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Understanding the 1989 Loma Prieta earthquake in an urban context

Geosphere articles posted online for September-October 2014

Geological Society of America

Boulder, Colo., USA - In an urban environment, the effect of a major earthquake such as the 17 Oct. 1989 Loma Prieta event can be pieced together by the infrastructure damaged or destroyed. This study by Kevin M. Schmidt and colleagues details the effects of the Loma Prieta earthquake still detectable 25 years on and sheds light on the potential damage to infrastructure from future earthquakes along the San Andreas fault or the neighboring Foothills thrust belt.

Despite the absence of primary surface rupture from the 1989 Loma Prieta earthquake, patterns of damage to pavement and utility pipes can be used to assess ground deformation near the southwest margin of the densely populated Santa Clara or "Silicon" Valley, California, USA. Schmidt and colleagues utilized more than 1,400 damage sites as an urban strain gage to determine relationships between ground deformation and previously mapped faults.

Post-earthquake surveys of established monuments and the concrete channel lining of Los Gatos Creek reveal belts of deformation consistent with regional geologic structure. The authors conclude that reverse movement largely along preexisting faults, probably enhanced significantly by warping combined with enhanced ground shaking, produced the widespread ground deformation.

Such damage, with a preferential NE-SW sense of shortening, occurred in response to the 1906 and 1989 earthquakes and will likely repeat itself in future earthquakes in the region.

FEATURED ARTICLE

Deformation from the 1989 Loma Prieta earthquake near the southwest margin of the Santa Clara Valley, California

Kevin M. Schmidt et al., U.S. Geological Survey, 345 Middlefield Road, MS-973, Menlo Park, California 94025, USA. Published online 7 Oct. 2014; http://dx.doi.org/10.1130/GES01095.1. Themed issue: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara ("Silicon") Valley.

Other GEOSPHERE articles published online between 5 September and 30 October 2014 are highlighted below.

All GEOSPHERE articles available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Active surface salt structures of the western Kuqa fold-thrust belt, northwestern China

Jianghai Li et al. (A. Alexander G. Webb, corresponding author), The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China; Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Published online 15 Oct. 2014; http://dx.doi.org/10.1130/GES01021.1.

Amongst solid Earth materials, salt stands apart, such that the contrast in dynamics between salt bodies and other rocks is comparable to distinctions between rock and ice. One consequence is that surface salt bodies are geologically ephemeral features. This study by Jianghai Li and colleagues offers a new prospect for the sub-aerial investigation of a range of salt structures. Specifically, Li and colleagues provide first-order analysis of the geometry, kinematics, and surface processes associated with the development of surface salt structures across the Kuqa fold-thrust belt of northwestern China. Previous analysis of Kuqa salt tectonics has been almost entirely restricted to seismic reflection work, but many of the surface structures are spectacular and merit field-based investigation. This new work by Li and colleagues confirms that active deformation spans the Kuqa fold-thrust belt from foreland to hinterland; provides observations of variations in salt and related strata that impact interpretations of regional structural geometry; and documents the surface geology of active sub-aerial salt walls, including an active strike-slip fault lubricated by salt extrusion. Additional highlights include various erosion-tectonics interactions, including stream deflection morphologies and the potential development of a salt "tectonic aneurysm."

Closing the Canada Basin: Detrital zircon geochronology relationships between the North Slope of Arctic Alaska and the Franklinian mobile belt of Arctic Canada

Eric S. Gottlieb et al., Geological and Environmental Sciences, Stanford University, 450 Sierra Mall, Building 320, Stanford, California 94305, USA. Published online 15 Oct. 2014; http://dx.doi.org/10.1130/GES01027.1

