1906 San Francisco Earthquake centennial Field Guides:

Field trips associated with the 100th Anniversary Conference, 18–23 April 2006, San Francisco, California

Edited by Carol S. Prentice, Judith G. Scotchmoor, Eldridge M. Moores and Jon P. Kiland


The twenty field trip guides in this volume represent the work of earthquake professionals from the earth science, engineering, and emergency management communities. The guides were developed to cross the boundaries between these professions, and thus reflect this diversity: trips herein focus on the built environment, the effects of the 1906 earthquake, the San Andreas fault, and other active faults in northern California. Originally developed in conjunction with the 100th Anniversary Earthquake Conference held in San Francisco, California, in April 2006, this book is meant to stand the test of time and prove useful to a wide audience for general interest reading, group trips, or self-guided tours.

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    This volume consists of twenty field guides that were created to cover the diverse interests of the 100th Anniversary Conference held in San Francisco, California, to mark the centennial of the 1906 San Francisco earthquake. The guides presented here represent the interests of earth scientists, engineers, and emergency planners, and reflect the cooperation between the Seismological Society of America, the Earthquake Engineering Research Institute, and the California Governor's Office of Emergency Services, the three organizations that jointly organized this unique conference. The field guides are specifically intended to cross the boundaries between these organizations and to be accessible to the general public.

    The locations of most of the field trips are shown on Figure 1, which shows the San Francisco Bay area as photographed from the International Space Station. However, the area shown on this figure is not big enough to include all of the trips: Chapters 11, 16, 19, and 20 spill over to the north, south, and east of the region shown in the figure.

    The geology of California is the direct result of the action of plate tectonics. Earth's crust is composed of six major (and many smaller) plates that are in constant motion with respect to each other. There are three kinds of boundaries between these plates: (1) divergent boundaries, where plates move apart, material wells up from Earth's interior, and new crust is created; these boundaries lie mostly along Earth's major mid-oceanic ridges; (2) convergent boundaries, or subduction zones, where plates collide and

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      The field trip covers three short walks through downtown San Francisco focusing on the events that occurred in the aftermath of the 1906 earthquake. The first walk is in the South of Market area, located on artificially filled ground of the old Mission Bay marshland. The second walk follows the path of the fire as it spread out of the South of Market area on to Market Street. The third walk is along Montgomery Street, located on the old shoreline of Yerba Buena Cove, and follows the progress of the fire as it crossed Market Street northward into the Financial District. The wetlands bordering the bay were prime real estate, and by 1906 about a sixth of the city was built on artificial fill. The highest concentration of damage to buildings by ground shaking and liquefaction caused by the earthquake occurred here. Throughout this area, water, sewer, and gas lines were ruptured, and it was the location of most of the 52 fires that flared up in the city after the earthquake. The main objective of the field trip is to evaluate the lessons we have learned from building on poorly engineered ground within a major metropolitan center in a seismically active area.

      The settlement of Yerba Buena was established in the 1830s along the margin of a sheltered cove in San Francisco Bay. The port attracted settlers, and by 1847 the population had gradually increased to almost 500. Early maps drawn of the town showed the streets crisscrossing the

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      This field trip consists of two stops at locations where it is possible to see damage from the 1906 earthquake and to gauge the intensity of the ground shaking that caused the damage. The first stop is at a cemetery in Colma, where the damage to monuments and headstones was photographed and roughly quantified in the Report of the State Earthquake Investigation Commission, Lawson (1908), commonly referred to as the “Lawson Report.” The Lawson Report represents the formal study of the earthquake and consists of a compilation of the reports of many investigators who gathered information about faulting, ground failure, and damage due to the 1906 earthquake. The second stop is at a brick office building at the southern limit of San Francisco that was damaged by the earthquake but repaired in such a fashion that the damage is still clearly evident.

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      This tour includes many of San Francisco's most interesting pre- and post-1906 buildings. We will investigate these buildings in the context of their urban setting and their earthquake-resistant architecture and engineering. Many of the buildings we are going to visit were considered earthquake-resistant when they were conceived, although they might not be judged earthquake-resistant today. We will examine their histories in relation to San Francisco's struggle for safety from earthquakes and fires, and particularly the earthquake and fire of 1906. (Note: The text of this tour is excerpted from Tobriner, 2006.)

