fault is more accessible than any other fault in the world. With California’s large population and temperate climate, there are many roads that snake along the fault. They are uncrowded and peaceful, perfect for family outings. There is abundant camping, bird watching, wild flowers and wildlife, rock collecting and natural beauty along the way. State and National parks are strung along the fault like beads on a string. All it takes is a good map, a comfortable car and a desire to see the world’s most famous fault. About the Author David K. Lynch, PhD, is an astronomer and planetary scientist living in Topanga, CA.

When not hanging around the fault or using the large telescopes on Mauna Kea, he plays fiddle, collects rattlesnakes, gives public lectures on rainbows and writes books (Color and Light in Nature, Cambridge University Press) and essays. Dr. Lynch's latest book is the Field Guide to the San Andreas Fault . The book contains twelve one-day driving trips along different parts of the fault, and includes mile-by-mile road logs and GPS coordinates for hundreds of fault features. As it happens, Dave's house was destroyed in 1994 by the magnitude 6.7 Northridge earthquake. More Earthquakes San Andreas Fault Facts San Andreas Fault Facts By Becky Oskin, Contributing Writer | March 10, 2015 10:16pm ET MORE Oblique aerial view of San Andreas Fault (between white arrows) in southeastern Coachella Valley, near Red Canyon; view to the west. Credit: USGS. California's sleeping giant, the San Andreas Fault, marks the slippery yet sticky boundary between two of Earth's tectonic plates. It is responsible for the biggest earthquakes in California, up to at least magnitude 8.1.

Beginnings Viewed from space, the San Andreas Fault looks like a long, narrow valley that marks where the North America plate meets the Pacific plate. This narrow break between the two plates is called a fault. But viewed up close, there are actually many fractures and faults that mark the zone where the two plates slide past one each other. Sometimes the boundary is a zone of several smaller faults, one or more of which may break during an earthquake. Sometimes it is a single fault. On the ground, one can find the San Andreas Fault by looking for landforms it created. For example, sharp cliffs called scarps form when the two sides of the fault slide past each other during earthquakes. "The dominant motion along the fault is primarily horizontal, but some areas also have vertical motion," noted Shimon Wdowinski, a geophysicist at the University of Miami's Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault. And stream channels with sharp jogs — the channels are offset across the fault line — can be visited in the central California's Carrizo Plain National Monument . On the west side of the fault sits most of California's population, riding the Pacific Plate northwest while the rest of North America inches south.

The Pacific Plate is moving to the northwest at 3 inches (8 centimeters) each year, and the North American Plate is heading south at about 1 inch (2.3 cm) per year. The San Andreas Fault was born about 30 million years ago in California, when the Pacific Plate and the North America plate first met. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone , another type of plate boundary. The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault . Researchers have measured identical rocks offset by 150 miles (241 kilometers) across either side of the fault. For example, the volcanic rocks in Pinnacles National Park south of Monterey match volcanic rocks in Los Angeles County (called the Neenach volcanics). Geologists think the total amount of displacement along the fault is at least 350 miles (563 km) since it formed. NorCal vs.

SoCal The San Andreas Fault is about 800 miles long (1,287 kilometers), stretching from the Mendocino coast south to the San Bernardino Mountains and the Salton Sea. Geologists divide the fault into northern and southern segments, separated in the middle by a curiously quiet portion that "creeps." [ Photo Journal: The Gorgeous San Andreas Fault ] The northern segment runs from Hollister north through the Point Reyes National Seashore, then eventually moves offshore. The southern segment stretches from Parkfield south through the Salton Sea. The central, creeping section includes everything from Parkfield to Hollister. In historical times, this creeping section has not generated powerful earthquakes similar to those on the "locked" sections. That's because the creeping section slowly, continuously moves, while the locked sections seem to get stuck. These stuck sections of the fault store energy like springs, slowly building up strain until — sproing! — they suddenly unzip and slide past one another in an earthquake. South of the creeping section, the fault also has a visible "Big Bend" that helps push up some of southern California's spectacular mountain ranges.

Near the town of Gorman, the fault suddenly swings east for about 6 miles (10 km), the turns south again. This bend creates geologic squeezing and stretching between the two tectonic plates. The stress on Earth's crust is relieved by building mountains (squeezing) and faulting, or breaking, the Earth's crust, such as the faults that slice up Los Angeles, Wdowinski said. Earthquake prediction The San Andreas Fault was the site of a massive effort to drill into Earth's crust and investigate a fault at depth. In 2004, work began near the town of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD) to drill nearly 2 miles (3.2 km) into the fault. Parkfield, in central California, pops off a moderate earthquake of around magnitude 6 every couple decades, and is a center for earthquake research. It was the site of the first official earthquake prediction by the U.S. Geological Survey. Scientists predicted another earthquake should occur in 1993, but it didn't happen until 2004.

