Unstable Slip Research Articles

Asperity Failure Control of Stick–Slip Along Brittle Faults

Stick–slips are spontaneous, unstable slip events during which a natural or man-made system transitions from a strong, sticking stage to a weaker, slipping stage. Stick–slips were proposed by Brace and Byerlee (Science 153:990–992, 1966) as the experimental analogue of natural earthquakes. We analyze here the mechanics of stick–slips along brittle faults by conducting laboratory experiments and by modeling the instability mechanics. We performed tens of shear tests along experimental faults made of granite and gabbro that were subjected to normal stresses up to 14.3 MPa and loading velocities of 0.26–617 µm/s. We observed hundreds of spontaneous stick–slips that displayed shear stress drops up to 0.66 MPa and slip-velocities up to 14.1 mm/s. The pre-shear and post-shear fault surface topography were mapped with atomic force microscopy at pixel sizes as low as 0.003 µm2. We attribute the sticking phase to the locking of touching asperities and the slipping phase to the brittle failure of these asperities, and found that the fault asperities are as strong as the inherent strength of the host rock. Based on the experimental observations and analysis, we derived a mechanical model that predicts the relationships between the measured stick–slip properties (stress-drop, duration, and slip-distance) and asperity strength.

Slipped Capital Femoral Epiphysis in Children without Obesity

To evaluate rates and characteristics of slipped capital femoral epiphysis (SCFE) in children who are not obese to prevent missed diagnoses and subsequent complications. A multicenter, retrospective review identified all patients with SCFE from January 1, 2003 to December 31, 2012. Patients were excluded if they received previous surgery at an outside institution, had no recorded height and weight, or had medical co-morbidity associated with increased risk of SCFE. Body mass index (BMI) percentile for age was calculated and categorized for each patient (patients without obesity vs with obesity). In total, 275 patients met inclusion criteria. Average BMI was 91.2 percentile (range: 8.4-99.7). Thirteen percent (34 patients) were considered "normal weight" (BMI 5%-85%), 17% (48 patients) were considered "overweight" (BMI 85%- 95%), and 70% (193 patients) were considered "obese" (BMI >95%). Average BMI percentile was higher in male than female patients (93.2±12.7 vs 88.5±21.4, P=.034). Patients without obesity were older compared with patients with obesity (12.2±1.7 vs 11.7±1.6years, P=.015). Fewer patients without obesity were seen at the hospital in the southwest. The southwest had fewer patients without obesity than the northeast (18.3% vs 36.1%, P=.002). Patients without obesity were more likely to present with a severe slip as graded by Wilson percent displacement (27.2% vs 11.4%, P=.007) and an unstable slip (32.9% vs 14.7%, P=.001). Rates of nonobese SCFE in this study are higher than reported in the previous literature. Normalweight patients with SCFE are more likely to be older, female, and present with a severe and unstable SCFE.

Fluid injection-induced seismicity considering secondary damage and heterogeneity in the surrounding rock

Rock is a heterogeneous material with primary damage and defects, which can greatly affect the mechanical properties of the rock and the slip on a fault. Additionally, slip on a fault can generate secondary damage in the surrounding rock. Therefore, this paper focuses on investigating injection-induced seismicity considering the heterogeneity and secondary damage in surrounding rocks with the FEM-based numerical code COMSOL. First, a heterogeneous model was established using the digital image processing technique, and the elemental microscale parameters were determined through comparison with testing results from a homogeneous model. Subsequently, using the defined damage variable and the rate and state friction law, numerical modeling was performed with the established homogeneous and heterogeneous models while considering the heterogeneity, fluid pressure, and generated secondary damage. The results showed that fluid pressure and heterogeneity can significantly influence injection-induced earthquakes. With increasing fluid pressure, the initial time for shear stress drop decreases, both the stress drop and the area of secondary damage increase, and the probability of unstable slip increases in a homogeneous rock matrix. Compared with the homogeneous numerical model, the heterogeneous model has a reduced time, a higher stress drop, a higher probability of seismicity, and a larger area of secondary damage. In addition, secondary damage is generated at two ends of the fault. The area of the secondary damage zone increases when unstable slip is induced, and the rate increases with slip velocity. The findings in this paper could facilitate better understanding of the mechanisms of fluid injection-induced seismicity and hence may be helpful for predicting, evaluating, and controlling induced seismicity.