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      This trip will visit the Ferry building, a classic icon of San Francisco that has recently been retrofitted to withstand the strong shaking from an earthquake. The building suffered moderate damage in the 1906 earthquake and only minor damage in the 1989 Loma Prieta earthquake.

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      This field trip consists of one location stop and a building tour of the 1996 historic rehabilitation and seismic retrofit of the Ninth District U.S. Court of Appeals building in San Francisco. This field guide provides an overview of the building's significant historic features, a brief presentation on the history of the facility, and a summary of the historic rehabilitation and seismic retrofit.

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      San Francisco's Civic Center ((Figs. 1) and 2) is on the National Register of Historic Places because it includes a magnificent collection of nineteenth and twentieth century Revival and Beaux Arts architecture and exemplifies the finest manifestation of the “City Beautiful” movement in the United States. The Civic Center is known as one of the most important national and international historic sites, as it is the birth place of the United Nations and has witnessed the drafting and signing of post World War II peace treaties with Japan. Major government and cultural buildings surround the Civic Center Plaza, including San Francisco City Hall, the Asian Art Museum, the new Main Library, the Bill Graham Civic Auditorium, as well as the State Supreme Court building. This trip will visit both the San Francisco City Hall and the Asian Art Museum to explore their recent seismic retrofits as well as their histories.

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      This five-building walking tour (Fig. 1) provides an overview of significant tall buildings in San Francisco that were constructed in the first few years of the twenty-first century and gives insight into the modern design and seismic innovations of today's skyscrapers in high seismic zones. The St. Regis Tower (42 story), 101 Second Street (26 story), the JP Morgan Chase Building (31 story), the Paramount (39 story), and the Four Seasons Hotel (40 story) will be surveyed in this tour. These buildings showcase a variety of important structural designs and use of materials including (1) reinforced concrete framed dual system, (2) structural steel framed dual system, (3) steel frame with sloped boxed columns and offsets, (4) precast hybrid moment resistant frame, and (5) steel framed dual system with nonlinear viscous damping.

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      This field trip consists of a 30-minute presentation by the California Department of Transportation (Caltrans) about the ongoing construction of the new, seismically upgraded Bay Bridge, followed by a guided boat tour of the ongoing bridge construction.

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      This field trip consists of stops in four locations (Fig. 1) that provide insight into the seismic retrofit and strong motion instrumentation of the Golden Gate Bridge ((Figs. 2) and 3). Only one of the four stops is normally open to the public (Stop 3a). The first stop at the Golden Gate Bridge Highway and Transportation District (GGBHTD) office board room will include an introduction to the bridge history and presentation of the seismic retrofit schemes, strong motion instrumentation of the bridge, and the data products available from the California Strong Motion Instrumentation Program (CSMIP) of the California Geological Survey (CGS). At the second stop, participants will see a free-field instrument in the maintenance area.

      At the third stop, we will observe retrofit work under way (in 2006) from a public overlook area. At the fourth stop, we will see the seismic sensors and instrumentation installed on the bridge. The locations of these four stops (Stops 1, 2, 3a, and 3b) are shown in Figure 4.

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      This field trip consists of a visit to the site of one of the 1906 earthquake relief camps and the City and County of San Francisco Emergency Communications Center facility, located at 1011 Turk Street in San Francisco.

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      This two-day trip explores the northern San Andreas fault in the Gualala area between Fort Ross and Point Arena (Fig. 1). The first stop overlooks the Golden Gate Bridge and includes a discussion of its in-progress seismic retrofit. Several subsequent stops are at paleoseismic sites on the San Andreas fault. The stop at Annapolis Road includes a short hike along the fault through the redwood forest. This section of the fault is locked and has not moved since the 1906 earthquake. Additional stops visit Quaternary marine terraces and include discussion of associated tectonic deformation.

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      The main destination of this field trip is the San Andreas fault in Marin County, where the ground rupture of the 1906 earthquake is well preserved within the boundaries and easements of Point Reyes National Seashore. In addition to three stops along the fault, the field guide also describes stops to view the Golden Gate Bridge and White's Hill slide on Sir Francis Drake Boulevard near the town of Fairfax, and it discusses the geology along the way. Figure 1 shows the location of the stops for this field trip. Excellent online fieldtrip guides to the geology of Point Reyes peninsula, the Marin Headlands, and the San Andreas fault are available on the Internet (Stoffer, 2005; Elder, 2005).

