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As preparation for a workshop on "The Dynamics of Fault Zones" (95 th Dahlem Workshop, Berlin, January 2005), specifically on the sub-topic "Rheology of Fault Rocks and Their Surroundings", we addressed critical... more
As preparation for a workshop on "The Dynamics of Fault Zones" (95 th Dahlem Workshop, Berlin, January 2005), specifically on the sub-topic "Rheology of Fault Rocks and Their Surroundings", we addressed critical research issues for understanding the seismic response of fault zones in terms of the constitutive response of fault materials. That requires new concepts and a host of new observations and experiments to document material response, to understand the shear localization process and the inception of earthquake instability, and especially to understand the mechanisms of fault weakening and dynamics of rupture tip propagation and arrest during rapid, possibly large, slip in natural events. We examine in turn the geological structure of fault zones and its relation to earthquake dynamics, the description of rate and state friction at slow rates appropriate to the interseismic period and earthquake nucleation, and the dynamics of fault weakening during rapid sl...
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We examine, and find evidence to support, the hypothesis that the width of Siple Coast ice streams in West Antarctica is set by the development of significant internal melting (i.e., development of temperate ice conditions) within the ice... more
We examine, and find evidence to support, the hypothesis that the width of Siple Coast ice streams in West Antarctica is set by the development of significant internal melting (i.e., development of temperate ice conditions) within the ice sheet at the margins. We first illustrate, from published ice sheet deformation data and from simple 1-D thermal modeling based on temperature-dependent flow and conduction properties, that most existing Siple Coast ice stream margins are in a state of partial melt, with temperate ice being present over a substantial fraction of the sheet thickness. We show that, although the margins sustain high lateral strain rates, they can support (and hence transmit to the cold ridges) notably less lateral shear stress than would the somewhat less rapidly deforming ice located inboard, away from the ridge. We then propose, and quantify approximately, a possible related mechanism of margin formation, that is, of locking the sheet to the bed at the margin. Shear...
Antarctic mass balance and contribution to sea level rise are dominated by the flow of ice through narrow conduits called ice streams. These regions of relatively fast flow drain over 90% of the ice sheet and generate significant amounts... more
Antarctic mass balance and contribution to sea level rise are dominated by the flow of ice through narrow conduits called ice streams. These regions of relatively fast flow drain over 90% of the ice sheet and generate significant amounts of frictional heat at the ice stream margins where there is a transition to slow flow in the ridge. This heat can generate temperate ice and a sharp transition in flow speed between the stream and the ridge. Within zones of temperate ice, meltwater is produced and drains to the bed. Here we model the downstream development of a temperate zone along an ice stream shear margin and the flow of meltwater through temperate ice into a subglacial hydrologic system. The hydrology sets the basal effective pressure, defined as the difference between ice overburden and water pressure. Using the southern shear margin of Bindschadler Ice Stream as a case study, our model results indicate an abrupt transition from a distributed to channelized hydrologic system wi...
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Injection‐induced seismicity is thought to be due primarily to increase in fluid pore pressure, which reduces the shear strength of a nearby fault. We address the modeling and prediction of the hydromechanical response due to fluid... more
Injection‐induced seismicity is thought to be due primarily to increase in fluid pore pressure, which reduces the shear strength of a nearby fault. We address the modeling and prediction of the hydromechanical response due to fluid injection, mainly as wastewater disposal. We consider the full poroelastic effects, including the changes in porosity and permeability of the medium due to changes in local volumetric strains. Our results consider effects of the fault architecture (low‐permeability fault core and anisotropic high‐permeability damage zones) on the pressure diffusion and the fault poroelastic response. We show that the high‐permeable damage zone, the poroelastic response, and the permeability evolution can accelerate the pore pressure diffusion process during and after wastewater injection. By studying a geologically based model of the Guy‐Greenbrier fault and of the earthquake sequence induced along it in Arkansas, United States, from October 2010 to July 2011, we show tha...
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A 2008 report by Das et al. documented the rapid drainage during summer 2006 of a supraglacial lake, of approximately 44×106 m3, into the Greenland ice sheet over a time scale moderately longer than 1 hr. The lake had been instrumented to... more
A 2008 report by Das et al. documented the rapid drainage during summer 2006 of a supraglacial lake, of approximately 44×106 m3, into the Greenland ice sheet over a time scale moderately longer than 1 hr. The lake had been instrumented to record the time-dependent fall of water level and the uplift of the ice nearby. Liquid water, denser than ice, was presumed to have descended through the sheet along a crevasse system and spread along the bed as a hydraulic facture. The event led two of the present authors to initiate modeling studies on such natural hydraulic fractures. Building on results of those studies, we attempt to better explain the time evolution of such a drainage event. We find that the estimated time has a strong dependence on how much a pre-existing crack/crevasse system, acting as a feeder channel to the bed, has opened by slow creep prior to the time at which a basal hydraulic fracture nucleates. We quantify the process and identify appropriate parameter ranges, part...
