[1] A combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a~100 m wide damage zone exists to the northeast with a~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P-wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter-to decimeter-sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro-scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory-derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault-parallel orientation of microfractures, resulting in the lowest P-wave velocity orientated in fault-perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge-like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro-scale damage. Fault-parallel damage fabrics are consistent with existing models for pulverized-rock generation that predict strong dynamic reductions in fault-normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods.

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.

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=1m/s) rotary-shear experiments at normal stresses of 3–20 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 μm) 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

We investigated the formation mechanisms of rounded clast-cortex aggregates, a composite grain found in the slipping zones of faults hosted in calcite- and clay-rich rocks. The natural aggregates contain a central clast commonly made of host-rock fragments or reworked cataclasite from the slipping zone. The central clasts are surrounded by an outer cortex of calcite or clay grains a few mu m or less in size. In laboratory experiments on calcite gouges using two rotary-shear apparatus we investigated the dependence of clast-cortex aggregate formation on the applied slip rate, normal stress, total displacement and ambient humidity. Clast-cortex aggregates formed at all investigated slip rates (100 mu m/s to 1 m/s) but only at relatively low normal stresses (<= 5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that clast-cortex aggregates in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces

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.

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. Plain Language Summary Tectonic pseudotachylytes are solidified melts produced by rapid sliding of faults during earthquakes. A long-lasting unsolved dispute in earthquake physics regards the abundance of pseudotachylytes in nature and the relevance of frictional melting as a seismic-related process. Although experimental and theoretical arguments indicate that frictional melts are easily generated at seismic deformation conditions, pseudotachylytes are apparently rare in the geological record and are often related to specific structural settings (i.e., water-deficient environments and high shear stresses). Such a discrepancy poses the problem whether pseudotachylytes are rarely generated or only rarely preserved in a recognizable form. Here we investigated the preservation potential of pseudotachylytes in the presence of fluids, by performing long-lasting (18-35 days) hydrothermal tests on fresh pseudotachylytes produced in the laboratory. After all tests, the pseudotachylytes were heavily altered with the generation of microstructures resembling those of very common fault rocks called cataclasites, which are unrelated to frictional melting. We suggest that pseudotachylytes are easily produced during earthquakes but they fade from the geological record in few weeks at most, in the presence of altering fluids percolating along the faults. This implies that frictional melting is a relevant process occurring on faults during earthquakes rupturing crystalline basement rocks.

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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.