Previous quakes hit in 1857, 1881, 1901, 1922, 1934 and 1966. Rocks retrieved from the deep drilling project revealed that slippery clays may be responsible for some of the "creeping" behavior along the San Andreas Fault. "They found some talc-like minerals," Wdowinski said. "This material has less friction than on the northern and southern section of the fault, so the central section of the fault can creep." Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America. Credit: USGS. The San Andreas Fault System Geology of California's Imperial Valley A Monograph by Eugene Singer CHAPTER 18 THE SAN ANDREAS FAULT SYSTEM The San Andreas fault is a major geologic feature of North America. Actually, the term San Andreas fault is a misnomer, suggesting that the fault is a single break. It is more accurately called the San Andreas fault system, or zone, with several major and numerous minor branches, particularly in southern California. The San Andreas Fault Despite its benign name (Saint Andrew), the San Andreas fault is a violent and destructive killer.

Over its long history, spasms of its enormous energy have been released in countless earthquakes along its great length. These have ranged in magnitude from slight tremors to cataclysmic upheavals and rupturing of Earth's surface. It is a well known and familiar geologic feature in western America. The remarkably straight fault trace is prominent on any California map. It looks like a knife slash from hell in a northwest-southeast direction that attempts to separate the Coast Ranges from central and southern California. The fault ranks among the longest such structures in the world. Overland, it is a visible break that can be traced for 625 miles from Point Arena, north of San Francisco, to the Salton Sea, near the border with Mexico. The inferred extensions of the fault are even more dramatic. Northward, the break is thought to extend under the sea another 400 miles, possibly merging into the Murray Fracture Zone on the deep sea floor. At the southern end, the fault is believed to follow the axis of the Gulf of California for almost 1,000 miles.

Because of the fault's great length, as much as 2,000 miles, movement is intermittent as to time, and random as to place. At any place along the fault, movement may be a one-time event, or movement may be recurrent over thousands of years. In any one episode, displacement may be a fraction of an inch or it may be many feet. The surface trace of the fault first appears at Point Arena, a peninsula north of San Francisco. It takes a straight path southeastward through a series of elongated valleys to San Francisco Bay, it crosses the bay with San Francisco on the western block and the bay cities on the eastern or continental block. In the San Francisco Bay area, the San Andreas has two major companion branches. Generally defining the east side of the Bay is the Hayward fault, and a few miles inland is the Calaveras fault. The Hayward fault presently carries a high probability forecast for an earthquake. In an undeviating course, the trace passes San Jose and enters the Coast Ranges. It runs parallel to the Gabilan Range, passes the towns of Coalinga and Parkfield (both with vivid earthquake histories), thence it defines the elongated Temblor Range to Tejon Pass.

This segment of the fault in central California is very old. Between the Bay area and the Tehachapi Mountains, rock formations appear to have been displaced as much as 160 miles. This suggests that the northern section of the fault may have originated about 30 to 40 million years ago, during Oligocene time. The fault planes of the faults in the system are usually near vertical. In some sections, however, the fault plane is locally tilted as much as 60 degrees to the northeast. A local example of this effect is the Banning fault near Cabazon in San Gorgonio Pass. Here, the fault tilted to the point where the moving western block slid under the eastern block, a thrust fault. The San Andreas fault is a shallow fault, generally extending into the earth's crust four to ten miles. Earthquakes associated with shallow faults are more destructive at the surface than those originating from deep-seated faults. The San Andreas is an uninterrupted zone of many braided fault traces. These fractures branch and interlace in a swath two to eight miles wide along the fault's great length.

With these multiple fractures creating great slices or slabs, and over countless episodes of earth movement, the rocks within the fault zone have been severely deformed, smashed and ground up. Thus weakened and broken, they become subject to rapid weathering and erosion. So, the fault course in central California has been eroded into a series of elongated valleys for more than one hundred miles through the Coast Ranges as a shallow trench only a few miles wide. This distinctive landform was made both from erosion of the shattered and weathered rocks and from the effect of recent faulting. The valleys contain many typical fault-related features such as offset streams, ponds, and scarps. This string of continuous valleys is recognizable on any large scale road map. In southern California, the fault is much younger. Major right-lateral movement began about 12 million years ago, or during late Miocene time. This considerably predates the opening of the present Gulf of California which began about 5.5 million years ago. The progressively younger age of segments of the fault from north to south is consistent with ideas concerning plate movement along the west coast of North America.

The southern California segment of the San Andreas fault is described from the vicinity of Gorman, about 60 miles northwest of Los Angeles. There, it bends abruptly to the east for six miles, then resumes its original southeast heading. This "Big Bend" area is possibly the most significant tectonic area in California today. Here, the San Andreas intersects the left-lateral Garlock fault, the only major east-west trending fault in southern California. Earthquakes are common in this area. The Fort Tejon earthquake in 1857 (magnitude 8-plus) is thought to have been at least as violent as the San Francisco earthquake 49 years later. The San Fernando earthquake of 1971 (magnitude 6.6) was also associated with this zone of intersection. From Tejon Pass, the main trace of the San Andreas fault passes through the high desert north of Los Angeles, defining the north face of the San Gabriel Mountains. It then separates the San Gabriel Mountains from the San Bernardino Mountains, creating Cajon Pass that takes Interstate 15 out of southern California. From the vicinity of Cajon Pass, the southern California segment of the San Andreas fault becomes very complex with no distinctive single trace as in the north.

