Unstable Slip Research Articles

Off-fault deformation feedback and strain localization precursor during laboratory earthquakes

Recent large-scale seismological observations have shown that off-fault strain localization and foreshock migration could serve as an early warning of an impending earthquake. However, this process is still largely unknown. In this study, state-of-the-art friction experiments were conducted in a oil-confined biaxial shear apparatus to investigate the link between stick-slip nucleation and off-fault deformation. Our findings indicate that there is a direct link between stick-slip nucleation and off-fault deformation, provided that the fault is conditionally unstable (a − b < 0). Inelastic off-fault deformation may trigger unstable slip by decreasing the stiffness of the surrounding rock volume, which favors earthquake nucleation. Additionally, the study presents laboratory observation of precursory strain localization around a fault during stick-slip cycles. These findings suggest that volumetric deformation processes could be a main factor in the nucleation of large ruptures and strain localization could be a reliable harbinger of large earthquakes.

Fault slip and seismic source response characteristics under induced stress wave

As coal mining depth and structural complexity increase, the failure severity of mining-induced fault activation leading to rock bursts escalates. Fault fracture zones, consisting of various secondary structures, are essential for understanding tectonic evolution and seismic wave propagation. The interaction between mining-induced stress waves and in-situ stress complicates the dynamics and activation markers of fault fracture zones. This paper aims to clarify the patterns of stick–slip instability and stress evolution in these zones under coal mining-induced stress waves. Using theoretical models and continuous-discrete coupling simulations, the study explores the dynamic response of fault fracture zones in the meta-instability stage, the micro-scale fracture force chain structure, and the distribution and synergistic instability of fault plane seismic sources. The research shows that fault fracture zones changes stress wave propagation path and energy redistribution due to its heterogeneity leads to multiple times decomposition of wave field. Minor delamination between the hanging wall and footwall can trigger unstable slip. Tensional seismic sources with extensive failure drive fault slip. Dynamic stress waves disrupt force chains near seismic sources, increasing hazard levels. Static in-situ stress affects shear seismic sources by influencing porosity and crack angles. Seismic sources along the fault strike transition from divergent to concentrated, trending horizontally. In the meta-instability stage, the initial fracture area locks, with strong structures causing cohesive fractures in the middle of the fault zone, releasing strain energy and forming a sudden increase in slip intensity. These findings provide crucial insights into fault fracture-development-activation markers, fault region stress deformation evolution mechanisms, and risk control of fault-induced rock bursts.

Frictional Properties of Simulated Fault Gouges Subject to Normal Stress Oscillation and Implications for Induced Seismicity

AbstractUnder critical conditions where experimental fault slip exhibits self‐sustained oscillation, effects of normal stress oscillation (NSO) on fault strength and stability remain poorly understood, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct‐shear testing to investigate the frictional behavior of a synthetic, slightly velocity‐weakening (SVW) fault gouge (characterized by self‐sustained oscillation under quasi‐static shear loading), when subjected to NSO at different amplitudes (5%–20% of 5 MPa) and frequencies (0.001–1 Hz). During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be triggered at NSO frequencies ranging from 0.03 to 0.1 Hz and amplitudes exceeding 5%. Interestingly, an amplified shear stress drop and weakening effect were observed when the NSO frequency fell in 0.05–0.1 Hz. Analysis of transmitted ultrasonic waves in tests on the SVW gouge revealed fault dilation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the “Chen‐Niemeijer‐Spiers [CNS]” model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiments on the SVW gouge was reproduced, suggesting an approach for modeling fault instability under upper crustal (SVW) conditions where normal stress is perturbed by subsurface operations, such as periodic gas storage stimulation of reservoir formations.

