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  • 11
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    Off-fault secondary failure induced by a dynamic slip pulse (2005)
    In:  Bull. Seism. Soc. Am., Luxembourg, National Academy of Sciences of the USA, vol. 95, no. 1, pp. 109-134, pp. 2131, (ISSN: 1340-4202)
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    Publication Date: 2005
    Keywords: Rock mechanics ; Fracture ; Dynamic ; Fluids ; flow ; gouge ; Two-dimensional ; Modelling ; BSSA
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    BIBLIOGRAPHY SEISMOLOGY
  • 12
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    Laboratory earthquakes along inhomogeneous faults: directionality and supershear (2005)
    In:  Science, Reykjavík, Icelandic Meteorological Office, Ministry for the Environment, University of Iceland, vol. 308, no. 5722, pp. 681-684, pp. L05306, (ISSN: 1340-4202)
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    Publication Date: 2005
    Keywords: Fracture ; Rock mechanics ; Laboratory measurements ; Velocity ; Inhomogeneity ; Friction ; Rayleigh waves
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    BIBLIOGRAPHY SEISMOLOGY
  • 13
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    Shear heating and weakening of the margins of West Antarctic ice streams (2015)
    Wiley
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    Publication Date: 2015-04-07
    Description: Ice streams are fast flowing bands of ice separated from stagnant ridges by shear margins. The mechanisms controlling the location of the margins remain unclear. We use published ice deformation data and a simple one-dimensional thermal model to show that West Antarctic ice stream margins have temperate ice over a substantial fraction of their thickness, a condition that may control their width. The model predicts a triple-valued relation between the thickness-averaged lateral shear stress and the lateral shear strain rate. Observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream. This requires enhanced basal resistance near the margin. We suggest, in agreement with the limited observations, the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice-till interface, thus increasing the shear stress acting on the yielding Coulomb-plastic bed.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 14
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    Subglacial hydrology and ice stream margin locations (2015)
    Wiley
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    Publication Date: 2015-06-23
    Description: Fast flowing ice streams in West Antarctica are separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins. It is presently uncertain what mechanisms control the location of shear margins, and possibly allow them to migrate. In this paper we show how subglacial hydrological processes can select the shear margin location, leading to a smooth transition from a slipping to a locked bed at the base of an ice stream. Our study uses a two-dimensional thermo-mechanical model in a cross-section perpendicular to the direction of flow. We confirm that the intense straining at the shear margins can generate large temperate regions within the deforming ice. Assuming that the melt generated in the temperate ice collects in a drainage channel at the base of the margin, we show that a channel locally decreases the pore pressure in the subglacial till. Therefore the basal shear strength just outside the channel, assuming a Coulomb-plastic rheology, can be substantially higher than that inferred under the majority of the stream. Results show that the additional basal resistance produced by the channel lowers the stress concentrated on the locked portion of the bed. Matching the model to surface velocity data we find that shear margins are stable when the slipping-to-locked bed transition occurs less than 500 m away from a channel operating at an effective pressure of 200 kPa and for a hydraulic transmissivity equivalent to a basal water-film of order 0.2 mm thickness.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 15
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    Determining conditions that allow a shear margin to coincide with a Röthlisberger channel (2016)
    Wiley
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    Publication Date: 2016-06-08
    Description: The mass loss from the West Antarctic Ice Sheet is dominated by numerous rapidly flowing ice streams, which are separated from stagnant ice in the adjacent ridges by zones of concentrated deformation known as shear margins. Because the discharge from a single ice stream depends sensitively on the ice stream width, determining the physical processes that control shear margin location is crucial to a full understanding of ice stream dynamics. Previous work has shown that the transition from a deforming to an undeforming bed within a shear margin concentrates large stresses on the undeforming bed beneath the ridge [ Jacobson and Raymond , 1998; Schoof , 2004; Suckale et al., 2014]. In this paper we investigate how the presence of a drainage channel collocated with the transition from a deforming to an undeforming bed perturbs the stress field within the shear margin. We show that the channel limits the maximum shear stress on the undeforming bed and alters the yield strength of the till by changing the normal stress on the ice-till interface. By comparing the maximum stress with the till strength, we show that the transition from a deforming to an undeforming bed can occur across a channel whenever the water flux in the channel exceeds a critical value. This critical flux is sensitive to the rheology and loading of the shear margin, but we conclude that there are some scenarios where the transition from a deforming to an undeforming bed can be collocated with a drainage channel, though this configuration is probably not typical.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 16
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    Strain localization driven by thermal decomposition during seismic shear (2015)
    Wiley
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    Publication Date: 2015-04-16
    Description: Field and laboratory observations show that shear deformation is often extremely localized at seismic slip rates, with a typical deforming zone width on the order of a few tens of microns. This extreme localization can be understood in terms of thermally driven weakening mechanisms. A zone of initially high strain rate will experience more shear heating and thus weaken faster, making it more likely to accommodate subsequent deformation. Fault zones often contain thermally unstable minerals such as clays or carbonates, which devolatilize at the high temperatures attained during seismic slip. In this paper, we investigate how these thermal decomposition reactions drive strain localization when coupled to a model for thermal pressurization of in-situground water. Building on Rice et al . [2014], we use a linear stability analysis to predict a localized zone thickness that depends on a combination of hydraulic, frictional, and thermochemical properties of the deforming fault rock. Numerical simulations show that the onset of thermal decomposition drives additional strain localization when compared with thermal pressurization alone, and predict localized zone thicknesses of ~7 and ~13 μ m for lizardite and calcite respectively. Finally we show how thermal diffusion and the endothermic reaction combine to limit the peak temperature of the fault, and that the pore fluid released by the reaction provides additional weakening of ~20–40% of the initial strength.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 17
