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On the relation between fault strength and frictional stability
No full textA fundamental problem in fault mechanics is whether slip instability associated with earthquake nucleation depends on absolute fault strength. We present laboratory experimental evidence for a systematic relationship between frictional strength and friction rate dependence, one of the key parameters controlling stability, for a wide range of constituent minerals relevant to natural faults. All of the frictionally weak gouges (coefficient of sliding friction, μ < 0.5) are composed of phyllosilicate minerals and exhibit increased friction with slip velocity, known as velocity-strengthening behavior, which suppresses frictional instability. In contrast, fault gouges with higher frictional strength exhibit both velocity-weakening and velocity-strengthening frictional behavior. These materials are dominantly quartzofeldspathic in composition, but in some cases include certain phyllosilicate-rich gouges with high friction coefficients. We also find that frictional velocity dependence evolves systematically with shear strain, such that a critical shear strain is required to allow slip instability. As applied to tectonic faults, our results suggest that seismic behavior and the mode of fault slip may evolve predictably as a function of accumulated offset. © 2011 Geological Society of America
The role of deformation bands in dictating poromechanical properties of unconsolidated sand and sandstone
Full text linkCataclastic shear bands in sands and sandstones are typically stronger, stiffer, and exhibit lower permeability than the surrounding matrix, and therefore act as barriers to fluid flow. Previous work has quantified the reduction in permeability associated with these features; however, little is known about the role of shear band structure in controlling the way they impact permeability and elastic properties. Here, we report on a suite of laboratory measurements designed to measure the poromechanical properties for host material and natural shear bands, over effective stresses from 1–65 MPa. In order to investigate the role of host material properties in controlling poromechanical evolution with stress, we sampled shear bands from two well-studied sandstones representing structurally distinct end-members: a poorly cemented marine terrace sand from the footwall of the McKinleyville thrust fault in Humboldt County, California, and a strongly-cemented sandstone from the hanging wall of the Moab Fault in Moab, Utah. The permeability-porosity trends are similar for all samples, with permeability decreasing systematically with increasing effective stress and decreasing porosity. The permeability of the host material is consistently >1 order of magnitude greater than the shear bands for both localities. For the unconsolidated case, shear bands are less permeable and stiffer than the host material, whereas for the consolidated case, shear bands are slightly less permeable, and wave speeds are slower than in the host. We attribute the differences between the McKinleyville and Moab examples to changes in structure of the nearby host material that accompanied formation of the shear band
Frictional and hydrologic properties of clay-rich fault gouge
No full text[1] The slip behavior of major faults depends largely on the frictional and hydrologic properties of fault gouge. We report on laboratory experiments designed to measure the strength, friction constitutive properties, and permeability of a suite of saturated clay-rich fault gouges, including: a 50:50% mixture of montmorillonite-quartz, powdered illite shale, and powdered chlorite schist. Friction measurements indicate that clay-rich gouges are consistently weak, with steady state coefficient of sliding friction of <0.35. The montmorillonite gouge (μ = 0.19-0.23) is consistently weaker than the illite and chlorite gouges (μ = 0.27-0.32). At effective normal stresses from 12 to 59 MPa, all gouges show velocity-strengthening frictional behavior in the sliding velocity range 0.5-300 μm/s. We suggest that the velocity- strengthening behavior we observe is related to saturation of real contact area, as documented by the friction parameter b, and is an inherent characteristic of noncohesive, unlithified clay-rich gouge. Permeability normal to the gouge layer measured before, during, and after shear ranges from 8.3 × 10 -21 m2 to 3.6 × 10-16 m2; permeability decreases dramatically with shearing, and to a lesser extent with increasing effective normal stress. The chlorite gouge is consistently more permeable than the montmorillonite and illite gouge and maintains a higher permeability after shearing. Permeability reduction via shear is pronounced at shear strains ≲5 and is smaller at higher strain, suggesting that shear-induced permeability reduction is linked to fabric development early in the deformation history. Our results imply that the potential for development of excess pore pressure in low-permeability fault gouge depends on both clay mineralogy and shear strain. Copyright 2009 by the American Geophysical Union
Frictional properties and sliding stability of the San Andreas fault from deep drill core
No full textThe strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We find that the fault is extremely weak (friction coefficient, μ = ∼ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of μ > ∼0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ∼10 MPa in the upper 5-8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes. © 2012 Geological Society of America
Effect of hydration state on the frictional properties of montmorillonite-based fault gouge
No full textWe report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge. We tested four mixtures of Ca-montmorillonite and quartz (100, 70, 50, and 30% montmorillonite) at four hydration states: dry (<4.50 wt% water), one water interlayer equivalent (4.5-8.7 wt% water), two layers (8.7-16.0 wt% water), and three layer (>16.0 wt% water). We controlled the hydration state using either oven drying (for <13 wt% H2O) or saline solutions (to achieve>13 wt% H2O under conditions of controlled relative humidity). For each clay/quartz mixture and hydration state, we measured frictional properties over a range of normal stresses (5-100 MPa) and sliding velocities (1-300 μm /s). We observe a systematic decrease in the coefficient of friction (μ) with increasing water content, normal stress, and clay content. Values of μ for 50/50 mixtures range from 0.57 to 0.64 dry and decrease to 0.21-0.55 for the most hydrated cases (wet). For layers of 100% montmorillonite, μ ranges from 0.41-0.62 dry to 0.03-0.29 wet. As water content is increased from 0 to 20.0 wt%, the friction rate parameter a-b becomes increasingly positive. Variation in a-b values decreases dramatically as normal stress increases. If our experimental results can be applied to natur al fault gouge, the combination of stress state, hydration state, and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite. Copyright 2007 by the American Geophysical Union