Inertial flow-induced fluid pressurization enhances the reactivation of rate-and-state faults

We reveal that inertial flow can introduce additional pressure perturbations which accelerate the instability of critically stressed faults. We integrate a poroelastic spring-slider model with rate-and-state friction (RSF), a nonlinear flow model characterized by the improved Izbash equation, and a universal visco-inertial permeability model. The effect of inertial flow depends on the fracture network properties, proximity to faults, and injection parameters. Significant inertial flow results in higher pressure magnitudes and pressurization rates, causing notable differences in predictions between linear and nonlinear flow models, as well as aging and slip laws. The impact of inertial flow is particularly pronounced within a specific permeability range and is enhanced by high-rate injections. The fault reactivation is delayed due to fault strengthening with RSF in response to increased pressurization rates. Conversely, the presence of inertial flow enhances fault reactivation and promotes unstable slip behaviors. Furthermore, the results demonstrate that cyclic injection increases the critical stiffness of faults without an adequate release of pore pressure in each cycle. The time interval of the cyclic injection is prolonged as inertial flow generates higher pressure levels. This study highlights the importance of inertial flow in the stability analysis of critically stressed faults.

Power law dependence of unstable slip velocity on decreasing shear loading stiffness

Shear loading stiffness plays a critical role in conditioning the stability of slip on reactivated faults. However, a relationship linking peak slip velocities and shear loading stiffness is lacking. To explore this, we shear granite faults in double direct shear with shear loading stiffnesses spanning two orders to define the effects of shear loading stiffness in conditioning the transition from stable to unstable slip. Our results show that peak slip velocity and acceleration decrease as power law relationships with respect to the shear loading stiffness ratio, with identical exponent, and with a linear relationship between the peak slip velocity and acceleration. The maximum acceleration occurs during the velocity weakening process that limits the peak slip velocity. The power law exponent increases linearly with increasing normal stress. Experimental results also indicate that slip magnitude, stress drop and recurrence time of unstable slip events increase, and slip durations decrease with decreasing stiffness ratios. The spans of limit cycles of velocity versus shear stress decrease, and their shapes evolve from triangular to semicircular with increasing loading stiffness ratio. Stress drops mostly occur during deceleration. The deceleration phase dominates the unstable slip duration that decreases as peak slip velocity increases. Our results indicate that the average stress drop rates over a slip duration increase as a power law relationship with reducing shear loading stiffness, which also contributes to a lower shear loading stiffness producing increased slip velocities and accelerations. Our findings highlight that loading stiffness ratio is an underlying mechanism defining unstable slip behaviors with the normal stress merely conditioning the exponent. The present relationship of unstable slip velocity with shear loading stiffness suggests a way to evaluate hazard of an impending instability event based on the initial shear loading stiffness ratio that can be calculated through the linear-elastic behaviors at the early quasi-static phase.

Kaolin from Poletaevskoye deposit (Chelyabinsk region, Russia) in ceramics production

The paper presents the results of a study of the properties of Poletaevsky kaolin with the aim of using it in the ceramic industry. Based on the results of determining the chemical, mineral and granulometric composition. The mineral composition of Poletaevsky kaolin is kaolinite 80–85 wt %, muscovite and illite 10–15 wt %, quartz up to 10–15 wt %. It has been established that the Hinckley index for Poletaevsky kaolin is 0.68, compared to 1.38 for Glukhovetsky one. The ratio of highly ordered kaolinite to low ordered kaolinite for Poletaevsky kaolin is 22 : 78, for Glukhovetsky kaolin the ratio of highly ordered kaolinite to low-ordered kaolinite is 57 : 43. It was established that Poletaevsky kaolin can be used in the production of ceramic materials by plastic or semi-dry molding. The determined low content of high-ordered kaolinite and the insignificant content of particles less than 2 μm according to the results of laser diffraction analysis can lead to the production of an unstable slip. When firing samples of Poletaevsky kaolin, a minimum water absorption of 5.5 % was achieved at a firing temperature of 1450 °C. In this case, the resulting maximum apparent density was 2.17 g/cm3, the maximum mullite content was 38 %, and the maximum compressive strength was 32 MPa.