      The great San Francisco earthquake of 18 April 1906 was generated by rupture of at least 435 km of the northern San Andreas fault (Lawson, 1908). The earthquake produced maximum horizontal offsets of 16–20 ft (5–6 m) along the San Andreas fault north of San Francisco and smaller offsets south of the city. In Marin County, there has been very little urbanization along the fault. Prior to the establishment of the National Seashore in 1962, most of the region was used for dairy farming and cattle ranching. Because the region remains largely as it was in the late nineteenth century, conditions are ideal for investigating how the morphology of the rupture has changed in the 100 years since the earthquake. Furthermore, this section of the San Andreas fault continues to yield important data about dates of prehistoric earthquakes and the slip rate of the fault.

      Two fundamentally different types of bedrock underlie Marin County (Fig. 2). Right-lateral shear along the San Andreas transform plate boundary during the late Cenozoic has juxtaposed Franciscan subduction zone rocks on the east against the Salinian terrane of Point Reyes peninsula to the west. The Franciscan Assemblage (Complex) is a highly deformed, lithologically heterogeneous sequence of metamorphosed volcanic and sedimentary rocks accreted to western North American during subduction of the Farallon plate in the Mesozoic. The Salinian terrane is a displaced fragment of continental crust that consists of Cretaceous plutonic and older metamorphic rock overlain by lower Eocene to Pliocene marine sedimentary rocks (Clark and Brabb, 1997). In between the Franciscan and Salinian terranes lies a valley created by the San Andreas fault zone that is characterized by Quaternary deposition and low ridges and depressions elongated parallel or subparallel to the fault.

      Along the route of this field trip on our way to the San Andreas fault, road cuts expose the world-famous, Franciscan Accretionary Complex rocks including oceanic pillow basalts (greenstone) overlain by radiolarian chert, graywacke sandstone, and “mélange” (from the French word for “mixture”), with inclusions of greenstone, chert, serpentinite, and graywacke. Isolated outcrops or knobs of erosion-resistant rocks within a surrounding matrix of highly sheared shale of the mélange typify the topography of grass-covered slopes of eastern Marin County. During the trip we will also travel through a forest of redwood trees near Samuel Taylor State Park en route to the Douglas-fir–covered Point Reyes peninsula.

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      This field trip consists of stops in four locations that provide insight into the San Andreas fault along the San Francisco peninsula. The first two stops provide an overview and close-up look at the fault where no urbanization has occurred. The last two stops are examples of areas where urbanization occurred directly over the fault prior to current regulations. The field trip also addresses the history of, and seismic hazard issues related to, an important part of the San Francisco Public Utilities Commission's (SFPUC) water-supply system, which is located along the San Andreas fault.

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      Leland Stanford (president of the Central Pacific Railroad and former governor of California) and his wife Jane established Stanford University in 1885 as a memorial to their only child, Leland Jr., who died from typhoid fever contracted while vacationing in Florence in 1884. In 1906, fifteen years after opening, the university had just completed an aggressive building program and was poised to refocus its attention on academics when, at 5:12 a.m. on 18 April, those plans were radically changed. The first shock waves of the earthquake did not cause immediate alarm, but the continued shaking intensified as the peninsula segment of the San Andreas fault, only a few miles away, ruptured. Several of the buildings, only recently completed, disintegrated. Chimneys in both the men's and women's dorms buckled and fell, carrying sections of floors down with them. Remarkably, there were only two fatalities on campus, a student and a university employee. In response to the damage, university President David Starr Jordan cancelled classes for the remainder of the year and closed the university. It was soon realized, however, that only the showier buildings built after Leland Stanford's death were badly damaged; the main buildings of the Quad were still functional. The university would reopen and resume classes on their normal schedule in the autumn.

      The 1906 earthquake prompted awareness at Stanford that its location so close to an active fault is no place for seismically unsafe monumental architecture. Over subsequent years, the university would not only build safer buildings, but would research earthquakes and engineering methods for withstanding earthquakes. In contrast to 1906, no Stanford buildings were destroyed in the 1989 earthquake (much smaller than that of 1906, but nonetheless a significant earthquake), and campus was closed for only one day.