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    Influence of plastic deformation on bimaterial fault rupture directivity (2011)
    Wiley
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    Publication Date: 2011-10-22
    Description: Material juxtapositions across mature faults are a common occurrence. Previous work has found that this elastic mismatch results in a rupture that will preferentially propagate in the direction of slip displacement on the more compliant side of the fault, with more off-fault damage in the stiffer material. This result has implications for inferring preferred rupture directions based on observations of damage zone asymmetry. We perform a complete numerical investigation of the role of the stress state on the distribution of plastic deformation and the direction of preferred rupture propagation. We show that there are important factors, in addition to the elastic mismatch, which control the preferred direction of propagation as well as the side of the fault in which damage predominately accumulates. The orientation of the most compressive principal stress is the controlling factor in determining the location of plastic deformation. For different orientations, plastic deformation can accumulate in either the stiffer or the more compliant material. For high angles of most compressive stress, the aforementioned preferred rupture direction prediction holds true. However, the off-fault plastic response can reverse that direction for low angles of most compressive stress so that rupture will preferentially propagate in the direction of slip displacement in the stiffer material.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 18
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    Influence of material contrast on fault branching behavior (2011)
    Wiley
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    Publication Date: 2011-07-26
    Description: Material contrasts across faults are a common occurrence, and it is important to understand if these material contrasts can influence the path of rupture propagation. Here we examine models, solved numerically, of rupture propagation through one type of geometric complexity, that of a fault branch stemming from a planar main fault on which rupture initiates. This geometry, with a material contrast across the main fault, could be representative of either a mature strike-slip fault or a subduction zone interface. We consider branches in both the compressional and extensional quadrants of the fault, and material configurations in which the branch fault is in either the stiffer or the more compliant material as well as configurations with no material contrast. We find that there are regimes in which this elastic contrast can influence the rupture behavior at a branching junction, but there are also stress states for which the branch activation will not depend on the orientation of the mismatch. For the scenarios presented here, both compressional and extensional side branches are more likely to rupture if the branch is on the side of the fault with the more compliant material versus the stiffer material. The stresses induced on the branch fault, by rupture traveling on the main fault, are different for the two orientations of material contrast. We show how the interactions between rupture on the two faults determine which faults are activated.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 19
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    Deformation-induced melting in the margins of the West-Antarctic ice streams (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: Flow of glacial ice in the West Antarctic Sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice, but our understanding of the physical processes that generate this morphology is incomplete. Here, we study the thermal and mechanical properties of ice-stream margins, where flow transitions from rapid to stagnant over a few kilometers. Our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation-induced melting. We propose a 2D model that represents a cross-section through the ice-stream margin perpendicular to the downstream flow direction. We limit temperature to the melting point to estimate melt rates based on latent heat. Using rheology parameters as constrained by laboratory data and observations, we conclude that a zone of temperate ice is likely to form in active shear margins.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 20
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    Nucleation of slip-weakening rupture instability in landslides by localized increase of pore pressure (2012)
    Wiley
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    Publication Date: 2012-03-17
    Description: We model landslide initiation as slip surface growth driven by locally elevated pore pressure, with particular reference to submarine slides. Assuming an elastic medium and friction that weakens with slip, solutions exist in which the slip surface may dynamically grow, without further pore pressure increases, at a rate of the order of the sediment shear wave speed, a situation comparable to earthquake nucleation. The size of the rupture at this transition point depends weakly on the imposed pore pressure profile; however, the amount of slip at the transition depends strongly on whether the pore pressure was broadly or sharply elevated. Sharper profiles may result in pore pressures reaching the total slope-normal stress before dynamic rupture is nucleated. While we do not account for modes of failure other than pure slip on a failure surface, this may be an indication that additional modes involving liquefaction or hydraulic cracking may be factors in the initiation of shallow slope failure. We identify two length scales, one geometrical (h, depth below the free surface) and one material (ℓ, determined by the frictional weakening rate) and a transition in nucleation behavior between effectively “deep” and “shallow” limits dependent on their ratio. Whether dynamic propagation of failure is indefinite or arresting depends largely on whether the background shear stress is closer to nominal peak or residual frictional strength. This is determined in part by background pore pressures, and to consider the submarine case we simplify a common sedimentation/consolidation approach to reflect interest in near-seafloor conditions.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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