Instead, it is divided into several right-lateral elements, all somewhat parallel to each other. Principal among these are the San Jacinto and Elsinore faults. Several branches go through the San Bernardino Mountains and along the north margin of San Gorgonio Pass into the Coachella Valley. One important associated fault is the San Gabriel fault, defining the southern edge of the San Gabriel Mountains in the Los Angeles basin. This fault was probably the most active strand for thousands of years, but there is no evidence of recent activity. Another important branch in the Los Angeles area is the Newport-Inglewood fault, the source of the destructive Long Beach earthquake of 1923. The Banning Fault The Banning fault, a subordinate branch of the San Andreas, first appears east of Riverside, trending almost due east. It defines the north side of San Gorgonio Pass where a section is unique in being a thrust fault. This thrust is responsible for the low, brown-colored hills along the north side of Interstate 10 between Cabazon and Whitewater Canyon. The fault has created a distinctive gash behind these hills.

In crossing Whitewater Canyon, it forms a barrier to the passage of groundwater down-canyon, supporting lush riparian growth in the floor of the canyon. The Banning fault enters the Coachella Valley near Whitewater, crossing Route 62 about 1.5 miles north of the interchange with Interstate 10. It intersects Indian Avenue one mile north of the freeway, and crosses Palm Drive at 20th Avenue, then continues to the southwest past Palm Springs. The surface trace of the San Andreas fault is conspicuous on the valley floor by the lineup of vegetation along its north side, a result of fault-dammed groundwater flow. This prominent feature is conpicuous as viewed from the observation post at the top of the aerial tramway. The Banning fault trace defines the southern margin of the Indio Hills, and supports several beautiful palm oases in the canyons where ground water reaches the surface as seepage along the fault trace. Similarly, the northern margin of the Indio Hills is defined by the Mission Creek fault, and here, too, may be found palm oases, principally Thousand Palms Oasis.

While the Banning fault is very well defined by surface features, its relationship to the main San Andreas fault has not been firmly established. The area in question is the complex fault geology of San Gorgonio Pass. The Mission Creek Fault About six miles north of the Banning fault, another branch, the Mission Creek fault, enters the Coachella Valley near the mouth of Morongo Valley. The Mission Creek fault passes directly through the town of Desert Hot Springs, and is responsible for the many warm springs in that area. It intersects Dillon Road near Wide Canyon Road, and marks the northern margin of the Indio Hills. Thousand Palms Oasis is on this fault. Locally, the Mission Creek fault is the most active branch in the system. Seismological data relating to recent earthquakes and measures from precise triangulation surveys show current persistent movement. Disrupted alluvial deposits, truncated older alluvial fans, and vegetation lines are evidence of its trace. In particular, low scarps may be seen where Indian Avenue meets Route 62, and again at Miracle Hill, east of Desert Hot Springs. The Banning fault and the Mission Creek fault join at Biskra Palms in the Indio Hills near the north end of Madison Street).

The San Andreas fault continues along the northwest shore of Salton Sea. At Salt Creek Wash, the surface trace of the great fault ends, more than 600 miles from its northern end. Current thinking is that the San Andreas fault, buried under alluvium, merges with the Imperial fault. It then continues southeastward to the Gulf of California, defining the western margin of the Gulf, finally merging into the East Pacific Rise. The South Pass Fault The inferred South Pass fault defines the southern margin of San Gorgonio Pass along the edge of the San Jacinto mountains. The fault has no surface trace, being deeply buried in alluvium. The principal rationale for placing a fault here is (1) the uniform straight mountain scarp, and (2) without a fault, it would be difficult to account for the pass structure. Where the fault enters the Coachella Valley, it curves to the southwest, past Windy Point to about Chino Canyon. The San Jacinto Fault The most seismically active fault in southern California today is the San Jacinto fault and its branches. It is also the fault that is potentially troublesome to the Salton Trough. Many earthquakes within historic time have been associated with it.

The San Jacinto fault is a major element of the San Andreas system. Separating from the San Andreas west of Cajon Pass, it takes a more southerly but generally parallel track. The trace bisects the city of San Bernardino, and passes directly under the four-level freeway interchange at Colton. Its path takes it behind the San Jacinto Mountains, passing the communities of San Jacinto and Hemet. Along the mountain front in this area, the fault has dammed groundwater channels, forcing water to the surface as hot springs. It lies west and south of the San Jacinto Mountains, and for several miles forms the northeast edge of Borrego Valley. The fault cuts the Ocotillo Badlands near Ocotillo Wells, then enters the Imperial Valley where its trace is buried under recent lake bed sediments. The known length of the fault is approximately 180 miles. Like the San Andreas, it is a complex system of many faults with local names. At its northwest end, following its separation from the San Andreas fault near San Bernardino, it generally has one or more well-defined breaks, especially where its trace cuts across rocks of somewhat recent age.

Southeastward, it includes several zones of subparallel breaks, separating masses of bed rock into slabs ranging in width from a few feet to thousands of feet. Although they are parts of the San Jacinto fault zone, many of these subsidiary faults are themselves major features and commonly are given individual names. One important subordinate fault is the Thomas Mountain fault that forms the southwest margin of Garner Valley. This pattern of multiple faults continues to broaden to the southeast where the San Jacinto zone enters the Salton Trough. There, at least six subordinate faults are spaced half a mile to three miles apart. This pattern continues through the Imperial Valley and into Baja California. The major breaks in the San Jacinto fault zone dip very steeply to nearly vertical, and the entire zone appears to be a very deep-rooted feature. There is abundant evidence of recency of movement along most of the length of the fault. In its central section, along the San Jacinto Mountains and the Santa Rosa Mountains, there are many fault scarps, elongate trenches, sag ponds, aligned canyons, and offset drainage. Many earthquakes, moderate to severe in scale, have been recorded from the City of San Jacinto south into Mexico.