      This field guide describes a walking tour (about one hour) of the Stanford campus showing selected effects of both the 1906 and 1989 earthquakes and describing how the Stanford community responded to the subsequent challenges. The tour is on paved paths and is accessible to pedestrians, bicyclists, and wheelchairs (Fig. 1).

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      On the southern part of the San Francisco Peninsula, the San Andreas fault traverses the actively uplifting Santa Cruz Mountains. The field guide is comprised of a hiking tour along the fault in Sanborn County fault, a visit to a winery and vineyards traversed by the fault, and visits to two wineries that provide vistas of the San Andreas Rift Valley and surroundings.

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      This field trip is along the central section of the San Andreas fault and consists of eight stops that illustrate surface evidence of faulting, in general, and features associated with active fault creep, in particular. Fault creep is slippage along a fault that occurs either in association with small-magnitude earthquakes or without any associated large-magnitude earthquakes. Another aspect of the trip is to highlight where there are multiple fault traces along this section of the San Andreas fault zone in order to gain a better understanding of plate-boundary processes.

      The first stop is along the Calaveras fault, part of the San Andreas fault system, at a location where evidence of active fault creep is abundant and readily accessible. The stops that follow are along the San Andreas fault and at convenient locations to present and discuss rock types juxtaposed across the fault that have been transported tens to hundreds of kilometers by right-lateral motion along the San Andreas fault. Stops 6 and 7 are examples of recent studies of different aspects of the fault: drilling into the fault at the depth of repeating magnitude (M) 2 earthquakes with the San Andreas Fault Observatory at Depth (SAFOD) and the geological, geophysical, and seismological study of M 6 earthquakes near the town of Parkfield.

      Along with the eight official stops on this field trip are 12 “rolling stops”—sites of geologic interest that add to the understanding of features and processes in the creeping section of the fault. Many of the rolling stops are located where stopping is difficult to dangerous; some of these sites are not appropriate for large vehicles (buses) or groups; some sites are not appropriate for people at all. We include photographs of or from many of these sites to add to the reader's experience without adding too many stops or hazards to the trip.

      An extensive set of literature is available for those interested in the San Andreas fault or in the creeping section, in particular. For more scientifically oriented overviews of the fault, see Wallace (1990) and Irwin (1990); for a more generalized overview with abundant, colorful illustrations, see Collier (1999). Although the presence of small sections of the San Andreas fault was known before the great 1906 San Francisco earthquake, it was only after that event and subsequent geologic investigations reported in Lawson (1908) that showed the fault as a long structure, extending all the way from east of Los Angeles into northern California. Prentice (1999) described the importance of the 1908 “Lawson report” and how it pivotally influenced the understanding of the San Andreas. Hill (1981) presented a wonderful introduction to the evolution of thought on the San Andreas. Geologic maps and maps of the most recently active fault trace in the creeping section, or large parts of it, include those by Brown (1970), Dibblee (1971, 1980), and Wagner et al. (2002); detailed geologic maps are discussed at various stops in this guide. Various aspects of the creeping section of the San Andreas fault have been the focus of many geologic field trips in the past few decades. Guidebooks for some of those trips include those by Gribi (1963a, 1963b), Brabb et al. (1966), Rogers (1969), Bucknam and Haller (1989), Harden et al. (2001), and Stoffer (2005).

