Frictional slip instability, resulting in intermittent “stick‐slip” rather than continuous sliding, is a phenomenon that depends on the frictional properties of the sliding area and the stiffness of ...the surrounding material. For geomechanical rock and sediment testing, the stiffness of the testing apparatus partially controls the occurrence of stick‐slip sliding behavior. Under a wide range of conditions, we directly measured the shear loading stiffness of five direct‐shear apparatuses in the Marine Geotechnics laboratory at MARUM, University of Bremen. Under constant normal stress, the shear loading and unloading curves are non‐linear and exhibit significant hysteresis. Shear stiffness values generally increase with increasing normal and shear stresses. Absolute values of stiffness as well as their dependency on shear and normal stress vary amongst the apparatuses despite the same basic apparatus design. For the application of stiffness concepts to stick‐slip sliding in the Earth, for example, earthquakes, the most appropriate stiffness value is obtained at a shear stress value comparable to the sample strength, and measured during stress unloading. Well‐characterized apparatus stiffness under a wide range of testing conditions is recommended to optimize analyses of laboratory friction data.
Plain Language Summary
When sliding occurs, it may occur smoothly or in a “jerky” motion called stick‐slip. Whether or not stick‐slip sliding occurs depends on the friction of the sliding surfaces and also the stiffness or “springiness” of the material around the sliding surface. In laboratory experiments that replicate sliding in rocks, stick‐slip represents earthquakes, the sliding surface represents a fault and the testing machine represents the rock mass around the fault. Under a wide range of conditions, we directly measured the stiffness of five direct‐shear apparatuses used in the Marine Geotechnics laboratory at MARUM, University of Bremen. The shear stress versus apparatus distortion curves are non‐linear and exhibit differences between loading and unloading. Machine stiffness is generally larger when the machine experiences larger pressures. Stiffness varies amongst the apparatuses despite the same basic apparatus design. For laboratory experiments designed to study fault motion and earthquakes, our data show that it is important to carefully identify the machine stiffness under the exact conditions of the experiment in order to best translate the results to the Earth.
Key Points
We systematically measured the stiffness of five direct‐shear apparatuses in the Marine Geotechnics laboratory at the University of Bremen
Apparatus stiffness depends on shear stress and normal stress and exhibits loading path hysteresis, but does not depend on loading rate
Laboratory studies of frictional stick‐slip behavior should consider apparatus stiffness variations as a function of testing conditions
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.
The evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip, nature of earthquake stress drop, and earthquake nucleation. Laboratory‐based rate‐ and ...state‐dependent friction (RSF) laws can describe changes in fault strength during slip, but the connections between fault strength and the mechanisms that dictate the mode of failure, from aseismic creep to earthquake rupture, remain poorly understood. The empirical nature of RSF laws remains a drawback to their application in nature. Here we analyze an extensive data set of friction constitutive parameters with the goal of illuminating the microphysical processes controlling RSF. We document robust relationships between: (1) the initial value of sliding (or kinetic) friction, (2) RSF parameters, and (3) the time rates of frictional strengthening (aging). We derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by: (1) an activation energy that controls the rate of asperity growth by plastic creep, and (2) an inverse relationship between material hardness and the activation volume of plastic deformation. Collectively, our results illuminate the physics expressed by the RSF parameters, and which describe the absolute value of frictional strength and its dependence on time and slip rate. Moreover, we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain‐scale asperity contacts.
Key Points:
Base level friction, and friction velocity‐ and time‐dependence all correlate
Macroscopic fault friction for is controlled by grain‐scale plastic deformation processes
Real area of contact, activation energy, hardness, activation volume are important
In addition to the velocity dependence of friction, slip dependence may play a major role before and during earthquake slip in fault zones. We performed laboratory friction experiments on simulated ...fault gouges, measuring both the velocity and slip dependence of friction in velocity step tests. The pure velocity‐dependent component of friction measured over short displacements shows both velocity strengthening and velocity weakening friction, depending on the amount of slip considered. However, we observe that increases in sliding velocity can induce slip weakening behavior which overwhelms the velocity dependence resulting in large overall weakening, especially at rates > 1 µm/s. On natural tectonic faults, this suggests that a velocity perturbation, such as coseismic rupture propagating onto a fault patch, could induce instability via large slip weakening. Therefore, a fault which is experiencing a transient slip or slow earthquakes may be more easily induced to slip coseismically if a dynamic rupture from large earthquake propagates onto the fault.
Key Points
Slip weakening can be large compared to velocity weakening
Slip weakening can be induced by velocity perturbations
Slow slip faults may be more prone to instability by slip weakening slip
The Nankai Trough is an exceptionally well-studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip. Outcrop studies of exhumed analogues ...for the modern subduction thrust, as well as seismic reflection images of the active subduction zone, suggest that the basaltic basement participates in plate-boundary shearing at depths of a few km and greater. We 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. The altered basalt exhibits predominantly velocity-strengthening frictional behavior, indicating a tendency for stable slip. For bare surface experiments, few instances of velocity-weakening friction occur and are restricted to slip rates of <10−6 m/s. Thin sections and XRD analyses of the starting material indicate that the velocity-strengthening behavior is likely associated with the presence of clay minerals, mostly Mg-smectite, which coat the rims of larger, stronger grains. Based our friction data, we suggest that in addition to creep the altered Nankai basalts may also allow the possibility of slow slip events for two reasons: (1) velocity-weakening friction may be expected at low slip rates, but velocity-strengthening at higher rates will damp any potential slip instabilities; and (2) based on a critical stiffness criterion for accelerating (stable) slip, SSEs are capable of nucleating in the altered Nankai basalt despite velocity-strengthening friction. In light of these observations, and given the depth at which altered basement appears to be incorporated along the plate interface, we raise the possibility that some shallow slow slip events in the Nankai accretionary prism could originate from shearing in altered basalt.