The most active branch at this time is the Imperial fault, the source of many tremors and quakes in recent years, some of them very destructive. Another active branch is the Superstition Hills fault along the west side of the Imperial Valley. The very active Cerro Prieto fault on the Mexican side of the border is a member of the San Jacinto system, and is probably an extension of the Imperial fault. The San Jacinto fault is a young, right lateral zone of seismic strain that has dominated fault movement in southern California for a least a century. Notwithstanding the notoriety of the San Andreas fault, since 1857 there have been 36 major earthquakes identified to faults in the San Jacinto system. Of these, 15 have originated along the Imperial fault in the Salton Trough. Between 1915 and 1954, five historic large quakes, all with magnitudes between 6.0 to 6.8 on the Richter scale, occurred along this fault between the City of San Jacinto and the Salton Sea.

While presently more active than the San Andreas fault, the San Jacinto fault is much younger in age, having slipped laterally only about 15 miles, compared to nearly 200 miles of displacement along the San Andreas fault. The age of the San Jacinto fault is uncertain, but it seems probable that the fault has been active since at least early Tertiary time. The Elsinore Fault A brief mention of the Elsinore Fault is made here only because it is one arm of a trilateral split of the San Andreas Fault. As a geographic feature it is not associated with the Salton Valley. The third major fault in the southland is the Elsinore fault, also a member of the San Andreas system. Its trace is parallel to the San Jacinto, and about 20 miles west. The fault originates beneath the alluvium of the Los Angeles Basin. Running southeast, it sharply defines Elsinore Valley and Lake Elsinore). Passing south of the Vallecito Mountains bordering Borrego Valley, the fault enters the Salton Trough near the Coyote Mountains in the extreme southwest corner of the Imperial Valley.

Of the three principal branches including the San Andreas and the San Jacinto faults the Elsinore fault has been considerably less active in historic time. Movement along the San Andreas fault The San Andreas is a right lateral fault, with the relative movement of adjacent land masses being sideways. Along the San Andreas system, movement of the western block has been consistently to the northwest. Over millions of years, the cumulative displacement amounts to several hundred miles. Or, as Armand Eardley put it, "The San Andreas marks such an important contact that rarely can it be crossed, except in recent alluvium, without passing into significantly different rocks. Displacement, or the amount of movement along lateral faults is determined by identifying distinctive rock units found on opposite sides of a fault, but are some distance apart. One such technique is identification of rock debris in a sedimentary basin that could not possibly have come from the local mountains. The geologist then attempts to match up the sedimentary material with the appropriate source on the other side of the fault. A local example of this occurs in Whitewater Canyon. A rock formation known as the Coachella Fanglomerate a coarse, bouldery ancient alluvial fan deposit is exposed where the Mission Creek fault crosses the canyon.

Geologists have studied the boulder fragments in the Coachella Fanglomerate in detail, finding several distinctive types south of the Mission Creek fault that do not appear north of the fault. These rock fragments appear to have been derived from a source area close to the Cargo Muchacho Mountains, near the Mexican border. Such a correlation requires 130 miles of right-lateral separation within the San Andreas fault system. Studies at many locations between Soledad Pass and the Salton Sea have established that the San Andreas fault has a total offset of at least 140 miles. As the southern section of the San Andreas fault can be dated back to the Miocene Epoch, about 12 million years ago, this suggests an average displacement of more than one inch per year. Offset of the rocks has also been observed at many points along the length of the San Andreas fault. Older rocks appear to have been displaced more than 350 miles, while progressively younger rocks are displaced progressively fewer miles. It is difficult to absorb the scale on which nature works! A displacement of hundreds of miles along the San Andreas fault can only be accepted when we understand that it is the result of an immense number of small steps occurring over a vast span of time.

San Andreas Fault Zone, Central California San Andreas Fault Zone, Central California by: Jesus Alvarez Abstract The San Andreas Fault delineates the boundary between the Pacific Plate and the North American Plate just west of the central California Valley. The fault is parallel by other smaller faults mostly on the west side running in a north-south direction and by Garlock Fault which runs perpendicular to San Andreas Fault along the Traverse Ranges (see map below). The geology on the central coast side of California is complex; in a generalized geologic map (see Fig 2), the geology of the eastern side of California is preserved intact dating back to Late Cretaceous while on the coast side, most geologic features now locked together appear to be out of place. The Monterey Terrane, which dominates the central coast California could date back to Late Cretaceous since it shows similarities to the Mojave-eastern arc located at lower latitudes. The tectonic blocks of the central coast could have been transferred northward several degrees of latitude via the tectonic activities in the last 20 million years. The tectonic activities continue today as the Pacific Plate and the North American Plate slip past each other reshaping the landscape and generating earthquakes along the fault.