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We report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch. Rupture is always found to be asymmetric bilateral. In one... more
We report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch. Rupture is always found to be asymmetric bilateral. In one direction, rupture always propagates at the generalized Rayleigh wave speed, whereas in the opposite direction it is subshear or it transitions to supershear. The lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in Parkfield, California, and in North Anatolia.
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We analyze the nucleation and propagation of shear cracks along nonplanar, kinked, and branched fault paths corresponding to the configurations used in recent laboratory fracture studies by Rousseau and Rosakis (2003, 2009). The aim is to... more
We analyze the nucleation and propagation of shear cracks along nonplanar, kinked, and branched fault paths corresponding to the configurations used in recent laboratory fracture studies by Rousseau and Rosakis (2003, 2009). The aim is to reproduce numerically those shear rupture experiments and from that provide an insight into processes which are active when a crack, initially propagating in mode II along a straight path, interacts with a bend in the fault or a branching junction. The experiments involved impact loading of thin Homalite‐100 (a photoelastic polymer) plates, which had been cut along bent or branched paths and weakly glued back together everywhere except along a starter notch near the impact site. Strain gage recordings and high‐speed photography of isochromatic lines provided characterization of the transient deformation fields associated with the impact and fracture propagation. We found that dynamic explicit 2‐D plane‐stress finite element analyses with a simple l...
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Faults often separate materials with different elastic properties. Nonuniform slip on such faults induces a change in normal stress. That suggests the possibility of self‐sustained slip pulses [Weertman, 1980] propagating at the... more
Faults often separate materials with different elastic properties. Nonuniform slip on such faults induces a change in normal stress. That suggests the possibility of self‐sustained slip pulses [Weertman, 1980] propagating at the generalized Rayleigh wave speed even with a Coulomb constitutive law (i.e., with a constant coefficient of friction) and a remote driving shear stress that is arbitrarily less than the corresponding frictional strength. Following Andrews and Ben‐Zion [1997] (ABZ), we study numerically, with a two‐dimensional (2‐D) plane strain geometry, the propagation of ruptures along such a dissimilar material interface. However, this problem has been shown to be ill‐posed for a wide range of elastic material contrasts [Renardy, 1992; Martins and Simões, 1995; Adams, 1995]. Ranjith and Rice [2000] (RR) showed that when the generalized Rayleigh speed exists, as is the case for the material contrast studied by ABZ, the problem is ill‐posed for all values of the coefficient ...
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What are the origins of earthquake complexity? The possibility that some aspects of the complexity displayed by earthquakes might be explained by stress heterogeneities developed through the self‐organization of repeated ruptures has been... more
What are the origins of earthquake complexity? The possibility that some aspects of the complexity displayed by earthquakes might be explained by stress heterogeneities developed through the self‐organization of repeated ruptures has been suggested by some simple self‐organizing models. The question of whether or not even these simple self‐organizing models require at least some degree of material heterogeneity to maintain complex sequences of events has been the subject of some controversy. In one class of elastodynamic models, previous work has described complexity as arising on a model fault with completely uniform material properties. Questions were raised, however, regarding the role of discreteness, the relevance of the nucleation mechanism, and special parameter choices, in generating the complexity that has been reported. In this paper, we examine the question of whether or not continuum complexity is achieved under the stringent conditions of continuous loading, and whether...
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On the basis of elastodynamic stress fields for singular crack and nonsingular slip‐weakening models of propagating rupture, we develop preliminary answers to such questions as follows: If a rupturing fault is intersected by another,... more
On the basis of elastodynamic stress fields for singular crack and nonsingular slip‐weakening models of propagating rupture, we develop preliminary answers to such questions as follows: If a rupturing fault is intersected by another, providing a possible bend in the failure path, when will stressing be consistent with rupture along the bend? What secondary fault locations and orientations, in a damaged region bordering a major fault, will be stressed to failure by the main rupture? Stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed, especially near the limiting speed (Rayleigh for mode II, shear for mode III). Whether a bend path, once begun, can be continued to larger scales depends on principal stress directions and ratios in the prestress field. Conditions should often be met in mode II for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger‐scale stressing, a basis for intermittent ruptu...
