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 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] The San Andreas and Other Bay Area Faults The San Andreas and Other Bay Area Faults Bay Area Faults The San Francisco Bay Area is transected by a series of subparallel faults that together accommodate the relative motion between the Pacific and North American plates. The San Andreas Fault and 6 other significant fault zones are present in the Bay Area: the Calaveras, Concord-Green Valley, Greenville, Hayward, Rodgers Creek, and San Gregorio Faults. The faults shown here are represented by simple lines which do not convey how complicated they can be. In reality, active faults can be very complex, consisting of multiple breaks along curved, complex traces. See for yourself by examining the detailed mapping of the Hayward Fault . This Bay Area Faults layer is intended solely as an educational tool. Official state Earthquake Fault Zone maps that govern construction setbacks and disclosure are produced by the California Geological Survey .

Historic Ruptures In the last half of the 1800’s, potentially damaging earthquakes (magnitude 6.0 or greater) occurred in the San Francisco Bay Area at an average rate of once every four years. However, seismicity rates dropped dramatically after the 1906 earthquake: scientists believe the large stress release of the 1906 earthquake also relieved stresses on faults throughout the Bay Area. Seismic activity was very low until the 1989 Loma Prieta (6.9 magnitude) earthquake. The level of seismic activity has not yet returned to that of the late 1800’s, but as stresses continue to build scientists expect more frequent and stronger earthquakes in the future. The following file is an interpretation of historic ruptures in the Bay Area from 1836 until today. For more information, see Historic Earthquakes by State . Santa Clara County Earthquake Hazard « ABAG Resilience Program About Santa Clara County Earthquake Hazard Several active faults present potential hazard to Santa Clara County. On the northwestern boundary, the San Andreas Fault runs through the hills separating the County from Santa Cruz County.

In the central county, the Hayward/Rodgers Creek and Central Calaveras dominate the earthquake threat. The Greenville fault dominates in the northeastern portion of the county. The last major earthquake with an epicenter within Santa Clara county was the 1984 Morgan Hill Earthquake (M 6.2), and the 1989 Loma Prieta Earthquake’s (M 7.1) epicenter was located just a few miles from the County, in Santa Cruz County. The Hayward fault has the greatest likelihood of rupturing in the next 30 years of all the faults in the Bay Area. Liquefaction may occur in the northern portion of the County along the Bay margins, while landslides are more likely in the eastern hills. 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 Western Region Geology and Geophysics Science Center Please Note: This is an archived website. It is recommended to search on-line for revised or newer information. Western Earth Surface Processes Team San Andreas Fault System in the Inland Empire and Salton Trough Southern California's Major Faults General statement As you can see by examining a fault map of southern California or a satellite image showing superimposed fault lines , southern California is traversed by numerous faults. Some of these, like the San Andreas Fault, are master players in the southern California fault mosaic; others are minor and not so well known.

satellite image of southern California showing superposed fault lines (image sources: (1) faults from Jennings, 1994; (2) Landsat image from Jet Propulsion Laboratory of the California Institute of Technology). [Jennings, C.W., 1994, Fault activity map of California and adjacent areas: California Division of Mines and Geology, California Geologic Data Map Series, Map No. 6, scale 1:750,000. ] Diagram of southern California showing the San Andreas Fault as a master player in a tectonic setting that includes other faults and compressional fold belts (image source: Southern California Earthquake Center) Many of these faults are capable of generating earthquakes-some small and local, others major and capable of impacting human activities throughout southern California.

In order to understand how and where earthquakes occur and to prepare for their effects, geologists and seismologists need to address several issues: geologic relations among the various faults (do their geographic distributions and geologic characteristics mean anything in terms of earthquake potential?) the long-term geologic history of these faults (what is the earthquake history of each fault [paleo-earthquake history] and how have they interacted with each other over time?) the role each fault plays in the strain budget of southern California (how are the minute stress and strain forces that result from southern California's location at an active plate margin distributed among the various faults and folds, and have there been any changes in this stress strain budget throughout the last several hundred thousand years?) A fault is "a fracture or zone of fractures [in the earth's crust] along which there has been [movement] of the sides relative to one another" (modified from the Glossary of Geology, Bates and Jackson, 1987, p. 223). A fault forms where rock of the earth's crust literally is broken as the result of accumulated stresses operating on the crust.