The current relative movement of the plates creates several small earthquakes in the upper crust and every so often the fault builds enough stress allowing the plates to move past the average rate generating moderate to large size earthquakes (see map below), some of which have been shown to be catastrophic in populated areas in the last 160 years. The current horizontal displacement between the Pacific Plate and North American Plate has reshaped surface features in recent years. Man-made structures that have been built in the last century along the fault and streams that flow on the landscape across the fault are continuingly being twisted and shifted in directions relative to the horizontal displacement. San Andreas Fault Map. The San Andreas Fault, other active faults and related earthquakes are shown for California (taken from USGS). San Andreas Fault The San Andreas Fault extends more than 1,100 km and is located along the western side of central California; the fault begins at the Sea of Cortez branching out from the East Pacific Rise and cuts across central California exiting at Point Reyes and continuing northwest until it meets with the Mendocino Fracture Zone.

The fault separates the North American Plate (east) from the Pacific Plate (west) and depths extend to at least 18 km (USGS, 1997). The San Andreas Fault is relatively young in geologic terms, it dates back to the Early Miocene about 15-20 million years ago (Ma). The strike-slip horizontal displacement of the fault in which the North American Plate moves in a southeast direction and Pacific Plate moves in a northwest direction (see Fig 1) has a displacement rate of approximately 5cm/year (USGS, 1997). As the plates on opposite sides of the fault move, hundreds of small earthquakes related to plate movement occur in the upper crust near the fault every year with some moderate-size catastrophic earthquakes with magnitudes > 6 occurring around every 20-22 years and magnitudes > 8 occurring around every 150 years based on historic events. USGS (1997) estimates the accumulated displacement caused by the fault to be 550 km over the past 15-20 million years.

By comparing the geology on opposite sides of the San Andreas Fault, the mismatch of geologic features indicate the terranes on the west side of the fault have been transported more than 200 km via the San Andreas Fault (see Fig 2). Figure 1. Central California coast false-color composite. Landsat 5 TM dataset from Sep. 09, 2009 with relatively low cloud cover were used to produce the composite using IDRISI Taiga using Bands 2,3,4. The active vegetation features mostly on the windward slopes and valleys are shown with red-bright red tones, water features are dark blue, bare soils, rocks, sediment, paved surfaces, and dry vegetation are shown in gray tones. The San Andreas Fault Zone is shown with a solid yellow line and arrows show direction relative to plate movement and the plates labeled in bold yellow. Major mountain ranges of the Pacific Plate are labeled in white along with the Salinas Valley labeled in green. The city of Parkfield is located just east of Cholame Hills on the edge of the North American Plate. Figure 2. California Geology (taken from Ducea, 2007).

The generalized geologic map of California shows the major tectonic features. The geology of Mojave-eastern arc east of the Peninsular Ranges Batholith (PRB) correlates with the Salinia arc west of the San Andreas Fault. Figure 1 (above) is located mostly west of the Great Valley forearc basin on the Franciscan Complex, which is overlapped geologically by the Salinia basement (see Fig 5). Central California Last 90 Million Years of Tectonic Events The complex geology of the modern California central coast can be summarized only by generalizing the tectonic events that have taken place in the last 90 million years. Assuming that the Farallon Plate was being subducted under the North American Plate in the Late Cretaceous period (see Fig 4) before the San Andreas Fault became active some 20 Ma. The subducting motion and relative direction of the Farallon Plate created the Eastern California arc followed by the Western arc. As the Farallon Plate subducted under the North American Plate, the existing Franciscan Complex wedged between the Western arc and the Farallon Plate.

The Great Valley forearc basin accumulated sediments from the ancient seas over the Franciscan Complex (see Fig 1), which now form most of the California Valley that extends from the Traverse Ranges to the south up to the northern California Valley. During the Late Oligocene period some 30 Ma, the older dense ocean crust of the east Pacific Plate split the Farallon Plate as it was moving east towards the North American Plate. The two new emerged plates formed the Juan de Fuca Plate to the North split from the Pacific Plate by the Mendocino Fracture and the Cocos Plate on the south Pacific split from the Pacific Plate by the East Pacific Rise Fracture. During the Early Miocene some 20 Ma the realignment of the plates on the west side of North America forced the relative movement of the Pacific Plate and the North American Plate into a strike-slip horizontal displacement forming the San Andreas Fault. Based on the relative movement of the Pacific Plate and North American Plate and geologic evidence present on terranes on opposite sides of the San Andreas Fault we can assume that the coast ranges maybe related to the south.

According to Ducea (2007), the Coast Ranges west of the San Andreas Fault could date back to the Late Cretaceous and may have been transferred to the Pacific Plate via the San Andreas Fault. The complex geology of the Monterey Terrane (see Fig 3) is composed Late Cretaceous basement blocks that have accreted over the Pacific Plate. Cross-sections of geology maps of central California show the Monterey Terrane and terranes of Southern California have similar tectonic underplating. The Underplating of the Monterey Terrane west of the San Andreas Fault (see Figs 3-5) is believed to be composed of sediments wedged by the early subducting Farallon Plate some which are exposed as the Schist of the Sierra de Salinas (see Figs 3 & 5) in the Salinas area between the Santa Lucia Range and the Gabilan Range (see Figs 1 & 5D). The faults parallel to the San Andreas Fault and the Salinas Shear Zone (see Fig 5C) mark the boundaries of the structurally collapsed accreted wedges that formed prior to the San Andreas Fault.

