ABSTRACT Ophiolite complexes represent fragments of ocean crust and mantle formed at spreading centers and emplaced on land. The setting of their origin, whether at mid-ocean ridges, back-arc basins, or forearc basins has been debated. Geochemical classification of many ophiolite extrusive rocks reflect an approach interpreting their tectonic environment as the same as rocks with similar compositions formed in various modern oceanic settings. This approach has pointed to the formation of many ophiolitic extrusive rocks in a supra-subduction zone (SSZ) environment. Paradoxically, structural and stratigraphic evidence suggests that many apparent SSZ-produced ophiolite complexes are more consistent with mid-ocean ridge settings. Compositions of lavas in the southeastern Indian Ocean resemble those of modern SSZ environments and SSZ ophiolites, although Indian Ocean lavas clearly formed in a mid-ocean ridge setting. These facts suggest that an interpretation of the tectonic environment of ophiolite formation based solely on their geochemistry may be unwarranted. New seismic images revealing extensive Mesozoic subduction zones beneath the southern Indian Ocean provide one mechanism to explain this apparent paradox. Cenozoic mid-ocean-ridge–derived ocean floor throughout the southern Indian Ocean apparently formed above former sites of subduction. Compositional remnants of previously subducted mantle in the upper mantle were involved in generation of mid-ocean ridge lavas. The concept of historical contingency may help resolve the ambiguity on understanding the environment of origin of ophiolites. Many ophiolites with “SSZ” compositions may have formed in a mid-ocean ridge setting such as the southeastern Indian Ocean.

The Plate Tectonic Revolution that transformed Earth science has occurred together with revolutions in imagery and planetary studies. Earth's outer layer (lithosphere: upper mantle and crust) comprises relatively rigid plates ranging in size from near-global to kilometer scale; boundaries can be sharp (a few kilometers wide to diffuse, hundreds of kilometers) and are reflected in earthquake distribution. Divergent, transform fault, and convergent (subduction) margins are present at all scales. Collisions can occur between several crustal types and at subduction zones of varying polarity. Modern plate processes and their geologic products permit inference of Earth's plate tectonic history in times before extant oceanic crust. Ophiolites provide an insight into the products and processes of oceanic crust formation. Ophiolite emplacement involves a tectonic process related to collision of crustal margins with subduction zones. The Earth's mantle comprises, from top to bottom, the lithosphere, asthenosphere, mesosphere, and a hot boundary layer . Plume-related magmatism may arise from bulges in the latter, which in turn may alternate with depressions caused by pronounced subduction, leading to assembly of supercontinents. Plate tectonic activity probably occurred on an early Archean, or even Hadean, Earth. Earth-like plate tectonic activity seems not to be present on other terrestrial planets, although strike-slip faulting is present in Mars's Valles Marineris. Possible extensional and compressional tectonics on Venus and an inferred unimodal hypsographic curve for early Earth suggest that Venus may be a modern analogue for a young Earth.

Abstract 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

Abstract 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

Abstract 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.

Abstract 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.)

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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 .

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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 ).

Abstract 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.