The resulting fault plane will have a dip direction and dip amount, and its intersection with the ground surface will define a trace that will have a compass trend (azimuth). Faults extend from the ground surface downward into the earth's crust; this downward dimension is called a fault's "subsurface" extent. Faults usually extend for several kilometers into the subsurface, until the brittle conditions that allow for rock-breakage give way to plastic (ductile) conditions that lead to rock- and crystal-flowage. The photograph below shows a profile (cross-sectional) view of a fault in a Guatemala roadcut. Were it not for this roadcut, geologists would have no direct information about the fault-plane dip or about the amount of movement (displacement). Photograph of faulted sedimentary rock layers exposed in a roadcut in Guatemala. The sedimentary rocks originally were deposited as horizontal layers of sand and gravel in streams and debris flows. After their deposition, the consolidated sediment was broken by a moderately-dipping normal-slip fault that has dropped layers left of the fault down about 3 meters relative to layers to the right; note the small subsidiary fault that dips even more shallowly and joins the main fault above the person's head.

These kinds of relations are common where rocks of the earth's crust are stretched and pulled apart (extended), yielding faults like these that have normal dip-slip geometry. Photograph copyright J.K. Nakata, August, 1988 (used with permission). Faults not only extend downward into the earth's crust, they also can be traced laterally across the landscape. The fault plane in the preceding photograph could be traced to the top of the roadcut, and if conditions permitted, its trace on the ground surface could be identified and mapped away from the roadcut. In the desert regions of southern California it is easy to recognize the geographic extent of faults that form clear traces across the desert landscape. Such is the case for the strand of the San Andreas Fault zone pictured in the next photograph. Oblique aerial photograph of the trace of a fault on the desert floor of southern California; view looking east at the Coachella Valley trace of the Banning Fault in the northern Coachella Valley. The linear trace on the desert floor occurs where a fault plane that is vertical in the subsurface intersects the land surface. Thus, a cross-sectional view of a fault in a road cut expresses translates into a trace on the ground surface when viewed from above in this photograph.

The linear is formed by scarps (topographic expression of fault movement), by vegetation concentrated along the fault trace where ground-water ponds up-slope from the fault, and by sand dunes that form where wind-blown sand is trapped by the vegetation. Photo by J.C. Matti, USGS, December, 1979. Links to basic information about faults (???) How are strain budgets determined for southern California? paleoseismological studies that attempt to identify evidence for the timing and character of earthquakes over the last few thousand years by examining the detailed geologic record of trench excavations (Sieh, 1978, 1984; Fumal and others, 1993; Seitz and Weldon, 1994; McGill and others, 1998a, b).

Studies like these lead to estimates of fault-slip rate and faulting recurrence, which in turn lead to probabilistic estimates of seismic risk regional geologic-mapping investigations like those of the Southern California Areal Mapping Project (SCAMP) that attempt to identify long-term movement histories for various fault zones (Crowell, 1962, 1981; Ehlig, 1981; Powell and Weldon, 1992; Powell, 1993; Matti and Morton, 1993; Weldon and others, 1993; Dillon and Ehlig, 1993; Dickinson, 1997). These investigations map out the distribution of rock masses that have been displaced by the faults, and work out the timing of movements among faults that may have been active for a time and then abandoned as the strain distribution in southern California shifted with time to other faults. From these kinds of studies, a picture is emerging of how faults in southern California interact with each other.

In this scenario, the master San Andreas Fault and its relatives are all components of a widespread fault-and-fold system that has developed in southern California in response to plate tectonic interactions along the western margin of North America. Diagram illustrating the plate-tectonic setting of southern California (image source: USGS general-interest publication "This dynamic earth: the story of plate tectonics" (Kious and Tilling, 1996).

Many kinds of geologic structures occur within the San Andreas System, including: Right-lateral strike-slip faults develop where blocks of Earth's crust are sliding past each other in a right-handed sense (as the observer looks across the fault) Left-lateral strike-slip faults develop where blocks of Earth's crust are sliding past each other in a left-handed sense (as the observer looks across the fault) Normal dip-slip faults develop where the Earth's crust is pulling apart (extending) and crustal blocks drop and rise vertically past each other Reverse dip-slip faults develop where the Earth's crust is being squeezed together (contracting) and crustal blocks ramp upward over adjacent blocks Fold belts develop where the crust is being squeezed and warped rather than broken by faults, thereby yielding folds in certain kinds of rock Index Hayward Fault Fact Sheet Page Image Page Content The Hayward Fault is considered one of the most dangerous in the world because scientists believe it is due for a large earthquake and because it runs under a densely populated area of California. The California Geological Survey was among the organizations contributing to the Uniform California Earthquake Rupture Forecast, Version 2, released in April 2008.