The modern theory of plate tectonics relies heavily upon robust observations of bathymetric variations, magnetic anomalies and other geophysical properties within ocean basins to reconstruct both oceanic and continental plate histories. In places where such data are ambiguous or non-existent, such as the Canada Basin of the Arctic Ocean, constraining the timing and kinematics of plate motions is significantly more challenging. In the case of the Canada Basin, debate over the timing and mode(s) of its formation has been ongoing since the advent of plate tectonics concepts in the 1960s. We have tested the various contemporary models for the opening of the Canada Basin by characterizing age populations of zircon grains in sedimentary rocks in northern Alaska that were deposited before the Canada Basin formed and matching these age population "fingerprints" to sedimentary rocks from Arctic Canada. Our results demonstrate a consistently similar "fingerprint" is seen in sedimentary rocks of northern Alaska for a period of ~150 million years, and is remarkably similar to the age populations documented in slightly older rocks of Arctic Canada, implying that the Arctic Canada strata were eroded and re-deposited in northern Alaska over this 150 million year period. This correlation, coupled with other known geologic relationships, provides robust evidence that the Canada Basin formed as a result of counterclockwise-sense rotational motion of northern Alaska away from the conjugate Arctic margin of Canada.

Bedrock erosion by sedimentary flows in submarine canyons

Neil C. Mitchell, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK. Published online on 5 Sept. 2014; http://dx.doi.org/10.1130/GES01008.1. Themed issue: Exploring the Deep Sea and Beyond, volume 2.

Just beyond the continental shelves of the continents lie deep canyons that, in places, are deeper even than the U.S. Grand Canyon. It has been unclear how they were formed; we are unable to monitor them in the same ways we can monitor rivers such as the Colorado to work out how the submarine canyon floors are eroded. Neil Mitchel of the University of Manchester, UK, presents a new study of the canyons where rock strata have been observed outcropping in their walls at one site in the Pacific and one site in the U.S. Atlantic coast. Flows in the form of dense clouds of suspended sediment or slurries of sediment are thought to be responsible for some of the erosion. Based on an assumption that the stresses produced by sedimentary flows passing down the canyons were similar to those in rivers where rocks are being eroded (i.e., the same forces are involved in moving slabs of rock), the weight of the sediment flows is worked out. From these computed weights, we can say that the flows would have had 'normal' densities or thicknesses (their values are within ranges that have been inferred by other methods). It seems possible therefore that some bedrock in submarine canyons is exposed by similar erosional processes to those occurring in rivers where rock is presently being exhumed.

Paleogene rim gravel of Arizona: Age and significance of the Music Mountain Formation

Richard A. Young, Dept. of Geological Sciences, 1 College Circle, State University of New York at Geneseo, Geneseo, New York 14454, USA; and Joseph H. Hartman, Harold Hamm School of Geology and Geological Engineering, University of North Dakota, 204 Leonard Hall, 81 Cornell Street, Grand Forks, North Dakota 58202, USA. Published online 15 Sept. 2014; http://dx.doi.org/10.1130/GES00971.1. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II.

River gravels from the Early Tertiary, scattered across the Colorado Plateau in northern Arizona, are artifacts of the northeast-flowing rivers associated with the marginal uplifts of central and western Arizona created during the time of the Laramide orogeny. These Laramide gravels and their subsequently reworked counterparts have been studied by many geologists for their potential relevance to the gradual evolution of the modern Colorado River and the Grand Canyon. Studies of freshwater Paleogene gastropod fossils now constrain the age of portions of the parent gravels to Early Eocene or Late Paleocene time. Additional data from paleomagnetic measurements, dated volcanic rocks, stratigraphic distribution, and structural relationships all support this early Tertiary age assignment for the oldest suite of arkosic sediments widely preserved south of the Colorado River, now designated as the Music Mountain Formation in northwestern Arizona. Similar reworked gravels that were spread southward from Utah during the development of the late Tertiary Colorado River drainage are significantly younger, and were more recently derived from erosion of formations such as the Claron and Canaan Peak Formations, rocks whose ages overlap with the older Laramide sediments of Arizona. A fundamental appreciation of the diverse origins, differing ages, and significance of the wide variety of primary and reworked fluvial gravels present on the Colorado Plateau is essential to developing a better understanding of the gradual emergence of the modern landscape and its relationship to the formation of the Grand Canyon.