      The creeping section of the San Andreas fault zone lies between areas that experienced large-displacement surface breakage during great earthquakes in 1857 and 1906 (Fig. 1 inset). Burford and Harsh (1980) divided the creeping section into three segments: (1) a northwest section where the creep rate increases to the southeast in step-like increments, (2) a central section where the creep rate is relatively constant at a maximum value of ∼30 mm/yr (∼1.2 in/yr), and (3) a southeast section where the creep rate decreases to the southeast (Fig. 2). The rate of slip along the creeping section of the fault zone has been measured using creepmeters, alignment arrays, and laser distance-measuring devices. The aperture of measurements over which these measurements are made ranges from 10 m (∼33 ft) (creepmeters) to 100 m (∼330 ft) (alignment arrays) to kilometers and tens of kilometers (laser measuring devices). Creepmeter and alignment-array measurements are here termed “near-fault” measurements; laser measurements over distances of 1–2 km (∼0.6–1.2 mi) are termed “intermediate-scale” measurements; laser measurements over tens of kilometers (miles) are termed “broadscale” measurements. Comparisons among near-fault, intermediate-scale, and broadscale measurements and geologic maps show that the northwest part of the creeping section of the fault is composed of two narrow zones of active deformation, one along the San Andreas fault and one along the Calaveras-Paicines fault, whereas the central and southeast sections are both composed of a single relatively narrow zone of deformation. The southeast section is transitional to a locked zone southeast of Cholame; a locked fault is one that slips only in association with a moderate to large earthquake. Throughout the creeping section of the San Andreas fault zone, broadscale measurements generally indicate more deformation than near-fault and intermediate-scale measurements, which are in reasonably close agreement except at Monarch Peak (Mustang Ridge), near the center of the creeping section and our Stop 5 ((Figs. 1) and 2).

      Features that we see on this trip include offset street curbs, closed depressions (sag ponds), fault scarps (steep slopes formed by movement along a fault), a split and displaced tree, offset fence lines, fresh fractures, and offset road lines (Fig. 3 is a sketch showing some of the landforms that represent deformation by an active fault). We also see evidence of long-term maturity of the San Andreas fault, as indicated by fault features and displaced rock types (Fig. 4). Finally, we will visit sites of ongoing research into the processes associated with earthquakes and their effects. Discussions include drilling into the San Andreas fault at the SAFOD drill site and the 2004 Parkfield earthquake and its effects and implications.

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      This field guide consists of eleven stops at sites that illustrate the geological, geophysical, geographic, and engineering aspects of the Hayward fault in the East Bay. Section I (Stops 1–4) consists of stops that are part of the University of California at Berkeley (UC-Berkeley), including research facilities, retrofit of campus buildings, and geomorphic features along the fault. Section II (Stops 5 and 6) consists of stops along the Hayward fault north of the UC-Berkeley main campus, and Section III (stops 7–11) consists of stops related to the Hayward fault south of the UC-Berkeley main campus (Fig. 1). Stops are designed to illustrate geomorphic features of the fault, the effects of fault creep on structures sited on the fault, and retrofit design of structures to mitigate potential future deformation due to fault rupture.

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      The Carquinez Strait (Alfred Zampa Memorial) Bridge ((Figs. 1) and 2) is the first new toll bridge in California built to the stringent post-1989 performance-based design standards. This field trip consists of two stops that provide opportunities to observe the bridge site and discuss details of the design of the structure. The first stop on the south overlook of the strait provides an overview of the site. The second stop on the north side of the strait will start on the bicycle and pedestrian path plaza and will continue across the bridge.

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      Contributors: Hans AbramsonWard, Geomatrix Consultants, Inc., 2101 Webster St., Suite 1200, Oakland, California 94612, USA; Julie Bawcom, California Geological Survey, 17501 North Highway 101, Willits, California 95490, USA; John Boatwright, U.S. Geological Survey, 345 Middlefield Rd., M.S. 977, Menlo Park, California 94025, USA; Todd Crampton, Geomatrix Consultants, Inc., 2101 Webster St., Suite 1200, Oakland, California 94612, USA; Wayne Goldberg, City Manager's Office, 100 Santa Rosa Ave., Rm. 10, Santa Rosa, California 95404, USA; Kathryn L.Hanson, Geomatrix Consultants, Inc., 2101 Webster St., Suite 1200, Oakland, California 94612, USA; Victoria E.Langenheim, U.S. Geological Survey, 345 Middlefield Rd., M.S. 989, Menlo Park, California 94025, USA; MortLarsen, Department of Geology, Humboldt State University, 1 Harpst St., Arcata, Cali-fornia 95521, USA; Gaye LeBaron, Press Democrat, P.O. Box 569, Santa Rosa, California 95402, USA; Darcy K.McPhee, U.S. Geological Survey, 345 Middlefield Rd., M.S. 989, Menlo Park, California 94025, USA; William V. McCormick, Kleinfelder, 2240 Northpoint Parkway, Santa Rosa, California 95407, USA; Robert J. McLaughlin, U.S. Geological Survey, 345 Middlefield Rd., M.S. 973, Menlo Park, California 94025, USA; Craig A.McCabe, U.S. Geological Survey, 345 Middlefield Rd., M.S. 973, Menlo Park, California 94025, USA; David P.Schwartz, U.S. Geological Survey, 345 Middlefield Rd., M.S. 977, Menlo Park, California 94025, USA; GarySimpson, SHN Consulting Engineers and Geologists, 812 W. Wabash Ave., Eureka, California 95501, USA; Frank H. (Bert)Swan, Consulting Geologist, 240 Laidley Street, San Francisco, California 94131, USA

