1,721,145 research outputs found
Stiffness evolution of granular layers and the origin of repetitive, slow, stick-slip frictional sliding
No full textWe demonstrate the frictional behaviors of steady
state sliding, stick-slip, and repetitive, slow stick-slip sliding
through a carefully-designed suite of laboratory experiments
focused on exploring the role of loading system stiffness in
controlling the frictional response to shear. We performed
tests on sheared layers of baking flour, with three configurations of loading blocks made of steel and cast acrylic to
achieve different stiffnesses. Slide-hold-slide and velocity
step tests were conducted and analyzed in a rate-and-state
friction framework.With compliant loading blocks, the material exhibits unstable stick-slip behavior with slow-slip events
of duration up to 20 s. Slow-slip has been difficult to achieve
in the lab and has only been observed for a narrow variety
of boundary conditions and materials. Our results suggest
that this behavior is strongly controlled by the stiffness of
the system, the strain history of the sample, and shear fabric
evolution. We describe a new suite of automated tools that
greatly improve friction analysis and provide insight to the
underlying mechanisms of slow stick-slip. We demonstrate
that layer stiffness evolves with shear strain and modifies the
mechanical behavior of stick-slip sliding. Our work suggests
that slow earthquakes in tectonic fault zones may be linked
to shear fabric development and associated changes in local
stiffness, likely in combination with variations in frictional
constitutive properties and effective stress
Foreshock properties illuminate nucleation processes of slow and fast laboratory earthquakes
Full text linkAbstract Understanding the connection between seismic activity and the earthquake nucleation process is a fundamental goal in earthquake seismology with important implications for earthquake early warning systems and forecasting. We use high-resolution acoustic emission (AE) waveform measurements from laboratory stick-slip experiments that span a spectrum of slow to fast slip rates to probe spatiotemporal properties of laboratory foreshocks and nucleation processes. We measure waveform similarity and pairwise differential travel-times (DTT) between AEs throughout the seismic cycle. AEs broadcasted prior to slow labquakes have small DTT and high waveform similarity relative to fast labquakes. We show that during slow stick-slip, the fault never fully locks, and waveform similarity and pairwise differential travel times do not evolve throughout the seismic cycle. In contrast, fast laboratory earthquakes are preceded by a rapid increase in waveform similarity late in the seismic cycle and a reduction in differential travel times, indicating that AEs begin to coalesce as the fault slip velocity increases leading up to failure. These observations point to key differences in the nucleation process of slow and fast labquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity
Experimental 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
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
Laboratory friction measurements of Nankai Trough basalts
No full textWe obtained altered basaltic upper basement from the modern Nankai Trough seaward of the trench, recovered by drilling on IODP Expedition 333. We performed laboratory friction experiments on bare surfaces and gouge powders of this material, in addition to and in combination with other materials for comparison. Based our friction data, we suggest that in addition to creep the altered Nankai basalts may also allow the possibility of slow slip events
Scale dependence of in-situ permeability measurements in the Nankai accretionary prism: The role of fractures
No full textModeling studies suggest that fluid permeability is an important control on the maintenance and distribution of pore fluid pressures at subduction zones generated through tectonic loading. Yet, to date, few data are available to constrain permeability of these materials, at appropriate scales. During IODP Expedition 319, downhole measurements of permeability within the uppermost accretionary wedge offshore SW Japan were made using a dual-packer device to isolate 1 m sections of borehole at a depth of 1500 m below sea floor. Analyses of pressure transients using numerical models suggest a range of in-situ fluid permeabilities (5E-15–9E-17 m2). These values are significantly higher than those measured on core samples (2E-19 m2). Borehole imagery and cores suggests the presence of multiple open fractures at this depth of measurement. These observations suggest that open permeable natural fractures at modest fracture densities could be important contributors to overall prism permeability structure at these scales
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