1,720,993 research outputs found
Physicochemical processes of frictional healing: Effects of water on stick-slip stress drop and friction of granular fault gouge
No full textExperimental constraints on the relationship between clay abundance, clay fabric, and frictional behavior for the central deforming zone of the San Andreas fault
No full textThe presence of smectite (saponite) in fault gouge from the Central Deforming Zone of the San Andreas Fault at Parkfield, CA has been linked to low mechanical strength and aseismic slip. However, the precise relationship between clay mineral structure, fabric development, fault strength, and the stability of frictional sliding is not well understood. We address these questions through the integration of laboratory friction tests and FIB-SEM analysis of fault rock recovered from the San Andreas Fault Observatory at Depth (SAFOD) borehole. Intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or SAFOD clasts extracted from the sample. Nano-textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges; such slip planes have multiples of random distribution (MRD) values of 3.0-4.9. The MRD values measured are higher than previous estimates (MRD 1.5) that show lower degrees of shear localization and clay alignment averaged over larger volumes. The intact fault rock exhibits less well-developed nano-clay fabrics than the experimentally sheared materials, and MRD values decrease with smectite content. We show that the abundance, strength, and shape of clasts all influence fabric evolution via strain localization: quartz clasts yield more strongly developed clay fabrics than serpentine-dominated SAFOD clasts. Our results suggest that (1) both clay abundance and the development of nano-scale fabrics play a role in fault zone weakening and (2) aseismic creep is promoted by slip along clay shears with >20 wt % smectite content and MRD values >= 2.7
Poromechanics of stick-slip frictional sliding and strength recovery on tectonic faults
No full textPorefluids influence many aspects of tectonic faulting including frictional strength aseismiccreep and effective stress during the seismic cycle. However, the role of porefluid pressure during earthquakenucleation and dynamic rupture remains poorly understood. Here we report on the evolution of porefluidpressure and porosity during laboratory stick-slip events as an analog for the seismic cycle. We sheared layers ofsimulated fault gouge consisting of glass beads in a double-direct shear configuration under true triaxialstresses using drained and undrainedfluid conditions and effective normal stress of 5–10 MPa. Shear stress wasapplied via a constant displacement rate, which we varied in velocity step tests from 0.1 to 30μm/s. We observenet pore pressure increases, or compaction, during dynamic failure and pore pressure decreases, or dilation,during the interseismic period, depending onfluid boundary conditions. In some cases, a brief period of dilationis attendant with the onset of dynamic stick slip. Our data show that time-dependent strengthening and dynamicstress drop increase with effective normal stress and vary withfluid conditions. For undrained conditions,dilation and preseismic slip are directly related to porefluid depressurization; they increase with effectivenormal stress and recurrence time. Microstructural observations confirm the role of water-activated contactgrowth and shear-driven elastoplastic processes at grain junctions. Our results indicate that physicochemicalprocesses acting at grain junctions together withfluid pressure changes dictate stick-slip stress drop andinterseismic creep rates and thus play a key role in earthquake nucleation and rupture propagatio
Evolution of permeability across the transition from brittle failure to cataclastic flow in porous siltstone
No full textPorous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformationto pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flowand permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Forma-tion, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostaticloading paths (r15r25r3) and triaxial loading paths (r1>r25r3) while maintaining constant effectivepressure (Pe5Pc–Pp). Samples were deformed with an axial displacement rate of 0.1lm/s (strain rate of 231026s21). Changes in pore water volume were monitored (drained conditions) to measure the evolutionof porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, wefind the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transitionfrom brittle behavior with shear localization and compaction to cataclastic-flow as confining pressureincreases. When samples fail by shear localization, permeability decreases abruptly without significantchanges in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with signif-icant porosity reduction. Postexperiment observation of brittle samples show localized shear zones charac-terized by grain comminution. Our data show how zones of shear localization can act as barriers to fluidflow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results haveimportant implications for deformation behavior and permeability evolution in sedimentary systems, and inparticular where the stress field is influenced by injection or pumping
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Seismic Characterization of Intra-basement Deformation and its Influence on the Overlying Sedimentary Strata: Implications for Tectonic Evolution and Induced Seismicity in Northern Oklahoma
Full text linkPatterns of recent seismogenic fault reactivation in the granitic basement of north-central Oklahoma necessitates an understanding of the structural characteristics of the inherited basement-rooted faults. We analyze the top-Arbuckle, top-basement, and intra-basement structures in 9 post-stack time-migrated 3D seismic reflection datasets in north-central Oklahoma using structural seismic attributes. Across all surveys, sub-horizontal intra-basement reflectors (igneous sills) are observed, which commonly terminate at the basement-sediment interface. Overall, our results reveal 146 fault traces at the top of the Precambrian basement with dominant trends along WNW-ESE, NE-SW, and N-S. Many of these faults were previously unmapped, supporting the assertion that there is a considerable cumulative seismic risk from unidentified faults. First examined are a subset of eight 3D seismic surveys in which 115 of the basement faults were interpreted. Proximal to the Nemaha Fault Zone (NFZ), faults dominantly strike N-S, are fewer (<10), have the lowest areal density, lowest areal intensity, and exhibit the most significant maximum vertical separation. However, with distance from the NFZ, faults exhibit NE-SW trends, areal fault density increases, areal fault intensity increases, and maximum vertical separation decreases. Of these 115 faults, ~49% are contained within the basement, ~28% terminate in the Arbuckle Group, and ~23% transect units above the Arbuckle Group. These observations suggest that proximal to the NFZ, deformation is accommodated along a few but longer fault segments, and with distance away from the NFZ, deformation is distributed across more relatively shorter fault segments. The existence of through-going faults suggests the potential for spatially pervasive fluid movement along faults. In the Harper Creek 3D seismic volume, a sequence of 56 relocated earthquakes is collocated with the survey. These events delineate the actual structure of an intra-basement fault zone. This provides a means of guidance and validation on attribute-assisted basement fault interpretation. When included in the interpretation of basement faults, it shows two critical steps beyond curvature interpretation at the top basement. The first is to couple to curvature interpretation with an aberrancy attribute visualization. When included, aberrancy improves the quantity and placement of interpreted fault traces. Second, it is essential to analyze the units directly above the basement. In the sedimentary units, the faults tend to express more clear signs of too subtle structures to identify at the top of the basement. Results have direct implications for wastewater injection and seismicity in north-central Oklahoma and southern Kansas. Additionally, they provide insight into the characteristics of basement-rooted structures around the NFZ region and suggest how to characterize basement structures where seismic data are available
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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