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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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San Andreas Fault Exploration Using Refraction Tomography and S ‐Wave‐Type and F ϕ ‐Mode Guided Waves
Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations
Late Holocene slip rate of the San Andreas fault and its accommodation by creep and moderate-magnitude earthquakes at Parkfield, California
The U.S. Geological Survey (USGS) acquired two 1.4-km-long, high-resolution (~5 m vertical resolution) seismic-reflection lines in 2006 that cross near the International Continental Scientific Drilling Program (ICDP)–USGS Eyreville deep drilling site located above the late Eocene Chesapeake Bay impact structure in Virginia, USA. Five-meter spacing of seismic sources and geophones produced high-resolution images of the subsurface adjacent to the 1766-m-depth Eyreville core holes. Analysis of these lines, in the context of the core hole stratigraphy, shows that moderate-amplitude, discontinuous, dipping reflections below ~527 m correlate with a variety of Chesapeake Bay impact structure sediment and rock breccias recovered in the cores. High-amplitude, continuous, subhorizontal reflections above ~527 m depth correlate with the uppermost part of the Chesapeake Bay impact structure crater-fill sediments and postimpact Eocene to Pleistocene sediments. Reflections with ~20–30 m of relief in the uppermost part of the crater-fill and lowermost part of the postimpact section suggest differential compaction of the crater-fill materials during early postimpact time. The top of the crater-fill section also shows ~20 m of relief that appears to represent an original synimpact surface. Truncation surfaces, locally dipping reflections, and depth variations in reflection amplitudes generally correlate with the lithostrati-graphic and sequence-stratigraphic units and contacts in the core. Seismic images show apparent postimpact paleochannels that include the first possible Miocene paleochannels in the Mid-Atlantic Coastal Plain. Broad downwarping in the postim-pact section unrelated to structures in the crater fill indicates postimpact sediment compaction.
Surface Fault Slip Associated with the 2004 Parkfield, California, Earthquake
Abstract 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 ). Figure 1. Index map showing creeping section of the San Andreas fault (from Cholame northwestward to San Juan Bautista), southern section of the Calaveras fault, and location of field trip stops (red dots) and rolling stops (yellow dots; labeled ‘RS’ in this figure and in Figs. 21 , 27 , and 38 ). Yellow—alluvium in valleys. Only selected faults, roads, and towns included for reference. Location of creeping section and surface rupture associated with great earthquakes, with dates, along San Andreas fault shown in inset. Figure 2. Comparison of slip rates along creeping section of San Andreas fault zone as determined by various distance-measuring techniques (modified from Lisowski and Prescott, 1981 ). Geodetic measures at northern end of creeping section of San Andreas fault are significantly greater than creepmeter and alignment array measures because the longer line lengths include slip on the Calaveras fault. Location of stops in this field guide marked with arrows at top. 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.
Preliminary Report on the 28 September 2004, M 6.0 Parkfield, California Earthquake
The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue
Triggered Surface Slips in the Salton Trough Associated with the 1999 Hector Mine, California, Earthquake
Geologic and Paleoseismic Study of the Lavic Lake Fault at Lavic Lake Playa, Mojave Desert, Southern California
Triggered Surface Slips in the Coachella Valley Area Associated with the 1992 Joshua Tree and Landers, California, Earthquakes
The San Salvador earthquake of 10 October 1986 and its historical context
Late Quaternary deposits beneath Clear Lake, California; Physical stratigraphy, age, and paleogeographic implications
Clear Lake, California, lies in a volcano-tectonic depression that received nearly continuous lacustrine deposition for the past 500,000 yr and probably longer. The lake has been shallow (<30 m) and eutrophic throughout its history. Sediments beneath the floor of the lake are fine grained (chiefly >7.0φ) and contain fossils of a large lacustrine biota, as well as a pollen record of land plants that lived in the basin. The sediments also contain tephra units of local and regional extent. The ages of the sediments in Clear Lake are determined from radiocarbon dates on the sediments, from correlation of regionally distrbuted tephra units, and from inferred correlation of oak-pollen spectra with the marine oxygen-isotope record. From the chronology of events recorded in the cores from Clear Lake, the late Quaternary history of the lake can be deciphered and the sediments correlated with other basins in northern California. Comparison of cores from Clear Lake with strata of the Kelseyville Formation, exposed south of the main basin, suggests a general northward migration of lacustrine sedimentation, which in turn suggests a northward tilt of the basin. Migration of the lake was a response to volcanism and tectonism. Volcanic rocks erupted from Mt. Konocti, on the southern margin of the lake, and displaced the shoreline to the west and north. Clear Lake is bounded by faults that are part of the San Andreas fault system. These faults strongly influenced the position, depth, and longevity of Clear Lake. Movement on these boundary faults deepened the Highlands and Oaks arms of the lake about 10 ka. Tectonic movement accompanying faulting was probably also largely responsible for hiatuses in the deposits beneath Clear Lake about 17 and 350 ka that have been inferred from the subbottom stratigraphy of the lake. Climate change, although responsible for large variation in the composition of the terrestrial flora of the Clear Lake drainage basin, has not influenced the areal extent, depth, or position of the lake.
Depositional environments of the Cache, Lower Lake, and Kelseyville Formations, Lake County, California
We describe the depositional environments of the Cache, Lower Lake, and Kelseyville Formations in light of habitat preferences of recovered mollusks, ostracodes, and diatoms. Our reconstruction of paleoenvironments for these late Cenozoic deposits provides a framework for an understanding of basin evolution and deposition in the Clear Lake region. The Pliocene and Pleistocene Cache Formation was deposited primarily in stream and debris flow environments; fossils from fine-grained deposits indicate shallow, fresh-water environments with locally abundant aquatic vegetation. The fine-grained sediments (mudstone and siltstone) were probably deposited in ponds in abandoned channels or shallow basins behind natural levees. The abandoned channels and shallow basins were associated with the fluvial systems responsible for deposition of the bulk of the technically controlled Cache Formation. The Pleistocene Lower Lake Formation was deposited in a water mass large enough to contain a variety of local environments and current regimes. The recovered fossils imply a lake with water depths of 1 to 5 m. However, there is strong support from habitat preferences of the recovered fossils for inferring a wide range of water depths during deposition of the Lower Lake Formation; they indicate a progressively shallowing system and the culmination of a desiccating lacustrine system. The Pleistocene Kelseyville Formation represents primarily lacustrine deposition with only minor fluvial deposits around the margins of the basin. Local conglomerate beds and fossil tree stumps in growth position within the basin indicate occasional widespread fluvial incursions and depositional hiatuses. The Kelseyville strata represent a large water mass with a muddy and especially fluid substrate having permanent or sporadic periods of anoxia. Central-lake anoxia, whether permanent or at irregular intervals, is the simplest way to account for the low numbers of benthic organisms recovered from the Kelseyville Formation. Similar low-oxygen conditions for benthic life are represented throughout the sedimentary history of Clear Lake. Water depths for the Kelseyville Formation of 10 to 30 m and 12 m near the margins of the basin are inferred both before and after fluvial incursions. These water-depth fluctuations cannot be correlated with major climatic changes as indicated by pollen and fossil leaves and cones; they may be due to faulting in this technically active region.