The Coast Ranges that now dominate the landscape along the coast are believed to have formed after the San Andreas Fault in the last 20 million years. Figure 3. Central California Coast Terrane (taken from Ducea, 2007). The map shows the three main basement rocks that compose the Monterey Terrane and the Late Cretaceous and Cenozoic faults west of the San Andreas Faul. Figure 4. Pre-San Andreas tectonic profile (taken from Ducea, 2007). A (95-80 Ma) shows the normal subduction of the Farallon Plate under the North American Plate, which generated the great California arc. B (76-73 Ma) shows a shallower subduction and accretion of wedge sediments. Figure 5. Tectonic reconstruction of the modern Monterey Terrane (taken from Ducea, 2007). C (74-68 Ma) shows the collapse of the margin that resulted in deposition of marine sediments.

D (modern) shows modern Monterey Terrane after the collapsed margin and other tectonic events that took place during the Late Cenozoic along the San Andreas Fault and after the current mountain ranges formed. Recent Horizontal Displacement The horizontal displacement continues in the central California coast. The Pacific Plate and North American Plate continue to slide past each other in opposite directions. Along San Andreas Fault the recent horizontal displacement is evident on surface features. The slow horizontal movement of the plates deforms the linear shapes of the streams that carry runoff from the adjacent mountain slopes. Driving along highway 46 towards Paso Robles oil fields signal the location or areas rich in sediments deposited prior to San Andreas (see Fig 6) towards the end of the North American Plate. A few kilometers before reaching the end of the North American Plate westbound highway 46 changes direction and runs parallel to the San Andreas Fault in a northwest direction. Driving along the fault the apparent displacement along the fault appears to be vertical along the Temblor Range, which runs parallel to the fault on the Pacific Plate side (see Figs 7-8). The displacement could be related to the fault since it is consistent throughout the range appearing as if someone sliced the range with a knife.

In addition to the vertical displacement shown on the mountain ranges, the depression formed along the fault shows the banks along North American Plate side to be slightly lower than those on the Pacific Plate side (see Figs 11-12). The vertical displacement does not have to be related to the tectonic activities along the fault, however, we could assume that the vertical displacement if not caused by subsidence as the plates move apart, could be influenced by the differences in weight of the buoyant plates and/or the accreted geology west of the San Andreas Fault. Of the several hundred earthquakes that are generated in the upper crust along the San Andreas Fault every year, only a few reach magnitudes greater than 6 around every 20-22 years in the Parkfield area (USGS, 1997). The most evident recent horizontal displacement can be measured on the man-made structures as they are twisted or modified by moderate-size earthquakes. About 25 km northwest of the intersection of highway 46 and highway 41 a small bridge crosses the San Andreas Fault to access the small city of Parkfield (see Figs 9-14).

Recent tectonic activity along the fault triggered strong 6 M + earthquake twisting the metal barriers and moving the columns that hold the Parkfield bridge in place (see Fig 9). The bridge shows evidence of horizontal displacement relative to the direction of the moving plates; the Pacific Plate sliding northwest and North American Plate sliding southeast twisted the bridge in the same direction. Tectonic activities along the San Andreas Fault will continued as the plates buildup stress moving in opposite directions. Based on historical events, the next moderate-size > 6 M earthquake is expected to hit the Parkfield area in the 15 years, while a big earthquake is overdue in the southern California area just east of the Mojave-eastern arc, south of where the Garlock Fault and San Andreas Fault intersect (see Fig 2), an area which has an expected > 8 M earthquake interval of 150 years based on historical events. Figure 6. Oil Field between Temblor Range and Lost Hills. The oil fields are located between the Temblor Range and the small city of Lost Hills along Highway 46. Several oil fields are located in the fossil rich areas located between the Great Valley forearc basin and the Franciscan Complex (photo by: J.

Alvarez). Figure 7. North end of Temblor Range. The Temblor Range is located at the edge of the Pacific Plate and runs parallel to the San Andreas Fault. Vertical and horizontal displacement is common along the San Andreas side of the Range. In this photograph the vertical displacement on the slopes is about 2-4 meters (photo by: J. Alvarez). Figure 8. North end of Temblor Range & beginning of Cholame Hills. At the end of the Temblor Range another the Cholame Hills emerge also running parallel to the San Andreas Fault on the edge of the Pacific Plate (photo by: J. Alvarez). Figure 9. Parkfield bridge crossing the San Andreas Fault. The sign marks the entrance to the North American Plate. The bridge crosses the San Andreas Fault just south of Parkfield. The bridge has been retrofitted after strong earthquakes >6 magnitude twisted the bridge and offset the columns, which were once aligned horizontally (photo by: J. Alvarez). Figure 10. Parkfield bridge showing the twisted metal barriers and offset columns caused by the horizontal displacement of the Pacific and North American Plates (photo by: J. Alvarez). Figure 11.