Fault Reactivation During Fluid Pressure Oscillations: Transition From Stable to Unstable Slip

AbstractHigh‐pressure fluid injection in deep georeservoirs can induce earthquakes. Recent observations suggest that cyclic injections might trigger less seismicity than monotonic injections. Here, we report triaxial laboratory experiments conducted on faulted quartz‐rich sandstone that provide new insight into the physics of fault‐fluid interactions subjected to cyclic fluid pressure variations. The experiments were performed at 30 and 45 MPa confining pressure, imposing constant or sinusoidal fluid pressure oscillations of amplitudes ranging from 0 to 8 MPa in addition to a far‐field constant loading rate (10−4 and 10−3 mm s−1). The results show that (i) in agreement with the Mohr‐Coulomb theory, faults reactivate at the static friction criterion, which is generally reached at the maximum fluid pressure during oscillations. (ii) Oscillating fluid pressure perturbations promote seismic behavior rather than aseismic slip, and (iii) increasing the oscillation's amplitude enhances the onset of seismic activity along the fault. We demonstrate that this behavior is caused by slip rate variations resulting from the fluid pressure oscillations. Without fluid pressure oscillations, increasing the far‐field loading rate also promotes seismic activity. Our experiments demonstrate that the seismicity intensification due to cyclic fluid injections could be promoted at shallow depth, where confining pressure is relatively low, resulting in large strain rate perturbations.

Delineation of fault segments in mines using seismic source mechanisms and location uncertainty

The identification and quantification of faults or other geological discontinuities is an important task in managing seismic hazard in mines. Unstable slip along these faults may lead to seismic events with fatal or major economic consequences. Current approaches for delineating faults that are employed in mines rely on the interpretation of geological mapping. This mapping, however, may be sparse and can miss structures of potential concern. Clustering techniques are often used to associate seismic events to a common source process, but as previously used make no connection to the underlying physical processes.The Expectation Maximisation Algorithm is used in this study to identify probabilistic kernels representing active segments of faults. This soft assignment clustering method describes the kernels according to the seismic source mechanism, location and location uncertainty of the microseismic events. The method is tested on synthetic data and real data from an underground mine with the aim to delineate a previously unknown structure. Results using synthetic data illustrate how the incorporation of the seismic source mechanism improves the association of events to kernels in the case of scattered events with large location uncertainty. Application of the method to the real data indicates that the results can be interpreted at different levels, a smaller number of kernels provides good and robust description of the overall seismic behaviour, while using more kernels can provide insight into local variations in the location and orientation of faults.The resultant kernels have physical meaning in terms of the location and orientation of the structures. This provides geotechnical engineers at mines with improved tools to identify potentially hazardous areas in the mine and therefore manage these risks.

Modeling of fault slip during hydraulic stimulation in a naturally fractured medium

Assessing the intensity of potential seismicity induced by fault slip is a mandatory task in the development of enhanced geothermal systems. In this paper numerical simulations are performed to investigate the propagation of hydraulic fractures and the slip behavior of existing faults due to fluid injection. The cohesive zone model is used in combination with finite cohesive elements to model the hydraulic fractures and the existing faults while the fault shear strength is assumed to follow the Coulomb friction law. Our focus is on the role of the friction conditions on the fault slip behavior. The simulation results show that faults with small friction coefficients tend to slip at low slip rates while faults with a higher friction coefficient tend to slip at rates that are higher than the unstable slip rate threshold. It is also demonstrated that under specific frictional conditions, the sequential stimulation mechanism of permeability enhancement is possible. The results suggest that a good characterization of the fault frictional conditions is required to successfully predict the fault slip pattern and that lowering the fault friction coefficient could potentially reduce the fault slip rate.