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Motivated by observations of the subglacial drainage of water, we consider a hydraulic fracture problem in which the crack grows parallel to a free surface, subject to fully turbulent fluid flow. Using a hybrid... more
Motivated by observations of the subglacial drainage of water, we consider a hydraulic fracture problem in which the crack grows parallel to a free surface, subject to fully turbulent fluid flow. Using a hybrid Chebyshev/series-minimization numerical approach, we solve for the pressure profile, crack opening displacement, and crack growth rate for a crack that begins relatively short but eventually becomes long compared with the distance to the free surface. We plot nondimensionalized results for a variety of different times, corresponding with different fracture lengths, and find substantial differences when free-surface effects are important.
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A line integral is exhibited which has the same value for all paths surrounding the tip of a notch in the two-dimensional strain field of an elastic or deformation-type elastic-plastic material. Appropriate integration path choices serve... more
A line integral is exhibited which has the same value for all paths surrounding the tip of a notch in the two-dimensional strain field of an elastic or deformation-type elastic-plastic material. Appropriate integration path choices serve both to relate the integral to the near tip deformations and, in many cases, to permit its direct evaluation. This averaged measure of the near tip field leads to approximate solutions for several strain-concentration problems. Contained perfectly plastic deformation near a crack tip is analyzed for the plane-strain case with the aid of the slip-line theory. Near tip stresses are shown to be significantly elevated by hydrostatic tension, and a strain singularity results varying inversely with distance from the tip in centered fan regions above and below the tip. Approximate estimates are given for the strain intensity, plastic zone size, and crack tip opening displacement, and the important role of large geometry changes in crack blunting is noted. ...
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Field and borehole observations of active earthquake fault zones show that shear is often localized to principal deforming zones of order 0.1-10 mm width. This paper addresses how frictional heating in rapid slip weakens faults... more
Field and borehole observations of active earthquake fault zones show that shear is often localized to principal deforming zones of order 0.1-10 mm width. This paper addresses how frictional heating in rapid slip weakens faults dramatically, relative to their static frictional strength, and promotes such intense localization. Pronounced weakening occurs even on dry rock-on-rock surfaces, due to flash heating effects, at slip rates above approximately 0.1 m s(-1) (earthquake slip rates are typically of the order of 1 m s(-1)). But weakening in rapid shear is also predicted theoretically in thick fault gouge in the presence of fluids (whether native ground fluids or volatiles such as H2O or CO2 released by thermal decomposition reactions), and the predicted localizations are compatible with such narrow shear zones as have been observed. The underlying concepts show how fault zone materials with high static friction coefficients, approximately 0.6-0.8, can undergo strongly localized sh...
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Design, Mass Spectrometry, Cancer stem cells, Research, Translation, and 6 moreEGI, IE, Open, M, E, and Short Report
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We summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes. For models of smooth faults between elastically deformable continua, it is critical that the friction laws involve a... more
We summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes. For models of smooth faults between elastically deformable continua, it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state. That results in a finite nucleation size, or coherent slip patch size, h*. Models of smooth faults, using numerical cell size properly small compared to h*, show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self-similar (power law) Gutenberg-Richter frequency-size statistics. This conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences. In contrast, some models of locally heterogeneous faults with quasi-independent fault segments, represented approximately by simulations with cell size larger than h* so that the model becomes &quo...
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Engineering, Materials Science, Mathematical Sciences, Physical sciences, Nitrogen, and 14 moreComposite Material, Grain size, Stress Intensity Factor, Stress distribution, Stress Distribution, Plane Stress, Yield stress, Fracture Toughness, Mild Steel, Stress (Linguistics), Toughness, Tensile Properties, Ultimate Tensile Strength, and Critical value
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Engineering, Physics, Mechanics, Fracture Mechanics, Numerical Method, and 14 moreCrack propagation, Numerical Simulation, Mathematical Sciences, Physical sciences, First-Order Logic, Mechanical Stress, Heterogeneous media, Dynamic Fracture, Time Domain, Fracture Energy, Fracture Propagation, non perturbative QCd, Frequency Domain, and Crack growth
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A crack paralleling a bonded plane interface between two dissimilar isotropic elastic solids is considered. When the distance of the crack from the interface is small compared to the crack length itself and to other length scales... more
A crack paralleling a bonded plane interface between two dissimilar isotropic elastic solids is considered. When the distance of the crack from the interface is small compared to the crack length itself and to other length scales characterizing the geometry, a simple universal relation exists between the Mode I and Mode II stress intensity factors and the complex stress intensity factor associated with the corresponding problem for the crack lying on the interface. In other words, if the influence of external loading and geometry on the interface crack is known, then this information can immediately be used to generate the stress intensity factors for the sub-interface crack. Conditions for cracks to propagate near and parallel to, but not along, an interface are derived.