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  • 1
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    The Relationship Between Microfracture Damage and the Physical Properties of Fault-Related Rocks: The Gole Larghe Fault Zone, Italian Southern Alps (2019)
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    Publication Date: 2020-11-26
    Description: Although geological, seismological, and geophysical evidence indicates that fracture damage and physical properties of fault‐related rocks are intimately linked, their relationships remain poorly constrained. Here we correlate quantitative observations of microfracture damage within the exhumed Gole Larghe Fault Zone (Italian Southern Alps) with ultrasonic wave velocities and permeabilities measured on samples collected along a 1.5‐km‐long transect across the fault zone. Ultrasonic velocity and permeability correlate systematically with the measured microfracture intensity. In the center of the fault zone where microfractures were pervasively sealed, P wave velocities are the highest and permeability is relatively low. However, neither the crack porosity nor the permeability derived by modeling the velocity data using an effective‐medium approach correlates well with the microstructural and permeability measurements, respectively. The applied model does not account for sealing of microfractures but assumes that all variations in elastic properties are due to microfracturing. Yet we find that sealing of microfractures affects velocities significantly in the more extensively altered samples. Based on the derived relationships between microfracture damage, elastic and hydraulic properties, and mineralization history, we (i) assess to what extent wave velocities can serve as a proxy for damage structure and (ii) use results on the present‐day physical and microstructural properties to derive information about possible postseismic recovery processes. Our estimates of velocity changes associated with sealing of microfractures quantitatively agree with seismological observations of velocity recovery following earthquakes, which suggests that the recovery is at least in part due to the sealing of microfractures.
    Description: Published
    Description: 7661-7687
    Description: 3T. Sorgente sismica
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 2
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    Influence of Effective Stress and Pore Fluid Pressure on Fault Strength and Slip Localization in Carbonate Slip Zones (2021)
    Wiley Agu
    Details
    Publication Date: 2021-06-16
    Description: The presence of pressurized fluids influences the mechanical behavior of faults. To test the roles of normal stress and fluid pressure on shear strength and localization behavior of calcite gouges, we conducted a series of rotary-shear experiments with pore fluid pressures up to 10.5 MPa and difference between normal stress and fluid pressure up to 11.2 MPa. Calcite gouges were sheared for displacements of 0.3 m to several meters at slip rates of 1 mm/s and 1 m/s. Drainage conditions in experiments were constrained from estimates of the hydraulic diffusivity. Gouges were found to be drained at 1 mm/s, but possibly partially undrained during sliding at 1 m/s. Shear strength obeys an effective-stress law with an effective-stress coefficient close to unity with a friction coefficient of ~0.7 that decreases to 0.19 due to dynamic weakening. The degree of comminution and slip localization constrained from experimental microstructures depends on the effective normal stress. Slip localization in calcite gouges does not occur at low effective normal stress. The presence of pore fluids lowers the shear strength of gouges sheared at 1 mm/s and causes an accelerated weakening at 1 m/s compared to dry gouges, possibly due to enhanced subcritical crack growth and intergranular lubrication. Thermal pressurization occurs only after dynamic weakening when friction is generally low and relatively independent of normal stress and therefore unaffected by thermal pressurization. The experimental results are consistent with the view that the presence of pressurized fluid in carbonate-bearing faults can facilitate earthquake nucleation.
    Description: ERC StG 205175 USEMS ERC CoG 614705 NOFEAR Ca.Ri.Pa.Ro Foundation Gesellschaft der Freunde der Ruhr‐Universität Bochum e.V.
    Description: Published
    Description: e2020JB019805
    Description: 3T. Sorgente sismica
    Description: JCR Journal
    Keywords: Fault ; Earthquakes ; Carbonates ; Gouges ; Earthquake mechanics ; Rock Friction
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 3
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    Pseudotachylyte Alteration and the Rapid Fade of Earthquake Scars From the Geological Record (2021)
    Wiley Agu
    Details
    Publication Date: 2021-01-25
    Description: Tectonic pseudotachylytes are solidified frictional melts produced on faults during earthquakes and are robust markers of seismic slip events. Nonetheless, pseudotachylytes are apparently uncommon fault rocks, because they are either rarely produced or are easily lost from the geological record. To solve this conundrum, long-lasting (18–35 days) hydrothermal alteration tests were performed on fresh pseudotachylytes produced by sliding solid rock samples at seismic slip rates in the laboratory. After all tests, the pseudotachylytes were heavily altered with dissolution of the matrix and neo-formation of clay aggregates. Post-alteration products closely resemble natural altered pseudotachylytes and associated ultracataclasites (i.e., fault rocks affected by fracturing in the absence of melting), demonstrating that the preservation potential of original pseudotachylyte microstructures is very short, days to months, in the presence of hydrothermal fluids. As a consequence, pseudotachylytes might be significantly underrepresented in the geological record, and on-fault frictional melting during earthquakes is likely to occur more commonly than generally believed
    Description: ERC CoG NOFEAR 614705
    Description: Published
    Description: e2020GL090020
    Description: 3T. Sorgente sismica
    Description: JCR Journal
    Keywords: Fault ; Earthquakes ; Pseudotachylyte ; Earthquake mechanics
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 4
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    Earthquake lubrication and healing explained by amorphous nanosilica (2020)
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    Details
    Publication Date: 2021-06-30
    Description: During earthquake propagation, geologic faults lose their strength, then strengthen as slip slows and stops. Many slip-weakening mechanisms are active in the upper-mid crust, but healing is not always well-explained. Here we show that the distinct structure and rate-dependent properties of amorphous nanopowder (not silica gel) formed by grinding of quartz can cause extreme strength loss at high slip rates. We propose a weakening and related strengthening mechanism that may act throughout the quartz-bearing continental crust. The action of two slip rate-dependent mechanisms offers a plausible explanation for the observed weakening: thermally-enhanced plasticity, and particulate flow aided by hydrodynamic lubrication. Rapid cooling of the particles causes rapid strengthening, and inter-particle bonds form at longer timescales. The timescales of these two processes correspond to the timescales of post-seismic healing observed in earthquakes. In natural faults, this nanopowder crystallizes to quartz over 10s–100s years, leaving veins which may be indistinguishable from common quartz veins.