Mechanisms of Rockburst Triggered by Slip and Fracture of Coal–Parting–Coal Structure Discontinuities

The existence of parting in coal can contribute to unstable slip and shear failure, and even rockburst of mining faces and sidewalls. In this work, the rockburst triggered by the slip and fracture of coal–parting–coal structure (CPCS) discontinuities subjected to dynamic stress was visually investigated by numerical simulation, and the stress and displacement as well as the corresponding vibration characteristics surrounding the parting during the rockburst were comprehensively analyzed. Eventually, the rockburst triggering mechanism was summarized. As a field testimony for verifying the numerical results, the “11.20” rockburst in the first parting zone of the 1307 working face of the Zhaolou coal mine (ZCM) was investigated in detail by microseismic (MS) tomography. The following main conclusions were obtained: (1) rockburst may be triggered by the slip and fracture of a CPCS subjected to dynamic stress with a violent ejection of a large number of parting fragments at a high speed. The peak particle velocity (PPV) 10 m/s can be regarded as a critical vibration velocity for rockburst warning; (2) during rockburst, the vibration characteristics of different positions surrounding the parting are closely associated with the coal and rock failure caused by the slip and fracture of the parting; and (3) based on the clustering of many high-energy MS sources in the parting zone combined with a high P-wave velocity, velocity gradient, stress concentration and strain energy, it was fully verified that the “11.20” rockburst was triggered by the slip and fracture of the wedge-shaped parting. Additionally, the sudden and sharp rise of the fault total area (FTA) can be regarded as a precursory sign to warn rockburst. The in situ observations lend credence to the numerical simulations.

Effect of Slip-Weakening Distance on Seismic\u2013Aseismic Slip Patterns

In order to explain the seismic–aseismic slip patterns observed on megathrust faults, numerical simulations were carried out using the quasi-dynamic asperity fault model with the slip-dependent friction and stress-dependent healing. Two friction law parameters, strength and slip-weakening distance, are interpreted in the subduction channel context. The parameters are treated as random fields with specified characteristics. Their distributions define heterogeneities of the interplate frictional coupling. The higher strength regions accumulate stresses, whereas the slip-weakening distance lengths control the stress release rates. The simulation results indicate that the slip-dependent asperity model reproduces key features of real megathrust fault behavior. First, stable and unstable slip movements can occur at the same locations, even if the friction parameters are fixed. Second, two rupture styles, single asperity breaks and wide, smooth, propagating rupture fronts, can be distinguished. The latter style is responsible for large slips near the free surface, where lower fault strengths are expected. The reason for these effects is that slip instabilities depend both on local friction and on the system stiffness, which is related to the slipping area size and distribution of slips. It is also shown that the high-strength interplate patches, such as subducted seamounts, can both promote and restrain large earthquakes, depending on the slip-weakening distance lengths.

Constraints on the Physical Mechanism of Frictional Aging From Nanoindentation

AbstractThe increase in the frictional strength of rocks with the time of quasi‐stationary contact, known as frictional aging, may ultimately determine whether unstable slip (i.e., earthquakes) can nucleate. In spite of its importance, the physical mechanism that underlies frictional aging in rocks is still uncertain. The widely held view is that aging results from an increase in contact area due to asperity creep. Here we show via nanoindentation testing that the hardness and creep rate of quartz are independent of relative humidity from <10−4% to 50%. This contrasts strongly with the standard interpretation of previous friction experiments on quartz tested over a similar humidity range, which reveal an absence of frictional aging for humidity <5%. Our results demonstrate that frictional aging in quartz cannot result from asperity creep and instead argue in favor of other mechanisms, including time‐dependent chemical bond formation or slip‐induced strengthening.

Unstable Slip Pulses and Earthquake Nucleation as a Nonequilibrium First-Order Phase Transition.

The onset of rapid slip along initially quiescent frictional interfaces, the process of "earthquake nucleation," and dissipative spatiotemporal slippage dynamics play important roles in a broad range of physical systems. Here we first show that interfaces described by generic friction laws feature stress-dependent steady-state slip pulse solutions, which are unstable in the quasi-1D approximation of thin elastic bodies. We propose that such unstable slip pulses of linear size L^{*} and characteristic amplitude are "critical nuclei" for rapid slip in a nonequilibrium analogy to equilibrium first-order phase transitions and quantitatively support this idea by dynamical calculations. We then perform 2D numerical calculations that indicate that the nucleation length L^{*} exists also in 2D and that the existence of a fracture mechanics Griffith-like length L_{G}<L^{*} gives rise to a richer phase diagram that features also sustained slip pulses.

