Dec 7 2015
U.S. Geological Survey scientists used 3D laser scanning to make repeat measurements of an area affected by the 2014 magnitude 6.0 South Napa earthquake in order to define in great detail the surface deformation that occurred both during and after the earthquake. The recent revolution in 3D laser measurement technology (LiDAR) allows scientists to collect detailed information about the shape of the land surface and the objects that sit upon it with unprecedented accuracy.
These spatially extensive measurement techniques provide new understanding of how earthquakes and other phenomena deform the shape of Earth's surface, reinforce the notion that not all surface deformation occurs during an earthquake itself, and provide insight into what can be expected following future earthquakes. When earthquakes strike, damage is expected to occur along the fault trace over a few seconds or perhaps minutes as Earth's tectonic plates shift, shake, and tear the ground.
However, in some cases, the damage to Earth's crust and what sits on top of it can unfold slowly over hours, days, weeks, and even years following an earthquake. This is often termed "afterslip." and it has been observed following many moderate earthquakes. Surface deformation following the South Napa quake occurred variously as discrete fault slip, rotation of a block of earth adjacent to the fault, and by vertical elevation changes. Comparison of the new 2014 terrestrial laser scanner data with 2003 airborne laser scanner data also indicate that the earthquake caused vertical warping across the fault zone rather than forming a distinct vertical scarp, challenging notions of how topography is created in moderate earthquakes.
FEATURED ARTICLE
Rates and patterns of surface deformation from laser scanning following the South Napa earthquake, California
Stephen B. DeLong et al., U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA. This article is OPEN ACCESS at http://dx.doi.org/10.1130/GES01189.1.
Other GEOSPHERE articles are highlighted below. All GEOSPHERE articles are available at http://geosphere.gsapubs.org/.
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A 6600 year earthquake history in the region of the 2004 Sumatra-Andaman subduction zone earthquake
Jason R. Patton et al., College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, 101 SW 26th Street, Corvallis, Oregon 97331, USA. This article is online at http://dx.doi.org/10.1130/GES01066.1.
Earthquakes and tsunamis are some of the most deadly natural disasters on planet Earth, with the 26 December 2004 Sumatra-Andaman earthquake and tsunami responsible for the deaths of nearly a quarter of a million people. Knowledge of the earthquake cycle, through many cycles, is fundamental to a deeper understanding of the seismogenic process and seismic hazard. GPS data yield detailed short-term strain information during one or two strain cycles, yet in many regions even a single cycle may span centuries. Paleoseismology is one method that garners information about faults as they behave through many cycles. In our paper we present a 6600 year long prehistoric record of subduction zone earthquakes in the region of the 2004 Sumatra-Andaman subduction zone earthquake. Our results triple the length in time of the longest paleoseismic record as inferred by paleotsunami deposits and extend the terrestrial paleoseismologic record by ~six fold. The mean emplacement time for turbidites (likely triggered by Great earthquakes of magnitude M > 8) in the 2004 rupture region for the past 6.6 ± 0.14 k. y. is 160 years for 43 seismoturbidites. We present convincing sedimentary evidence of a turbidite that was deposited as a result of the 2004 earthquake. The likely 2004 turbidite has a distinctive stacked structure of three major fining-upward sequences observed at several basin and trench sites, similar to the pattern of moment release in the 2004 earthquake.
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Factors influencing non-expert term usage during a disaster: An analysis of the 2004 Indian Ocean tsunami
Xai Her et al. (Scott K. Clark, corresponding author), Department of Geology, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, Wisconsin 54701, USA. This paper is online at http://dx.doi.org.10.1130/GES01170.1. Themed issue: Human Dimensions in Geoscience.
