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In situ measurement of seismic shear-wave absorption in the San Francisco Holocene Bay Mud by the pulse-broadening method

By the pulse-broadening method, seismic absorption of San Francisco Holocene Bay Mud has been determined with the use of an impulsive shear-wave source at the surface, two receivers embedded in clay (Holocene Bay Mud), and digital seismic recording. The seismic anelastic quality factor is determined from 20 measurements to be 16.0 with a random error of ±0.9. Three sources of systematic error are examined. The fractional systematic error on the anelastic quality factor is estimated to be ∼ 11%. Details of experimental arrangement and error analysis are given.

Bulletin of the Seismological Society of America↗

What can seismology say about hotspots?

Seismological methods offer the highest-resolution views of the structure of the mantle. Since deep mantle plumes were proposed to explain melting anomalies (“hotspots”), increasingly powerful seismologic studies have sought to detect them, but so far without definitive success. This paper summarizes the relevant seismological methods and results for Earth scientists who are not seismologists.

Special Papers of the Geological Society of Americ↗

The active southwest margin of the Colorado Plateau: Uplift of mantle origin

During Cenozoic time, the Colorado Plateau was raised about 2 km above sea level. The most-recent and best-documented uplift of the plateau (∼1 km) has been concentrated at its southwest margin between 6 and 1 Ma, whereas the eastern Colorado Plateau may have been at high elevations since Eocene time. To better understand the recent tectonic activity at the southwest margin of the Colorado Plateau, we compile detailed crustal thickness and density information from seismic and gravity data for a region that includes northwest Arizona and the southern tip of Nevada. This information is used to isolate the mantle contribution to uplift. We find that there is relatively low density mantle underlying the southern margin of the plateau in northwest Arizona, which could result from about 60–80 km of thinning of the dense mantle lithosphere combined with about 100 °C of heating through a 100-km-thick mantle layer. The available estimates from earthquake-source seismology in or near the study area are compatible with this estimate of lithospheric thinning. We speculate that uplift may result from subduction-related thinning of the continental lithosphere.

Geological Society of America Bulletin↗

Preface

This book grew out of a topical session on “Central Virginia Earthquakes of 2011: Geology, Geophysics, and Significance for Seismic Hazards in Eastern North America” at the 2012 The Geological Society of America (GSA) Annual Meeting in Charlotte, North Carolina (USA). It also benefitted from related sessions at other meetings. The goal of this volume, The 2011 Mineral, Virginia, Earthquake, and Its Significance for Seismic Hazards in Eastern North America , is to bring together as much information as possible on lessons learned from this rare event. Chapters encompass a wide range of geoscience, engineering, and related studies of this earthquake and its effects from the epicentral area in central Virginia to Washington, D.C., and beyond. The intended audience is a broad spectrum of geoscientists, engineers, and decision makers interested in understanding earthquakes and seismic hazards in eastern North America and other intraplate settings. Chapters by Berti et al. (21), Chapman (2), Costain (8), Davenport et al. (15), Green et al. (9), Heller and Carter (10), Horton et al. (14), Hughes et al. (19), Powars et al. (23), Pratt et al. (16), Roeloffs et al. (7), Shah et al. (17), Stephenson et al. (3), Walsh et al. (18), and Wells et al. (12) are expansions of presentations at the 2012 GSA meeting. The volume also contains chapters from recent studies that were not presented at the GSA meeting, including those by Bobyarchick (22), Burton et al. (20), Dreiling and Mooney (5), Li et al. (11), McNamara et al. (4), Pollitz and Mooney (6), and Shahidi et al. (13). Following an overview and synthesis by the volume editors (1), chapters are arranged under the topical headings “Seismology and Regional Effects,” “Earthquake Damage, Geotechnical, and Engineering Investigations,” “Aftershocks, Geophysical Imaging, and Modeling,” “Geologic Investigations—Epicentral Area,” and “Geologic Investigations— Central Virginia Seismic Zone and Nearby Faults.” We thank the authors for their contributions and the many scientists and engineers who contributed time and expertise in reviewing manuscripts to substantially improve the quality of the volume. These reviewers include Gail Atkinson, Christopher Bailey, Richard Berquist, Kimberly Blisniuk, Paul Bodin, Aaron Bradshaw, Clive Collins, Ariel Conn, Randy Cox, Haitham Dawood, James Dewey, John Ebel, David Fenster, Alexander Gates, Kathleen Haller, Gregory Hancock, Robert Hatcher, William Henika, Paul Hsieh, Steven Jaumé, Jeffrey Kimball, Charles Langston, Jongwon Lee, Andrea Llenos, John McBride, Scott Olson, Michael Oskin, Brent Owens, Gilles Peltzer, Mark Quigley, Dhananjay Ravat, David Saftner, Arthur Snoke, Jamison Steidl, Kevin Stewart, Alice Stieve, Danielle Sumy, Ertugrul Taciroglu, Roy Van Arsdale, Mason Walters, Chiyuen Wang, Yang Wang, Richard Whittecar, Lorraine Wolf, Clint Wood, Liam Wotherspoon, and some anonymous reviewers.