Laboratory studies of slip along faults as a physical basis for a new approach to short-term earthquake prediction

The physical effects that may prove useful for developing a new approach to short-term earthquake prediction have been studied in laboratory conditions. In seismology and earthquake foci mechanics, one of the major challenges is searching for indicators of an upcoming seismic event and attempting to reliably record such indicators by available instruments. In this regard, the best result of the laboratory studies of dynamic slip along faults would be the identification of specific macroscopic parameters controlling the deformation process, which are measurable in field. Dynamic stiffness of a fault zone seems to be an appropriate parameter. The recent laboratory experiments have shown that the value of this parameter predetermines the slip mode along the fault (unstable slip, creep, tremor, etc.), and a radical decrease in shear stiffness takes place as the fault zone reaches the metastable state. The effect discovered in the laboratory conditions gives grounds to suggest that changes in the stress-strain state of the fault zone at the final stage of earthquake preparation are detectable from the parameters of microseismic noise in the low-frequency range. Apparently, the noise records during and after the arrival of surface waves from distant earthquakes can provide the best opportunity for determining the parameters characterizing the study area. The wave oscillations with a period of a few dozen seconds have significant amplitudes and duration, which contributes to the excitation of resonance oscillations of the blocks. There are problems requiring additional laboratory experiments: estimating the size of a fault, which predetermines regularities in decreasing of the own frequency of the block-fault system; determining the ratio of the mechanical parameters of the fault in the nucleation zone and on the periphery of the future rupture, etc. Having analyzed the results of experimental studies carried out by other researchers, we conclude that laboratory experiments under normal conditions and low pressures can successfully address a number of fundamental issues on the way to creating a new approach to short-term earthquake prediction. Increasing pressure and temperature to values characteristic of seismogenic depths does not lead to the occurrence of any fundamentally new features in the behavior of the block-fault system at the stage when dynamic slip is being prepared. During slip, friction reduces due to melting, physical and chemical transformations at the micro- and nanoscales and other processes on the slipping surface, but these effects play no role at the stage when dynamic rock failure and the onset of slip are being prepared.

Fundamental aspects of a new micromechanical model of rate and state friction

The foundations of a new micromechanical model of rate and state friction are established. Based on the assumption that the relative sliding of two material surfaces is accommodated by the viscoplastic deformation of junctions in their entire volume, the model parameters are characterized in terms of loading conditions, geometrical contact characteristics and rheological properties of the bulk material. A relationship is established between the state parameter of the friction law and the mean microscopic normal stress exerted on the real contact area. Original results are obtained for the transition from stable to unstable slip regimes and applications to geological faults are considered.

Frictional Properties of Subduction Input Sediments at an Erosive Convergent Continental Margin and Related Controls on Décollement Slip Modes: The Costa Rica Seismogenesis Project

AbstractThe spectrum of slip modes occurring along shallow portions of the plate boundary décollement in subduction zones includes aseismic slip, slow slip, and seismogenic slip. The factors that control slip modes directly influence the hazard potential of subduction zones for generating large‐magnitude earthquakes and tsunamis. We conducted an experimental study of the frictional behavior of subduction input sediments, recovered from two Integrated Ocean Drilling Program expeditions to the erosive subduction margin offshore Costa Rica (Expeditions 334 and 344), employing rotary shear under hydrothermal conditions. The velocity dependence of friction was explored, using simulated gouges prepared from all major lithologies, covering a wide range of conditions representative for the initial stages of subduction. Temperature, effective normal stress, and pore fluid pressure were varied systematically up to 140 °C, 110 MPa, and 120 MPa, respectively. Sliding velocities up to 100μm/s, relevant for earthquake rupture nucleation and slow slip, were investigated. The only sediment type that produced frictional instabilities (i.e., laboratory earthquakes) was the calcareous ooze carried by the incoming Cocos Plate, which by virtue of its slip‐weakening behavior is also a likely candidate for triggering slow slip events. We evaluate this mechanism of producing unstable slip and consider alternatives. Therefore, locking and unlocking of plate boundary megathrusts are not only related to variations in pore fluid pressure but may also depend on the presence of pelagic carbonate‐rich lithologies. Subduction systems containing such input are likely low latitude, where extensive deposition of carbonates takes place above the carbonate compensation depth.