Effective communication can be challenging in the aftermath of a natural disaster with communication needed between emergency responders, relief organizations, disaster experts, government officials, the media, and the public. Understanding how the general public uses science terminology has implications for improving communication during a disaster and for improving science literacy, in general. In a paper by Xai Her and colleagues, the researchers analyzed quotes from 131 non-tsunami experts published by media sources in the aftermath of the 2004 Indian Ocean tsunami. The research team found that individuals who were in a country that was directly affected by the tsunami were as likely as not to call the event a tidal wave. This contrasts with individuals who were not in a directly impacted country; only 2% of those individuals were quoted as referring to the tsunami as a tidal wave. The data set suggests that many people were aware of both the colloquial term (tidal wave) and the scientific jargon (tsunami), and that the term tsunami is more widely known than had been previously assumed. While many factors play a role in how people describe a natural disaster, the researchers’ favored interpretation for this pattern in term usage is that the emotional stress induced by the tsunami caused people to use a more familiar, colloquial, and linguistically simpler term instead of more accurate terminology. The implication for disaster communication efforts is that while a term that was once considered scientific jargon can become widely known and adopted, many people will resort to a more familiar term unless the scientific jargon resonates with their personal, conceptual image of the disaster.
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Kinematics of shallow backthrusts in the Seattle fault zone, Washington State
Thomas L. Pratt et al., U.S. Geological Survey, 12201 Sunrise Valley Drive, MS 905, Reston, Virginia 20192, USA. This article is online at http://dx.doi.org/10.1130/GES01179.1.
From the abstract: Near-surface thrust fault splays and antithetic backthrusts at the tips of major thrust fault systems can distribute slip across multiple shallow fault strands, complicating earthquake hazard analyses based on studies of surface faulting. The shallow expression of the fault strands forming the Seattle fault zone of Washington State shows the structural relationships and interactions between such fault strands. Paleoseismic studies document an ~7000 yr history of earthquakes on multiple faults within the Seattle fault zone, with some backthrusts inferred to rupture in small (M ~5.5 to 6.0) earthquakes at times other than during earthquakes on the main thrust faults. We interpret seismic-reflection profiles to show three main thrust faults, one of which is a blind thrust fault directly beneath downtown Seattle, and four small backthrusts within the Seattle fault zone. We then model fault slip, constrained by shallow deformation, to show that the Seattle fault forms a fault propagation fold rather than the alternatively proposed roof thrust system. Fault slip modeling shows that back-thrust ruptures driven by moderate (M ~6.5 to 6.7) earthquakes on the main thrust faults are consistent with the paleoseismic data. The results indicate that paleoseismic data from the back-thrust ruptures reveal the times of moderate earthquakes on the main fault system, rather than indicating smaller (M ~5.5 to 6.0) earthquakes involving only the backthrusts. Estimates of cumulative shortening during known Seattle fault zone earthquakes support the inference that the Seattle fault has been the major seismic hazard in the northern Cascadia forearc in the late Holocene.
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Validation of meter-scale surface faulting offset measurements from high-resolution topographic data
J. Barrett Salisbury et al., School of Earth and Space Exploration, Arizona State University, P.O. Box 876004, Tempe, Arizona 85287-6004, USA. This article is online at http://dx.doi.org/10.1130/GES01197.1.
Landscape features such as stream channels that cross active faults can be used to document displacements in past earthquakes. Such information is valuable documentation of the sizes and ages of past events - data crucial to assessing future earthquake hazard. Studies of known earthquake-generating faults (and the search for previously undiscovered ones) are shifting from field-based research to work conducted remotely on computers with high-resolution topographic maps. While the shift to studying digital topography along faults increases efficiency, provides many measurements, and mimics aspects of fieldwork, it may also introduce ambiguous observations. We argue that digital topography analysis is a natural and necessary progression of surface processes research. It is an effective approach provided an investigator is aware of common problems and the limitations of specific measurement tools. Accurate and repeatable measurement correlates well with experience, but as is also the case in the field, most discrepancies arise from inadequate understanding of pre-earthquake landform shape and post-earthquake modification rather than from uncertainty in the measurement process itself. In this paper, we explore key challenges faced when remotely measuring fault-offset geomorphic features, suggest preferred measurement methods and reporting protocols, and encourage the continued use of high-resolution topography for fault zone analyses where possible.
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Photogrammetric digital outcrop reconstruction, visualization with textured surfaces, and three-dimensional structural analysis and modeling: Innovative methodologies applied to fault-related dolomitization (Vajont Limestone, Southern Alps, Italy)
Andrea Bistacchi et al., Department of Earth and Environmental Sciences, Universitá degli Studi di Milano Bicocca, Piazza della Scienza 4, 20126 Milano, Italy. This paper is online at http://dx.doi.org/10.1130/GES01005.1.
