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  • Wiley  (5)
  • 2015-2019  (5)
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  • 1
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    On the evolution of elastic properties during laboratory stick-slip experiments spanning the transition from slow slip to dynamic rupture (2016)
    Wiley
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    Publication Date: 2016-12-10
    Description: The physical mechanisms governing slow earthquakes remain unknown, as does the relationship between slow and regular earthquakes. To investigate the mechanism(s) of slow earthquakes and related quasi-dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration. We reproduced the full spectrum of slip behavior, from slow to fast stick-slip, by altering the elastic stiffness of the loading apparatus (k) to match the critical rheologic stiffness of fault gouge (k c ). Our experiments show an evolution from stable sliding, when k 〉 k c , to quasi-dynamic transients when k ~ k c , to dynamic instabilities when k 〈 k c . To evaluate the micro-physical processes of fault weakening we monitored variations of elastic properties. We find systematic changes in P-wave velocity (V p ) for laboratory seismic cycles. During the coseismic stress drop, seismic velocity drops abruptly, consistent with observations on natural faults. In the preparatory phase preceding failure, we find that accelerated fault creep causes a V p reduction for the complete spectrum of slip behaviors. Our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults, depending on frictional properties. In agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by V p changes during the seismic cycle. The observed reduction in V p during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Frictional properties of the active San Andreas fault at SAFOD: Implications for fault strength and slip behavior (2015)
    Wiley
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    Publication Date: 2015-06-20
    Description: We present results from a comprehensive laboratory study of the frictional strength and constitutive properties for all three active strands of the San Andreas Fault penetrated in the San Andreas Observatory at Depth (SAFOD). The SAFOD borehole penetrated the Southwest Deforming Zone (SDZ), the Central Deforming Zone (CDZ), both of which are actively creeping, and the Northeast Boundary Fault (NBF). Our results include measurements of the frictional properties of cuttings and core samples recovered at depths of ~2.7 km. We find that materials from the two actively creeping faults exhibit low frictional strengths (μ = ~0.1), velocity-strengthening friction behavior, and near zero or negative rates of frictional healing. Our experimental dataset shows that the center of the CDZ is the weakest section of the San Andreas Fault, with μ = ~0.10. Fault weakness is highly localized and likely caused by abundant magnesium-rich clays. In contrast, serpentine from within the SDZ, and wall rock of both the SDZ and CDZ, exhibit velocity-weakening friction behavior and positive healing rates, consistent with nearby repeating microearthquakes. Finally, we document higher friction coefficients (μ 〉0.4) and complex rate dependent behavior for samples recovered across the NBF. In total, our data provide an integrated view of fault behavior for the three active fault strands encountered at SAFOD, and offer a consistent explanation for observations of creep and microearthquakes along weak fault zones within a strong crust.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Dynamically triggered slip leading to sustained fault gouge weakening under laboratory shear conditions (2016)
    Wiley
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    Publication Date: 2016-02-05
    Description: We investigate dynamic-wave triggered slip under laboratory shear conditions. The experiment is comprised of a 3-block system containing two gouge layers composed of glass beads and held in place by a fixed load in a bi-axial configuration. When the system is sheared under steady state conditions at a normal load of 4 MPa, we find that shear failure may be instantaneously triggered by a dynamic wave, corresponding to material weakening and softening if the system is in a critical shear-stress state (near failure). Following triggering, the gouge material remains in a perturbed state over multiple slip cycles as evidenced by the recovery of the material strength, shear modulus and slip recurrence time. This work suggests that faults must be critically stressed to trigger under dynamic conditions and that the recovery process following a dynamically triggered event differs from the recovery following a spontaneous event.
    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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  • 4
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    Laboratory observations of time-dependent frictional strengthening and stress relaxation in natural and synthetic fault gouges (2016)
    Wiley
    Details
    Publication Date: 2016-01-28
    Description: Interseismic recovery of fault strength (healing) following earthquake failure is a fundamental requirement of the seismic cycle and likely plays a key role in determining the stability and slip behavior of tectonic faults. We report on laboratory measurements of time- and slip-dependent frictional strengthening for natural and synthetic gouges to evaluate the role of mineralogy in frictional strengthening. We performed slide-hold-slide (SHS) shearing experiments on nine natural fault gouges and eight synthetic gouges at conditions of 20 MPa normal stress, 100% relative humidity (RH), large shear strain (~15), and room temperature. Phyllosilicate-rich rocks show the lowest rates of frictional strengthening. Samples rich in quartz and feldspar exhibit intermediate rates of frictional strengthening and calcite-rich gouges show the largest values. Our results show that: (1) the rates of frictional strengthening and creep relaxation scale with frictional strength, (2) phyllosilicate-rich fault gouges have low strength and healing characteristics that promote stable, aseismic creep, (3) most natural fault gouges exhibit intermediate rates of frictional strengthening, consistent with a broad range of fault slip behaviors, and (4) calcite-rich fault rocks show the highest rates of frictional strengthening, low values of dilation upon reshear, and high frictional strengths, all of which would promote seismogenic behavior.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Kinetic Models for Healing of the Subduction Interface based on Observations of Ancient Accretionary Complexes (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Sand‐shale mélanges from the Kodiak Accretionary Complex and Shimanto Belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350°C. We use observations from these mélanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid‐filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: 1) a transient drop in fluid pressure ∆Pf , and 2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than ∆Pf, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack‐spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which are loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab‐top geotherms. The model provides: 1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and 2) demonstration that kinetically‐driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment‐dominated convergent margins.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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