South view of the San Andreas Fault. The photograph was taken from Parkfield bridge which runs perpendicular to the fault. Cholame Creek runs along the fault. The banks on the Pacific Plate (right) seem to be about 3 meter higher than the banks on the North American Plate (left) (photo by: J. Alvarez). Figure 12. North view of the San Andreas Fault. The photograph was taken from Parkfield bride facing north and it shows the depression along the fault filled with sediment from runoff and mountain ranges that emerge on both sides of the fault (photo by: J. Alvarez). Figure 13. Entrance sign to the city of Parkfield. The small city of Parkfield is assumed to be located on the edge of the North American Plate (photo by: J. Alvarez). Figure 14. Southwest view from Reason Mountain. The photograph was taken from the peak of Reason Mountain located on the North American Plate about 10 km north of Parkfield.

Parkfield is to the left, three mountain ranges are visible in the background; Middle Mountain (front) runs along the San Andreas Fault, the Cholame Hills are on the background and the Santa Lucia Range are on the far background (photo by: J. Alvarez). References Alvarez, J., 2010. San Adreas Fault Zone Image Composite and Site Photographs. Ducea, M. N. 2007. The Monterey Terrane Crustal Section, Central California: Tectonic Underplating of Trench Sediments Beneath A Magmatic Arc, p. 1-18. World Wide Web homepage < > [retrieved on 31 March 2010] Figures used with permission. IDRISI Taiga. GIS/Image processing software, World Wide Web homepage < > USGS, 1997. The San Andreas Fault, World Wide Web homepage < > [retrieved on 29 March 2009] USGS, Earth Resources Observation and Science (EROS).

Landsat 5 TM Satellite Data, World Wide Web homepage < #/Find_Data/Products_and_Data_Available/Satellite_Products > [retrieved on 29 March 2010] San Andreas Fault Map - Zoom In on the Fault! - GEOLOGY.COM Home » Plate Tectonics » San Andreas Fault Map San Andreas Fault Map - Zoom in for a Closer Look! Map by David K. Lynch, author of SanAndreasFault.org and Bradley M. Cole of Geology.com This satellite view shows the approximate trace of the San Andreas Fault across California. A fault trace is a line on a map where the fault is believed to intersect Earth's surface. Use the buttons in the upper left corner of the map window to zoom in and out and follow the fault across California. You can switch between map, satellite, and hybrid views by clicking the buttons in the upper right corner of the map window. Details about this map, how it was made and how accurate it might be are given below. The San Andreas Fault trace shown on this map is approximate.

It was created by measuring the latitude and longitude for many points along the fault trace from maps published by the United States Geological Survey, California Geological Survey and other sources (a detailed list of these publications can be seen here ). These points are shown on the map as small red squares. The light red lines that connect the squares are linear interpolations. This yields a map that shows the approximate location of the fault. The points and the lines shown on this map are generally within about one hundred feet (actual ground distance) of where the authors of the source documents plotted them - however there are locations where greater divergence occur. This map provides a visual presentation of the fault in a format that can be easily transmitted via the web to a very large number of people who might otherwise know very little about the fault's location. We have chosen to represent the fault as a thin line, not because the location is known that precisely, but because we do not want to obscure ground features and evidence of the fault's location that can be seen in the satellite images. The concept of this map was created by Dr. David K. Lynch, author of Field Guide to the San Andreas Fault and Dr. Hobart M. King, publisher of Geology.com .

The Google map was created by Bradley Cole, cartographer, Geology.com . We thank Google and authors of the publications used to obtain the latitude and longitude estimates that serve as the basis for this map. Find it on Geology.com San Andreas Fault San Andreas Fault The San Andreas fault, California's major earthquake rift zone, is one natural feature that elicits a strong response from most visitors -- many of us have experienced earthquakes and all of us have seen photos of how powerful they can truly be. This world-famous fault runs along much of the length of the state, skirting on and just offshore California's coast, and it clearly comes ashore at Ross where its 1906 legacy is still visible. Fort Ross is an excellent destination to witness first-hand the powerful effects of plate tectonics and the lasting effect of big earthquakes. The park provides several clearly visible markers of the 1906 temblor, all of which are fairly accessible to the road just uphill from Highway One.

Even visitors with an untrained eye can see how the sag ponds left behind from the rift caused by the 1906 quake; clusters of redwood treetops directly on the fault are still damaged and cracked, and there is a clear offset to the gently sloping coastal hill. Also in our Fort Ross Visitor Center, we have photos documenting the sizable damage the 1906 quake caused to the historic fort compound, in particular the Fort Ross chapel. All of this is a testament to the incomparable power that these forces of nature can unleash, making the park one of the more accessible destinations at which to witness first-hand the power of plate tectonics. Fort Ross sits on the ocean side of The San Andreas fault zone. If you look carefully at the lay of the land above the fort you can see it, and if you walk up Fort Ross Road, the faultline is clearly visible. There is a red marker on the road, marking the actual faultlline. The 1906 earthquake that destroyed San Francisco was centered very near to Fort Ross, and many of the Russian-era buildings were destroyed in that 1906 event.