•We sheared altered basaltic upper basement from the modern Nankai Trough.•Alteration to clay minerals on rims of larger, stronger grains controls friction.•Altered basalt exhibits mostly velocity-strengthening frictional behavior.•Altered Nankai basalts may produce slow slip events at a few km depth.
•Sediment lithification is controlled in laboratory experiments.•Sufficient lithification enables velocity-weakening friction.•Velocity-weakening requires advanced porosity loss and cohesive ...strengthening.•Lithified samples show less damaged shear surfaces compared to powders.
Regarding the occurrence of seismicity on major plate-boundary fault zones, one leading hypothesis is that the processes of lithification is responsible transforming loose, unconsolidated sediment that does not host earthquake nucleation into the frictionally unstable rocks that inhabit the seismogenic zone. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis. However, systematically quantifying frictional behavior as a function of lithification remains a challenge. Here, we simulate the lithification process in the laboratory by consolidating mixtures of halite and shale powders with halite-saturated brine, which we then desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We quantify lithification by: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. We observe that powdered samples of each halite-shale proportion exhibit predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Analysis of the individual rate-dependent friction parameters shows that the occurrence of velocity weakening is due to relatively low values of a for lithified samples. Larger velocity weakening is associated with cohesion of >∼1 MPa, and porosity reduction of >∼50 vol%. Microstructural images reveal that the shear surfaces for powders tend to exhibit small cracks not seen on the lithified sample shear surfaces. Our results suggest that lithification via cementation and porosity loss can facilitate slip instability, supporting the lithification hypothesis for seismogenic slip.
Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is ...especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1-2 years after the 2011
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9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior.
Determining the friction during an earthquake is required to understand when and where earthquakes occur.
Drilling into the Tohoku fault showed that friction during the earthquake was low.
Dynamic friction during the earthquake was lower than static friction.
Complete stress drop is possible, and stress reversal is plausible.
The role of phyllosilicate fabrics in fault gouge is a poorly understood component of the mechanical and hydrologic behavior of brittle fault zones. We present 90 fabric intensity measurements using ...X‐ray texture goniometry on 22 natural clay‐rich fault gouges from low‐angle detachment faults (Death Valley area detachments, California; Ruby Mountains, Nevada; West Salton Detachment Fault, California) and the Peramola thrust in NE Spain. Natural fault gouges have uniformly weak clay fabrics (multiples of a random distribution (MRD) = 1.7–4.5, average MRD = 2.6) when compared to phyllosilicate‐rich rocks found in other geologic settings. Clay fabric intensities in natural gouges do not vary significantly either as a function of tectonic environment or of dominant clay mineralogy in the gouge. We compare these natural samples with 69 phyllosilicate fabric intensities measured in laboratory experiments on synthetic clay‐quartz mixtures. Clay fabric intensities from laboratory samples are similar to those in natural gouges (MRD = 1.7–4.6), but increase systematically with increasing shear strain and normal stress. Total phyllosilicate content does not significantly affect clay fabric intensity. Shear strain is important for developing stronger fabrics; samples subjected solely to compression exhibit uniformly weak fabrics (MRD = 1.6–1.8) even when compressed at high normal stresses (150 MPa). The weak fabrics found in natural fault gouge indicate that if anisotropic and overall low fault zone permeability allow elevated pore fluid pressures and fault weakening, this anisotropy must be a transient feature that is not preserved. Our data also reinforce the idea that clay fabric development in sedimentary rocks is primarily a function of authigenic mineral growth and not of compaction‐induced particle rotation.
The 2011 Tohoku-Oki earthquake demonstrated that the shallowest reaches of plate boundary subduction megathrusts can host substantial coseismic slip that generates large and destructive tsunamis, ...contrary to the common assumption that the frictional properties of unconsolidated clay-rich sediments at depths less than ∼5km should inhibit rupture. We report on laboratory shearing experiments at low sliding velocities (<1mm/s) using borehole samples recovered during IODP Expedition 343 (JFAST), spanning the plate-boundary décollement within the region of large coseismic slip during the Tohoku earthquake. We show that at sub-seismic slip rates the fault is weak (sliding friction μs=0.2–0.26), in contrast to the much stronger wall rocks (μs>∼0.5). The fault is weak due to elevated smectite clay content and is frictionally similar to a pelagic clay layer of similar composition. The higher cohesion of intact wall rock samples coupled with their higher amorphous silica content suggests that the wall rock is stronger due to diagenetic cementation and low clay content. Our measurements also show that the strongly developed in-situ fabric in the fault zone does not contribute to its frictional weakness, but does lead to a near-cohesionless fault zone, which may facilitate rupture propagation by reducing shear strength and surface energy at the tip of the rupture front. We suggest that the shallow rupture and large coseismic slip during the 2011 Tohoku earthquake was facilitated by a weak and cohesionless fault combined with strong wall rocks that drive localized deformation within a narrow zone.
•We perform shearing experiments on borehole samples from the Tohoku earthquake zone.•The fault zone is weak and exhibits partially velocity weakening slip behavior.•Large contrast in smectite content and strength between the fault zone and wall rock.•The fault is cohesionless, reducing fracture energy and favoring earthquake rupture.
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