Virginia↗

Table-top earthquakes; a demonstration of seismology for teachers and students that can be used to augment lessons in earth science, physics, math, social studies, geography

The apparatus consists of a heavy object that is dragged steadily with an elastic cord. Although pulled with a constant velocity, the heavy object repeatedly slides and then stops. A small vibration sensor, attached to a computer display, graphically monitors this intermittent motion. 2 This intermittent sliding motion mimics the intermittent fault slippage that characterizes the earthquake fault zones. In tectonically active regions, the Earth's outer brittle shell, which is about 50 km thick, is slowly deformed elastically along active faults. As the deformation increases, stress also increases, until fault slippage releases the stored elastic energy. This process is called elastic rebound. Detailed instructions are given for assembly and construction of this demonstration. Included are suggested sources for the vibration sensor (geophone) and the computer interface. Exclusive of the personal computer, the total cost is between $125 and $150. I gave a talk at the Geological Society of America's Cordilleran Section Centennial meeting on June 2, 1999. The slides show how this table-top demonstration can be used to help meet many of the K-12 teaching goals described in Benchmarks for Science Literacy (American Association for the Advancement of Science, 1993).

Open-File Report↗

A seismic-refraction survey of crustal structure in central Arizona

The U.S. Geological Survey conducted a seismic-refraction study of the earth's crust and upper mantle near the Tonto Forest Seismological Observatory (TFO), located 10miles south of the Mogollon Rim near Payson in central Arizona. Two recording lines 400 km long intersect in the approximate form of a cross at TFO; one line trends southeast and the other northeast. The sedimentary layer at most places southwest of the rim is less than 1 km thick, but north of the rim it is 2 to 3 km thick. The velocity in this uppermost layer ranges from 2.6 to 4.7 km/sec, with the higher limit measured near or north of the rim. Arrivals refracted in the upper crust (P g ) can be attributed to two layers for all the shot points south of the rim. The velocity in the upper layer is about 5.9 km/sec with thickness ranging from 2 to 8 km; beneath the upper layer the velocity is about 6.1 km/sec. The upper layer seems to be absent northeast of the rim, where two shot points generated P g arrivals that show only a velocity of 6.2 km/sec. A Poisson ratio of 0.22 for the upper crustal layers was measured from shear and compressional arrivals. The lower crust could not be identified from the first and later refraction arrivals; however, minimum depths to the intermediate layer were determined. An average crustal velocity of 6.2 km/sec was measured from wide-angle reflection alignments. A thin intermediate layer would explain the seismic measurements. A delay-time method was used to map the configuration of the M-discontinuity. The depth below sea level is about 36 km along the northwest-trending line. The northeast-trending line shows a shallow depth of 21 km near Gila Bend, increasing depth to about 34 km under TFO, and a flat M-discontinuity at 40 km depth under the Mogollon Mesa northeast to Sunrise Springs. There is evidence of an abrupt depth change of about 4 km on the M-discontinuity in the vicinity of TFO. The velocity in the upper mantle is 7.85 km/sec. The relation of topographic elevation to crustal thickness suggests an approach to isostatic equilibrium, which is deduced from a near-zero regional free-air gravity anomaly. However, lateral density change in the upper mantle is required to make the crustal-refraction model fit the observed gravity-anomaly values, provided that velocity and density are linearly related.

Arizona↗