The San Andreas fault ( see map ) runs offshore from around Tomales bay in Marin, but comes ashore at Fort Ross. From Fort Ross the fault continues overland, forming the valley through which the Gualala River flows. It goes back offshore at Point Arena. California’s major earthquake rift zone, passes through San Francisco and comes ashore two miles south of Fort Ross, then runs north and through the orchard ( see map ). The fault is the result of the boundaries of the Pacific and North American plates passing by each other. On this portion of the fault the movement tends to be in sudden large events, and it is thought that this section of the San Andreas Fault moves significantly only every few hundred years. California’s 1906 earthquake was the sudden result of such motion. The fort itself lies on marine sediments to the seaward side of the fault. These sediments were deposited underwater on the Pacific plate forty to sixty million years ago, and have moved from the south about three hundred miles up the California coast. In 1906 the land at Fort Ross shifted 12.6 feet along a narrow well-defined area above the plate movement. The resulting surface features can still be seen today.

Offset creeks, sag ponds (depressions along the fault which often become filled with water in winter), escarpments, shifted fences, and damaged trees are lasting evidence of the quake. At 5:12 AM on April 18, 1906 the San Andreas Fault shifted and caused damage from San Juan Bautista to Cape Mendocino. The epicenter was just south of San Francisco. This is San Francisco City Hall after the earthquake. (Steinbrugge Collection, UC Berkeley) Violent shocks punctuated the strong shaking which lasted up to a minute. The quake is remembered for the San Francisco fires; 2831 acres burned, destroying 28,188 structures. The approaching fire on Sacramento Street, San Francisco. (Steinbrugge Collection) The 1906 quake was felt from southern Oregon to Los Angeles. Damage was severe in many places along the fault rupture. Santa Rosa's business district was built on an alluvial plane 20 miles from the fault line. The city was shaken to rubble and ravaged by fire. The Sonoma County Courthouse in Santa Rosa, 1906.

(Steinbrugge Collection) The San Andreas Fault runs through the Fort Ross area. At that time the Call Ranch was a shipping and social center for the surrounding community. The old Russian structures at Fort Ross had been maintained and modified by successive owners. The Russian fort had been made a California State Historic Site in early 1906, but less than a month later the April 18 earthquake damaged the old Russian structures. Fort Ross pictured in Thompson & West's Atlas, 1877. The earthquake ruptured the ground at Fort Ross. "There were several shocks, quickly following each other; the first was not the strongest. They seemed to increase in force up to the third or fourth and come from different directions. . ." Report of the California Earthquake Commission, 1908. Rosa Call standing by the surface trace, 1906. Panorama of Fort Ross with the collapsed chapel on the left, 1906." . . . Chimney tops were thrown off . . . Many redwood and pine trees were broken off . . . All loose furniture was turned over, and a few frame buildings set upon unbraced posts were shaken down." 1908 Report.

"The dwelling of G. W. Call, proprietor of the place, was violently shaken . . . In cleaning up the wreck after the shock, six wheelbarrow loads of broken objects were picked up off the floors of the rooms. In Mr. Call's room a high case was thrown across the bed in which he was sleeping." 1908 Report. "At Fort Ross, 0.75 mile from the fault, the intensity of the shock was probably greater than the actual damage would indicate. The old Russian church and several other buildings suffered thru collapse of their underpinning, but all in a fair state of repair stood the shock, as did the more recently built dwellings." 1908 Report. The walls of the old Russian chapel collapsed, but the roof remained intact. Chapel and barns from behind the fort, 1906. These photos were taken after the 1906 Earthquake in the vicinity of the Doda Ranch, near Mill Creek just south of Fort Ross. The surface rupture and the offset fence resulting from the earthquake are pictured. (Bancroft Library) Repair of the earthquake damage to the Russian chapel began in 1916.

The Russian officials quarters, later used as the Fort Ross Saloon, was torn down and the salvaged lumber was used to rebuild the chapel. The fort lies on marine sediments to the seaward side of the fault. These sediments were deposited underwater on the Pacific plate forty to sixty million years ago, and have moved from the south about three hundred miles up the California coast. In 1906 the land at Fort Ross shifted 12.6 feet along a narrow well-defined area above the plate movement. The resulting surface features can still be seen today. Offset fences and creeks, sag ponds (depressions along the fault which often become filled with water in winter), escarpments, and damaged trees are lasting evidence of the quake. The San Andreas Fault traverses the length of the Fort Ross orchard in an approximate north-south direction. The orchard is situated east of Highway One 1/4 mile up Fort Ross Road. The dramatic effects of the earthquake are still visible in the orchard. Among the fruit trees there are redwoods damaged in 1906. Escarpments, or pressure ridges, such as this one in the orchard form when two tectonic plates collide.

Between trail markers 7 and 8 there is a sag pond. Across the road from the orchard on the trail beginning at the Stanley Spyra Memorial Grove there are many more visible remnants of the 1906 earthquake. Damaged giant redwoods, escarpments, and sag ponds. Redwoods damaged in 1906 continued to grow in a deformed manner. Trunks were split and the top branches snapped off. The sag ponds are still apparent in 2006. Geologists from the U.S. Geological Survey (USGS) continue to investigate the North Coast segment of the fault line at Fort Ross. Studies conducted in the Fort Ross Orchard, the former Doda Ranch, and other locations are aimed at determining methods to accurately forecast earthquakes.

Question: What is the name of the fault line that runs for about 800 miles in California?
Answer: San Andreas Fault