Comparison of smectite- and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts
No full textAlong plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ∼150°C is one of the key mineralogical changes thought to control the updip limit of seismicity. If correct, this hypothesis requires illite-rich gouges to exhibit frictionally unstable (velocity-weakening) behavior. Here, we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay-rich gouges. We sheared 5-mm-thick layers of commercially obtained pure Ca-smectite, a suite of smectite-quartz mixtures, and natural illite shale (grain size ranging from 2 to 500 μm) in the double-direct shear geometry to shear strains of ∼7-30 at room humidity and temperature. XRD analyses show that the illite shale contains dominantly clay minerals and quartz; within the clay-sized fraction (<2 μm), the dominant mineral is illite. Thus, we consider this shale as an appropriate analog for fine-grained sediments incoming to subduction zones, within which smectite has been transformed to illite. We observe a coefficient of friction ( μ ) of 0.42-0.68 for the illite shale, consistent with previous work. Over a range of normal stresses from 5 to 150 MPa and sliding velocities from 0.1 to 200 μm/s, this material exhibits only velocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity-weakening behavior of illite. Smectite sheared under identical conditions exhibits low friction ( μ =0.15-0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at higher normal stress (>40 MPa). Our data, specifically the velocity-strengthening behavior of illite shale under a wide range of conditions, do not support the hypothesis that the smectite-illite transition is responsible for the seismic-aseismic transition in subduction zones. We suggest that other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing, may play an important role in changing the frictional properties of subduction zone faults, and that these processes, in addition to clay mineralogy, should be the focus of future investigation. © 2003 Elsevier B.V. All rights reserved
Weakness of the San Andreas Fault revealed by samples from the active fault zone
No full textUnderstanding the strength and slip behaviour of tectonic faults is a central problem in earthquake physics and seismic-hazard assessment. Many major faults, including the San Andreas Fault, are weak compared with the surrounding rock, but the cause of this weakness is debated. Previous measurements of the frictional strength of San Andreas Fault rocks are too high to explain the observed weakness. However, these measurements relied on samples taken at a distance from the active fault or from weathered surface samples. Recent drilling into the San Andreas Fault has provided material from the actively slipping fault at seismogenic depths. Here we present systematic measurements of the frictional properties and composition of the San Andreas Fault at 2.7 km depth, including the wall rock and active fault. We find that the fault is weak relative to the surrounding rock and that the fault rock exhibits stable sliding friction behaviour. The fault zone contains the weak mineral smectite and exhibits no frictional healing-bonds in the material do not heal after rupture. Taken together, the low inherent strength and lack of healing of the fault-zone material could explain why the San Andreas Fault slips by aseismic creep and small earthquakes in central California, rather than by large, destructive earthquakes. © 2011 Macmillan Publishers Limited. All rights reserved
Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes
Full text linkSlow earthquakes represent an important conundrum in earthquake physics. While regular
earthquakes are catastrophic events with rupture velocities governed by elastic wave speed,
the processes that underlie slow fault slip phenomena, including recent discoveries of tremor,
slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse
laboratory observations have provided insights, but the physics of slow fault rupture remain
enigmatic. Here we report on laboratory observations that illuminate the mechanics of
slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the
threshold between stable and unstable failure, and is governed by frictional dynamics via the
interplay of fault frictional properties, effective normal stress and the elastic stiffness of the
surrounding material. This generalizable frictional mechanism may act in concert with other
hypothesized processes that damp dynamic ruptures, and is consistent with the broad range
of geologic environments where slow earthquakes are observed
Laboratory results indicating complex and potentially unstable frictional behavior of smectite clay
No full textA central problem in explaining the apparent weakness of the San Andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake-like unstable rupture. Our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress (<30 MPa). Our data are consistent with previous work, which has focused on higher normal stress conditions (50 MPa and greater) and found only velocity strengthening. If natural fault zones contain significant smectite, one key implication of our results is that localized zones of high pore pressure, which reduce effective normal stress, could be important in controlling potential sites of earthquake nucleation. Our experiments indicate that friction of smectite is complex, and depends upon both sliding velocity and normal stress. This complexity highlights the need for detailed experiments that reflect in-situ conditions for fault gouges
Slip weakening as a mechanism for slow earthquakes
No full textSlow slip forms part of the spectrum of fault behaviour between stable creep and destructive earthquakes. Slow slip occurs near the boundaries of large earthquake rupture zones and may sometimes trigger fast earthquakes. It is thought to occur in faults comprised of rocks that strengthen under fast slip rates, preventing rupture as a normal earthquake, or on faults that have elevated pore-fluid pressures. However, the processes that control slow rupture and the relationship between slow and normal earthquakes are enigmatic. Here we use laboratory experiments to simulate faulting in natural rock samples taken from shallow parts of the Nankai subduction zone, Japan, where very low-frequency earthquakes - a form of slow slip - have been observed. We find that the fault rocks exhibit decreasing strength over millimetre-scale slip distances rather than weakening due to increasing velocity. However, the sizes of the slip nucleation patches in our laboratory simulations are similar to those expected for the very low-frequency earthquakes observed in Nankai. We therefore suggest that this type of fault-weakening behaviour may generate slow earthquakes. Owing to the similarity between the expected behaviour of slow earthquakes based on our data, and that of normal earthquakes during nucleation, we suggest that some types of slow slip may represent prematurely arrested earthquakes. © 2013 Macmillan Publishers Limited. All rights reserved
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