Hydrothermal dolomitization -- the transformation of limestone into dolostone due to the circulation of relatively hot Mg-rich fluids -- is a process that influences very much the porosity and permeability of rocks, and might result in significant reservoirs of geofluids (hydrocarbons, water, etc.). Quite often the infiltration of hot, Mg-rich fluids in a low-permeability "tight" limestone takes place along networks of faults and fractures, resulting in dolostone geobodies with very complex geometries. In this study we have characterized in 3D, with photogrammetric techniques, a large outcrop in the Vajont Gorge (730 m x 360 m x 270 m), where dolostone geobodies with porosity up to 25% are present in the tight Vajont Limestone. This study allowed evidencing the relationships between faults, fractures, bedding, and the dolomitization process, and enabled reconstructing 3D models of dolostone geobodies with realistic geometries, which could be used as input in more advanced models and simulations. Moreover, in the paper innovative photogrammetric and 3D geological modelling techniques are discussed, which could be applied also to the quantitative study of other geological processes.
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How does the connectivity of open-framework conglomerates within multi-scale hierarchical fluvial architecture affect oil-sweep efficiency in waterflooding?
Naum I. Gershenzon et al., Department of Earth and Environmental Sciences, Wright State University, 3640 Colonel Glenn Highway, Dayton, Ohio 45435, USA. This article is online at http://dx.doi.org/10.1130/GES01115.1.
We studied the effects on oil-sweep efficiency of the proportion, hierarchical organization, and connectivity of high-permeability open-framework conglomerate (OFC) cross-sets within the multi-scale stratal architecture found in fluvial-type reservoirs. The effective permeability of the reservoir exhibits large-scale anisotropy created by the organization of OFC cross-sets. As a result, oil-sweep efficiency critically depends on the direction of the pressure gradient. However, contrary to expectations, the total amount of trapped oil due to the effect of capillary trapping does not depend on the magnitude of the pressure gradient within the examined range. Hence the pressure difference between production and injection wells does not affect sweep efficiency. Whether or not clusters of connected OFC span the domain affects only the absolute rate of oil production—not sweep efficiency.
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40Ar/39Ar ages of muscovites from modern Himalayan rivers: Himalayan evolution and the relative contribution of tectonics and climate
Peter Copeland et al., Department of Earth and Atmospheric Sciences, University of Houston, 3507 Cullen Boulevard, Houston, Texas 77204, USA. This article is online at http://dx.doi.org/10.1130/GES01154.1.
Copeland et al. present analysis of the ages of sand grains (the mineral muscovite) found in modern rivers draining the Himalaya in central and western Nepal. The two main rivers studied are the Karnali in the west and the Narayani in central Nepal. These results suggest that tectonics in central Nepal Narayani basin has been more vigorous during the past 10 million years than in the Karnali basin. Variations in climate over this interval are not likely to be the explanation for the differences in the observed data.
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Hornbrook Formation, Oregon and California: A sedimentary record of the Late Cretaceous Sierran magmatic flare-up event
Kathleen D. Surpless, Department of Geosciences, Trinity University, One Trinity Place, San Antonio, Texas 78212, USA. This article is online at http://dx.doi.org/10.1130/GES01186.1.
The Late Cretaceous history of the Sierra Nevada and western Idaho is characterized by voluminous magmatism and uplift; more than 70% of the volume of the Sierran magmatic arc was emplaced during the Late Cretaceous, and these rocks were uplifted and eroded soon after forming. Surprisingly, Cretaceous sedimentary basins in California, west of these uplifted magmatic regions, lack abundant Late Cretaceous detrital zircon derived from the voluminous Late Cretaceous magmatic rocks. Detrital zircon ages and geochemistry compiled in this study of the Hornbrook Formation suggest that river systems may have transported sediment eroded from Late Cretaceous rocks of the Sierra Nevada north and northwest into the Hornbrook basin of southern Oregon and northern California, rather than delivering sediment westward into California basins. Sierran-sourced drainage systems may have joined those draining westward from the western Idaho region, resulting in abundant Late Cretaceous detrital zircon in the Hornbrook basin. The limited present-day outcrop extent of the Hornbrook Formation may represent only a sliver of a much larger Late Cretaceous Hornbrook basin system that was deposited on subsiding terranes of the Klamath Mountains. Complete characterization of the episodic magmatic history of continental arcs requires integration of age distributions from the arc itself and from detrital zircon eroded from the arc that may have been distributed to sedimentary basins not directly linked to present-day arc exposures.