      This guidebook is for a two-day trip: the first part (Day 1) takes place in and near the city of Santa Rosa and on the Rodgers Creek fault in Sonoma County; the second part (Day 2) will go to stops in the town of Willits, on the northern Maacama fault, in Mendocino County.

      The Rodgers Creek and Maacama faults are major strands of the San Andreas fault system in northern California. The two faults are separated by a right step and may be considered the northern extension of the Hayward and Calaveras faults, which branch from the San Andreas fault south of the San Francisco Bay area (Fig. 1A). This system of faults accommodates almost a quarter of the total right-slip motion between the Pacific and North American tectonic plates. Slip is released in large, episodic earthquakes and, on some faults, such as the northern Maacama fault, by slow, steady creep.

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      A vital lesson of plate tectonics is that there is no validity to any assumption that the simplest and therefore most acceptable interpretation demands a proximal rather than a distant origin. (Coombs, 1997, p. 763).

      This field trip steps back to provide the very long term and large-scale tectonic history that one might call the broader tectonic context of the 1906 San Francisco earthquake. In effect, the field trip follows a cross section of northern California, with stops that illustrate the geologic history of the region. The field guide also discusses several archaeological stops of significance to California's prehistory. The entire field trip is meant to be taken over a period of four days, with overnight stops in Davis and in Quincy. Day one comprises Stops 1–9; Day 2, Stops 10–18; Day 3, Stops 19–28; and Day 4, Stops 29–33.

      Northern California geology is the result of an extended history of active plate margin interactions spanning some 500 million years (m.y.). Over the course of this period, countless numbers of large earthquakes of different types no doubt accompanied tens of thousands of kilometers of movement between tectonic plates and microplates that eventually came together to form the rocks of northern California as we see them today.

      From ca. 500–18 million years ago (Ma), subduction, a process still active north of Cape Mendocino, dominated the geologic history of northern California. During this period, several subduction zones and volcanic arcs were active, and subduction zones, whose former positions we will see on our trip, consumed ocean basins thousands of kilometers wide, sweeping together a vast collage of rocks from far-flung locations in the process.

      Remnants of the most ancient of these subduction zones and collided blocks (typically called terranes) are preserved in the Sierra Nevada. The most recent subduction history along the North American margin involved the Farallon plate, a plate that lay east of the Pacific plate and was separated from it by a spreading mid-oceanic ridge. The products of this subduction episode are preserved in the rocks of the Coast Ranges, and in the granitic and younger volcanic rocks of the Sierra Nevada ((Figs. 1) and 2).

      For the past 18 m.y., the plate margin has been dominated by right-lateral faults of a transform plate margin, the most famous of which, the San Andreas fault, produced the 1906 earthquake. The present plate boundary between the Pacific plate on the west and stable North America on the east is a broad one consisting of two active zones: (1) the San Andreas fault system, whose right-lateral faults occupy the California Coast Ranges; and (2) a zone east of the Sierra Nevada including the right-lateral faulting associated with the Walker Lane and the Eastern California Shear Zone, and, east of the Walker Lane, the extensional faults of the Basin and Range province (Fig. 3).

      These zones of active faults, shown in Figure 3, are responsible for generating most of the earthquakes in the area traversed by our field trip. The preexisting complexity of the crust resulting from the earlier tectonic history probably influenced the development and location of these more recent fault zones, in addition to giving us some exceedingly interesting and complex geology to examine on our trip.

      This long and complex history requires a great deal of discussion for complete understanding. In this guide, we present first the stops in sequence, including brief descriptions for each site. To augment these brief discussions, we follow the field trip guide with an Appendix in which we discuss the tectonic development of northern California more fully.

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