That report stated that there’s a 31 percent chance the Hayward Fault will produce a magnitude 6.7 or greater earthquake in the next three decades. Where is the fault? The fault runs parallel to and east of the San Andreas Fault. It extends from San Jose about 74 miles northward along the base of the East Bay Hills to San Pablo Bay. Communities on or near the fault include San Jose, Oakland, Fremont, Richmond, Berkeley, Hayward, San Leandro, San Lorenzo, El Cerrito, Emeryville, Kensington and Milpitas. Among other sites, the fault runs directly under the now-abandoned old city hall in downtown Hayward, the University of California-Berkeley football stadium, the Mira Vista Golf Course near Berkeley, Lake Temescal, Contra Costa College, and Port Pinole Shoreline Regional Park. The 1868 earthquake The three decades mentioned earlier may be an optimistic time span. By digging trenches across the fault, scientists have determined that the last five earthquakes large enough to cause damage have occurred about every 140 years (the “recurrence rate”). That means the Hayward Fault may be due.

The biggest earthquake on the fault in recorded history was an estimated magnitude 7 at 7:53 a.m. October 21, 1868. The fault moved northward from the Warm Springs area of Fremont possibly as far as Berkeley. The maximum horizontal displacement was about six feet. It was known as “the Great San Francisco earthquake” until 1906, damaging almost every building in Hayward and doing significant damage in San Francisco, in Fremont, San Jose, San Leandro and elsewhere. Cracks appeared in walls as far away as Napa, Santa Rosa, and Hollister. At a time when the greater Bay Area had a population of only about a quarter-million and infrastructure was minimal, damage was estimated at $350,000 in 1868 dollars. Thirty people perished in the quake, including five in San Francisco. Other quakes of note Earthquakes estimated at magnitude 5.9 occurred on the Hayward Fault in 1864 and 1870, and a magnitude 5.6 event struck in 1889.

In October 2007, a magnitude 5.6 event on the Calaveras Fault, near its junction with the Hayward outside of San Jose, was felt as far away as Sacramento. It caused minimal damage. In March 2008, the 1868 Hayward Earthquake Alliance and Risk Management Solutions estimated that a repeat of the 1868 earthquake would impact more than five million people – leaving 100,000 or more homeless -- and cause $165 billion in damage to residential and commercial properties. Much of the damage would be uninsured. And that figure does not include potential post-quake fire losses, damage to infrastructure, or the disruption of business. The California Geological Survey’s 1987 Earthquake Planning Scenario for a Magnitude 7.5 Earthquake on the Hayward Fault in San Francisco Bay Area anticipated 1,500 to 4,500 deaths, depending on the time of occurrence, and three times that number of nonfatal casualties.

While a significant amount of structural retrofitting work has been done in the Bay Area, the Hayward Fault runs under numerous lifelines, such as freeways, the Hetch Hetchy Aqueduct, and Bay Area Rapid Transit tracks. A sizable earthquake could cut off water supplies – not only for drinking, but also for fighting fires -- to nearly half the Bay Area. The aforementioned CGS scenario noted that eight large (99 beds or more) acute care hospitals in Alameda and Contra Costa counties are within a mile of the fault. More than a thousand roads (including the trans-bay bridges) could be closed for significant amounts of time. Liquefaction could render local airports and port facilities useless. Secondary hazards such as landslides and wildfires are possible. How large an earthquake can the fault produce? A fault’s length is related to the maximum strength of an earthquake it can produce. The Hayward Fault is thought capable of generating a magnitude 7.5 quake. However, many scientists believe that the Hayward Fault is connected to the Calaveras Fault to the south, the Rodgers Creek Fault to the north and to the Maacama Fault still farther north. If that is the case, the longer fault system could produce larger temblors.

Fault creep Aside from size and reputation, a major difference between the Hayward and San Andreas faults is “aseismic creep”. The San Andreas Fault is locked in many places; much of its energy is released in the form of earthquakes. However, creep occurs in spots along the Hayward Fault. The ground consistently moves a few millimeters each year, pulling apart sidewalks, pipelines and other structures that sit astride the fault. UC Berkeley’s Memorial Stadium was built in 1923. Since then, creep has caused the two sides of the stadium to be offset more than a foot, requiring retrofitting with expansion joints. Creep accounts for a small part of the total motion that takes place on a fault over geologic time; earthquakes account for the rest.

Question: What major fault line runs through California?
Answer: San Andreas Fault