Stratigraphic trends in detrital zircon geochronology of upper Neoproterozoic and Cambrian strata, Osgood Mountains, Nevada, and elsewhere in the Cordilleran miogeocline: Evidence for early Cambrian uplift of the Transcontinental Arch

Gwen M. Linde et al., Dept. of Geological Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA. Published online 7 Oct. 2014; http://dx.doi.org/10.1130/GES01048.1.

Researchers have discovered new and interesting aspects of the history of the ancient western North American continent of nearly 550 million years ago. At the University of Arizona LaserChron laboratory, they determined the ages of zircon crystals in rocks found in north-central Nevada, and matched these ages to source areas in the eastern and central parts of the continent. They found that the older rocks in the field study area came from a different source area than younger rocks. Before Early Cambrian time (approximately 540 million years ago), the rivers carrying the sand that would be deposited in present-day Nevada originated in the eastern part of the continent; after that time they originated in the central continent. The research team compared results of several other studies of rocks of the same age from across the Great Basin, and found the same trend. Researchers concluded that this shift in source area was caused by the uplift of the Transcontinental Arch, a feature of much speculation over the past century, in the middle of the ancient continent.

Correlating the Arperos Basin from Guanajuato, central Mexico, to Santo Tomás, southern Mexico: Implications for the paleogeography and origin of the Guerrero terrane

Michelangelo Martini et al., Instituto de Geología, Universidad Nacional Autónoma de México, 04510, México D.F., México. Published online 7 Oct. 2014; http://dx.doi.org/10/1130/GES01055.1.

The present-day Pacific coast of Mexico consists of Upper Jurassic-Lower Cretaceous igneous and sedimentary rocks representative of a more than 5000 km-long volcanic belt, namely the Guerrero arc, which was developed by prolonged subduction of oceanic lithosphere. The origin of the Guerrero volcanic arc has been debated since the early eighties, and is fundamental in the reconstruction of the paleogeographic evolution of North America. Some authors proposed that the Guerrero arc is exotic relative to North America. In this scenario, the Guerrero arc was developed in the paleo-Pacific realm, far from North America, and subsequently was accreted to the North American continental margin. Alternatively, other authors proposed a para-autochthonous origin for the Guerrero arc. According to this alternative scenario, the Guerrero arc was built directly on the Pacific continental margin of North America. In order to test the North American and Pacific hypothesis proposed for the Guerrero arc, we performed a petrologic study of volcaniclastic sandstones from the eastern boundary of the Guerrero arc in southern Mexico. Our results indicate that during the Late Jurassic inception of its magmatic activity, the eastern edge of the Guerrero arc was laterally contiguous to the North American continental interior. This supports that the Guerrero arc is para-autochthonous that was built along the North American continental margin, and discards the possibility of an exotic pacific origin for such a volcanic chain.

Upper flow regime bedforms on levees and continental slopes: Turbidity current flow dynamics in response to fine-grained sediment waves

Svetlana Kostic, Computational Science Research Center, 5500 Campanile Drive, San Diego State University, San Diego, California 92182, USA. Published online 7 Oct. 2014; http://dx.doi.org/10.1130/GES01015.1.

Our knowledge of deep-sea bedforms associated with supercritical turbidity currents is very limited. This study examines the hydrodynamic response of fine-grained sediment waves on levees and continental slope to a wide range of underflow conditions. The results unambiguously demonstrate that fine-grained sediment waves in turbidite systems with slope breaks most likely form and evolve as supercritical bedforms, i.e., cyclic steps, transitional bedforms between cyclic steps and antidunes, or antidunes. The study provides valuable information for the reconstruction of formative flow conditions for these bedforms in diverse submarine environments.