The transition from steady frictional sliding to inertia-dominated instability with rate and state friction

Unstable frictional slip motions are investigated with a rate and state friction law across the transitions from stable, quasi–static slip to dynamic, stick–slip motion, and finally to, inertia dominated quasi–harmonic vibration. We use a novel numerical method to capture the full dynamics and investigate the roles of inertial and quasistatic factors of the critical stiffness defining the transition to instability, Kc. Our simulations confirm theoretical estimates of Kc, which is dependent on mass and velocity. Furthermore, we show that unstable slip motion has two distinct dynamic regimes with characteristic limit cycles: (i) stick–slip motions in the quasi–static (slowly loaded) regime and (ii) quasi–harmonic oscillations in the dynamic (rapidly loaded) regime. Simulation results show that the regimes are divided by the dynamic frictional instability coefficient, η = MV2/σaDc and stiffness of the system K. The quasi–static regime is governed by the ratio K/Kc and both the period and magnitude of stick–slip cycles decrease with increasing loading rate. In the dynamic regime, slip occurs in harmonic limit cycles, the frequency of which increases with loading velocity to a limit set by the natural frequency of the system. Our results illuminate the origin of the broad spectrum of slip behaviors observed for systems ranging from manufacturing equipment to automobiles and tectonic faults, with particular focus on the role of elasto–frictional coupling in dictating the transition from slow slip to dynamic instability. We highlight distinct characteristics of friction–induced slip motions (stick–slip and friction–induced vibration) and show that the dynamic frictional instability coefficient (η) is a key parameter that both defines the potential for instability and determines the dynamic characteristics of instability.

Frictional Behavior of Input Sediments to the Hikurangi Trench, New Zealand

AbstractThe Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes.

Study of the Nankai seismogenic fault using dynamic wave propagation modelling of digital rock from the Nobeoka Fault

To understand the characteristics of the Nankai seismogenic fault in the plate convergent margin, we calculated the P- and S-wave velocities (VP and VS) of digital rock models constructed from core samples of an ancient plate boundary fault at Nobeoka, Kyushu Island, Japan. We first constructed 3D digital rock models from microcomputed tomography images and identified their heterogeneous textures such as cracks and veins. We replaced the cracks and veins with air, water, quartz, calcite and other materials with different bulk and shear moduli. Using the Rotated Staggered Grid Finite-Difference Method, we performed dynamic wave propagation simulations and quantified the effective VP, VS and the ratio of VP to VS (VP/VS) of the 3D digital rock models with different crack-filling minerals. Our results demonstrate that the water-saturated cracks considerably decreased the seismic velocity and increased VP/VS. The VP/VS of the quartz-filled rock model was lower than that in the water-saturated case and in the calcite-filled rock model. By comparing the elastic properties derived from the digital rock models with the seismic velocities (e.g. VP and VP/VS) around the seismogenic fault estimated from field seismic data, we characterised the evolution process of the deep seismogenic fault. The high VP/VS and low VP observed at the transition from aseismic to coseismic regimes in the Nankai Trough can be explained by open cracks (or fractures), while the low VP/VS and high VP observed at the deeper coseismic fault zone suggests quartz-filled cracks. The quartz-rich fault zone characterised as low VP/VS and high VP in this study could partially relate to the coseismic behaviour as suggested by previous studies, because quartz exhibits slip-weakening behaviour (i.e. unstable coseismic slip).

Implications of basal micro-earthquakes and tremor for ice stream mechanics: Stick-slip basal sliding and till erosion

The Whillans Ice Plain (WIP) is unique among Antarctic ice streams because it moves by stick-slip. The conditions allowing stick-slip and its importance in controlling ice dynamics remain uncertain. Local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick-slip and a window into current basal conditions, but the spatial extent and importance of this basal seismicity are unknown. We analyze data from a 2010–2011 ice-plain-wide seismic and GPS network to show that basal micro-seismicity correlates with large-scale patterns in ice stream slip behavior: Basal seismicity is common where the ice moves the least between unstable slip events, with small discrete basal micro-earthquakes happening within 10s of km of the central stick-slip nucleation area and emergent basal tremor occurring downstream of this area. Basal seismicity is largely absent in surrounding areas, where inter-slip creep rates are high. The large seismically active area suggests that a frictional sliding law that can accommodate stick-slip may be appropriate for ice stream beds on regional scales. Variability in seismic behavior over inter-station distances of 1–10 km indicates heterogeneity in local bed conditions and frictional complexity. WIP unstable slips may nucleate when stick-slip basal earthquake patches fail over a large area. We present a conceptual model in which basal seismicity results from slip-weakening frictional failure of over-consolidated till as it is eroded and mobilized into deforming till.