    Description: Published
    Description: 3T. Sorgente sismica
    Description: 2IT. Laboratori analitici e sperimentali
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 5
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    Influence of Effective Stress and Pore Fluid Pressure on Fault Strength and Slip Localization in Carbonate Slip Zones (2021)
    Details
    Publication Date: 2021-07-04
    Description: The presence of pressurized fluids influences the mechanical behavior of faults. To test the roles of normal stress and fluid pressure on shear strength and localization behavior of calcite gouges, we conducted a series of rotary‐shear experiments with pore fluid pressures up to 10.5 MPa and difference between normal stress and fluid pressure up to 11.2 MPa. Calcite gouges were sheared for displacements of 0.3 m to several meters at slip rates of 1 mm/s and 1 m/s. Drainage conditions in experiments were constrained from estimates of the hydraulic diffusivity. Gouges were found to be drained at 1 mm/s, but possibly partially undrained during sliding at 1 m/s. Shear strength obeys an effective‐stress law with an effective‐stress coefficient close to unity with a friction coefficient of ~0.7 that decreases to 0.19 due to dynamic weakening. The degree of comminution and slip localization constrained from experimental microstructures depends on the effective normal stress. Slip localization in calcite gouges does not occur at low effective normal stress. The presence of pore fluids lowers the shear strength of gouges sheared at 1 mm/s and causes an accelerated weakening at 1 m/s compared to dry gouges, possibly due to enhanced subcritical crack growth and intergranular lubrication. Thermal pressurization occurs only after dynamic weakening when friction is generally low and relatively independent of normal stress and therefore unaffected by thermal pressurization. The experimental results are consistent with the view that the presence of pressurized fluid in carbonate‐bearing faults can facilitate earthquake nucleation.
    Description: Key Points: Normal stress and fluid pressure equally affect shear strength of calcite gouges at relatively low effective normal stresses (≤11 MPa). The degree of slip localization in calcite gouges sheared at seismic slip rates increases with effective normal stress. Thermal pressurization has small effect on shear stress as it occurs after change from pressure‐ to temperature‐controlled slip behavior.
    Description: European Research Council http://dx.doi.org/10.13039/501100000781
    Keywords: 551.8 ; rotary‐shear experiments ; calcite gouges ; seismic slip rates ; strain localization ; effective‐pressure law ; microstructures
    Type: article
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    CITATION GEO-LEO
  • 6
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    The effect of water on strain localization in calcite fault gouge sheared at seismic slip rates (2018)
    Details
    Publication Date: 2022-06-06
    Description: Strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating, the earthquake energy budget and the evolution of fault rock microstructure. To assess the nature of strain localization during shearing of calcite fault gouges, high-velocity (vmax ¼ 1 m=s) rotary-shear experiments at normal stresses of 3e20 MPa were conducted under room-dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers. When sheared at 1 m/s, the room-dry gouges showed a prolonged strengthening phase prior to dynamic weakening, whereas the wet gouges weakened nearly instantaneously. Microstructural analysis revealed that a thin (〈600 mm) high-strain layer and through-going principal slip surface (PSS) developed after several centimeters of slip in both dry and wet gouges, and that strain localization at 1 m/s occurred progressively and rapidly. The strain accommodated in the bulk gouge layer did not change significantly with increasing displacement indicating that, once formed, the high-strain layer and PSS accommodated most of the displacement. Thus, a substantial strain gradient is present in the gouge layer. In water-dampened gouges, localization likely occurs during and after dynamic weakening. Our results suggest that natural fault zones in limestone are more prone to rapid dynamic weakening if water is present in the granular slipping zones.
    Description: Published
    Description: 104-117
    Description: 7T. Struttura della Terra e geodinamica
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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