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Latest Quaternary sedimentation in the northern Gulf of Mexico Intraslope Basin Province: I. Sediment facies and depositional processes
John E. Damuth and Hilary Clement Olson, Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, USA. This article is online at http://dx.doi.org/10.1130/GES01090.1. Part of themed issue " Exploring the Deep Sea and Beyond, Volume 2."
From the abstract: Very high-resolution seismic facies, classified, mapped, and interpreted from 3.5 kHz echograms, reveal that turbidity-current, mass-transport, and bottom-current depositional processes have all contributed to the regional sediment distribution in the intraslope basin province of the northwest Gulf of Mexico. Piston cores from these deposits confirm the interpretations of the processes. Turbidity currents transport sands into the intraslope mini-basins via channels and canyons. A few turbidity-current pathways, such as Bryant Canyon, allow extensive volumes of terrigenous sediment to bypass through many mini-basins and be deposited beyond the Sigsbee Escarpment to form large submarine fans, such as Bryant Fan. Bryant Fan is a large mud-rich fan that extends hundreds of kilometers from the mouth of Bryant Canyon but has only one meandering channel on the modern seafloor that extends down the length of the fan. In contrast, the much smaller Rio Grande Submarine Fan is deposited on a plateau area of the continental slope…
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Application of detrital zircon U-Pb geochronology to surface and subsurface correlations of provenance, paleodrainage, and tectonics of the Middle Magdalena Valley Basin of Colombia
Brian K. Horton et al., Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. This article is online at http://dx.doi.org/10.1130/GES01251.1.
U-Pb (uranium-lead) ages of zircon minerals from clastic deposits of Colombia demonstrate the utility of surface and subsurface samples in constraining regional source-to-sink relationships for sedimentary basins, the drainage evolution of paleorivers, and tectonic reconstructions of adjacent mountain ranges. New results for three key wells (21 subsurface samples) are integrated with previously reported data from six exposed intervals (90 surface samples) to reconstruct the provenance histories for key stratigraphic localities within the Middle Magdalena Valley Basin, a north-trending intermontane basin in the northern Andes of Colombia. U-Pb age distributions for modern rivers of Colombia highlight the distinctive signatures of several competing sediment source regions, including two major contiguous ranges (Central Cordillera and Eastern Cordillera) and two localized block uplifts (Santander Massif and San Lucas range). Results from Jurassic through Neogene stratigraphic units spanning nine surface and subsurface sites enable comparisons of provenance shifts at specific sites and spatial variations among key stratigraphic intervals across multiple sites. The surface and subsurface localities reveal temporal changes in U-Pb age distributions consistent with (1) Jurassic growth of extensional subbasins fed by local igneous sources, (2) Cretaceous deposition in an extensive post-rift setting, and (3) protracted Cenozoic growth of basin-bounding ranges during Andean crustal shortening. Provenance shifts of mid-Paleocene and latest Eocene-earliest Oligocene age are consistent with incipient uplift of the flanking Central Cordillera and Eastern Cordillera, respectively. U-Pb age spectra for Oligocene through Pliocene basin fill underscore complex north-south and east-west variations reflective of compartmentalized transverse deposystems demarcated by point-source contributions from the Central Cordillera and Eastern Cordillera. The late Miocene appearance of 100 - 0 Ma grains and a regional switch to broad, multimodal age distributions suggest the initial integration of the longitudinal proto–Magdalena River, linking the Middle Magdalena Valley with its southern headwaters and likely driving increased sedimentation rates farther north in the offshore Magdalena submarine fan of the southern Caribbean margin.
Source: http://www.geosociety.org