Pore Fluid Pressure Development in Compacting Fault Gouge in Theory, Experiments, and Nature

AbstractThe strength of fault zones is strongly dependent on pore fluid pressures within them. Moreover, transient changes in pore fluid pressure can lead to a variety of slip behavior from creep to unstable slip manifested as earthquakes or slow slip events. The frictional properties of low‐permeability fault gouge in nature and experiment can be affected by pore fluid pressure development through compaction within the gouge layer, even when the boundaries are drained. Here the conditions under which significant pore fluid pressures develop are analyzed analytically, numerically, and experimentally. Friction experiments on low‐permeability fault gouge at different sliding velocities show progressive weakening as slip rate is increased, indicating that faster experiments are incapable of draining the pore fluid pressure produced by compaction. Experiments are used to constrain the evolution of the permeability and pore volume needed for numerical modeling of pore fluid pressure build up. The numerical results are in good agreement with the experiments, indicating that the principal physical processes have been considered. The model is used to analyze the effect of pore fluid pressure transients on the determination of the frictional properties, illustrating that intrinsic velocity‐strengthening behavior can appear velocity weakening if pore fluid pressure is not given sufficient time to equilibrate. The results illustrate that care must be taken when measuring experimentally the frictional characteristics of low‐permeability fault gouge. The contribution of compaction‐induced pore fluid pressurization leading to weakening of natural faults is considered. Cyclic pressurization of pore fluid within fault gouge during successive earthquakes on larger faults may reset porosity and hence the capacity for compaction weakening.

Seismogenic Potential of a Gouge‐filled Fault and the Criterion for Its Slip Stability: Constraints From a Microphysical Model

AbstractPhysical constraints for the parameters of the rate‐and‐state friction (RSF) laws have been mostly lacking. We presented such constraints based on a microphysical model and demonstrated the general applicability to granular fault gouges deforming under hydrothermal conditions in a companion paper. In this paper, we examine the transition velocities for contrasting frictional behavior (i.e., strengthening to weakening and vice versa) and the slip stability of the model. The model predicts a steady state friction coefficient that increases with slip rate at very low and high slip rates and decreases in between. This allows the transition velocities to be theoretically obtained and the unstable slip regime (Vs→w &lt; V &lt; Vw→s) to be defined. In a spring‐slider configuration, linear perturbation analysis provides analytical expressions of the critical stiffness (Kc) below which unstable slip occurs and of the critical recurrence wavelength (Wc) and static stress drop (Δμs) associated with self‐sustained oscillations or stick slips. Numerical implementation of the model predicts frictional behavior that exhibits consecutive transitions from stable sliding, via periodic oscillations, to unstable stick slips with decreasing elastic stiffness or loading rate, and gives Kc, Wc, Δμs, Vs→w, and Vw→s values that are consistent with the analytical predictions. General scaling relations of these parameters given by the model are consistent with previous interpretations in the context of RSF laws and agree well with previous experiments, testifying to high validity. From these physics‐based expressions that allow a more reliable extrapolation to natural conditions, we discuss the seismological implications for natural faults and present topics for future work.

Seismic potential of weak, near-surface faults revealed at plate tectonic slip rates.

The near-surface areas of major faults commonly contain weak, phyllosilicate minerals, which, based on laboratory friction measurements, are assumed to creep stably. However, it is now known that shallow faults can experience tens of meters of earthquake slip and also host slow and transient slip events. Laboratory experiments are generally performed at least two orders of magnitude faster than plate tectonic speeds, which are the natural driving conditions for major faults; the absence of experimental data for natural driving rates represents a critical knowledge gap. We use laboratory friction experiments on natural fault zone samples at driving rates of centimeters per year to demonstrate that there is abundant evidence of unstable slip behavior that was not previously predicted. Specifically, weak clay-rich fault samples generate slow slip events (SSEs) and have frictional properties favorable for earthquake rupture. Our work explains growing field observations of shallow SSE and surface-breaking earthquake slip, and predicts that such phenomena should be more widely expected.