Ice sheet surface lineaments as nonconventional indicators of East Antarctica bedrock tectonics

Paola Cianfarra and Francesco Salvini, Dipartimento di Scienze, Sezione Geologia, Università Roma Tre, Largo San Leonardo Murialdo, 1, I-00146 Roma, Italy. Published online 15 Oct. 2014; http://dx.doi.org/10.1130/GES01074.1

Radarsat mosaic of Antarctica revealed the existence of regional scale lineaments on the East Antarctic Ice Sheet (EAIS) surface. These lineaments cluster around preferential orientations, called domains that develop on regions spanning over thousands of square kilometers. The spatial and azimuthal analysis of the found lineament domains in the Vostok-Dome C-Adventure Basin region showed their relation with the buried morphotectonic features of bedrock. The EAIS records the tectonic processes of the more brittle upper crust, despite differences in the velocity (to two orders of magnitude) between ice dynamics and tectonics. The lineament domain analysis proved an effective tool to explore the crustal architecture of ice covered regions of our planet as well as the crustal tectonics of other planetary bodies such as Europa, the moon of Jupiter.

Vesuvian area using GIS and remote sensing

Marina Bisson et al. (Claudia Spinetti, corresponding author), Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola 32, 56126 Pisa, Italy. Published online 30 Oct. 2014; http://dx.doi.org/10.1130/GES01041.1

The Circum-Vesuvian area is one of the Italian zones most affected by extremely dangerous natural events such as lahars (mixture of water and sediments that flows along volcano slopes driving by the force of gravity). The most important event occurred on 5-6 May 1998 in the Sarno, Quindici, Siano and Bracigliano municipalities and caused the loss of many lives and heavy damage to infrastructures. In fact, the steep hillslopes of the area combined with the presence of fallout deposits coming from the past eruptions of the Neapolitan volcanoes (Ischia, Campi Flegrei, and Somma-Vesuvius) provide conditions strongly favorable to generate these calamitous events during intensive or persistent precipitations. The present work, in a contest of mitigation and prevention by such natural disaster, has significantly improved the hazard map by classifying 1,100 drainage basins, recognized as potential sources of lahars, using the Disruption Proneness Index (DPI) ranked in four classes: very high, high, moderate, and low hazard. The results highlight that a considerable portion of investigated territory (240 square kilometers on the total area of 340 square kilometers) is characterized by very high and high DPI. In particular, 34 municipalities result in the most exposed to such events and need future planning to mitigate such hazard.

The Argentine Precordillera: A foreland thrust belt proximal to the subducted plate

Richard W. Allmendinger and Phoebe A. Judge, Dept. of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA. Published online 30 Oct. 2014; http://dx.doi.org/10.1130/GES01062.1.

The Argentine Precordillera is a zone of active continental deformation on the eastern side of the Andes that has frequently experienced large earthquakes. We describe the faults that make up the Precordillera and relate their geometry to the subduction of the Pacific Ocean floor, known as the Nazca Plate, beneath South America. We show that the crustal shortening in the Precordillera over the last 13 million years is indistinguishable from the rate measured by the Global Positioning System (GPS) measured over the last two decades. This work provides the background scientific context for understanding earthquakes and also hydrocarbon potential in the region.

Delivery of terrigenous material to submarine fans: Biological evidence of local, staged, and full-canyon sediment transport down the Ascension-Monterey Canyon system

Mary McGann, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Published online 30 Oct. 2014; http://dx.doi.org/10.1130/GES01019.1. Themed issue: Exploring the Deep Sea and Beyond, volume 2.

Submarine canyons are prevalent in the world's oceans and are instrumental in transporting sediment from coastal regions to deep-sea fans. Typically, mineral grains have been used to characterize these deposits, but they provide little information on sediment source or the delivery processes involved. Fortunately, transported along with the mineral grains are the remains of microorganisms living within the sediment, each with unique environmental signatures. They may be used to identify local, staged, or full canyon sediment transport. A 19,000 year record from a core obtained at 3500 m water depth in Ascension-Monterey Canyon system demonstrates that by using these biological constituents, we can determine that the displaced deposits originated in the estuarine to inner shelf, outer shelf, upper slope, or upper middle slope. These deposits may have microorganisms from several depths, reflecting staged sediment transport with storage occurring behind slumps that act as barriers to movement until finally released. Sediment bypassing, typically of the deeper biofacies, and full canyon flushing, were also evident. Identifying and interpreting the distribution of displaced microorganisms in marine deposits is a powerful tool in the investigation of sediment transport that can be applied to other submarine canyon systems.

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