Characterization of seismicity from different glacial bed types: machine learning classification of laboratory stick-slip acoustic emissions

Abstract Subglacial seismicity presents the opportunity to monitor inaccessible glacial beds at the epicentral location and time. Glaciers can be underlain by rock or till, a first order control on bed mechanics. Velocity-weakening, necessary for unstable slip, has been shown for each bed type, but is much stronger and evolves over more than an order of magnitude longer distances for till beds. Utilizing a de-stiffened double direct shear apparatus, we found conditions for instability at freezing temperatures and high slip rates for both bed types. During stick–slip stress-drops, we recorded acoustic emissions with piezoelectric transducers frozen into the ice. The two populations of event waveforms appear visually similar and overlap in their statistical features. We implemented a suite of supervised machine learning algorithms to classify the bed type of recorded waveforms and spectra, with prediction accuracy between 65–80%. The Random Forest Classifier is interpretable, showing the importance of initial oscillation peaks and higher frequency energy. Till beds have generally higher friction and resulting stress-drops, with more impulsive first arrivals and more high frequency content compared to rock emissions, but rock beds can produce many till-like events. Seismic signatures could enhance interpretation of bed conditions and mechanics from subglacial seismicity.

Fast Assessment Method for Transient Voltage Stability of Photovoltaic Receiving‐End Grid

The incorporation of renewable energy on a broad scale into power grids increases the complexity to the issue of transient voltage stability in power systems. This research presents a rapid evaluation technique for assessing the stability of voltage fluctuations in power grids that receive electricity from photovoltaic sources. At first, a receiving‐end system model was developed, which includes photovoltaics, and an alternative circuit of the virtual induction motor (IM) is obtained by utilizing the Thevenin equivalent. Second, the torque balance equation and the Kirchhoff voltage equation are interconnected to determine the unstable slip of the IM following a malfunction. Finally, by merging the unstable slip with the conventional transient stability discriminant index, the effects of different photovoltaic outputs, IM ratios, and system contact impedances on transient voltage stability are investigated. The proposed method avoids the computational burden of solving the differential‐algebraic equations describing the complex transient processes of the IM. It also obviates the necessity of iteratively modifying the receiving load or fault clearance time in the simulation platform to achieve the constrained stability condition of the system’s transient voltage. The transient stabilization of voltage results is efficiently obtained by directly solving algebraic equations.

Induced pattern of high and steep slope landslides under rainfall conditions

Abstract Owing to the deep concave mining in Bayan Obo stope, the slope angle is steep, the terrain is high, the outcrop width of the crushing belt is large, the stability of many slopes is poor, and there are potential sliding surfaces. In this paper, through on-site investigation and sampling, the main factors affecting the landslide of the high and steep slopes of Bayan Obo are analyzed. Uniaxial compression tests were carried out to obtain the mechanical parameters of dolomite and slate. With the help of the three-dimensional digital speckle system, the whole process of slope landslide under rainfall conditions was studied through similar simulation and numerical simulation experiments. The influence of rainfall on the slope of Bayan Obo and the induced pattern of landslide were revealed. The experimental results show that rainfall is the key to inducing instability, the slippage at the edge of the slope is obvious, and there is seepage in the depths but the effect is not significant. The landslide can be roughly divided into the damage accumulation stage; the deformation development and expansion stage; and the unstable slip stage.

Frictional and poromechanical properties of the Delaware Mountain Group: Insights into induced seismicity in the Delaware Basin

The Delaware Basin in West Texas and Southeast New Mexico has experienced a proliferation in seismic activity since 2016. The seismic activity is primarily due to subsurface injection of wastewater into shallow and deep reservoirs. However, the precise mechanisms connecting fluid injection to seismic activity are not well understood. To shed light on these processes, we measured rate-state friction and poromechanical properties of rocks sampled from the Delaware Mountain Group (DMG) at pressures and stresses representative of in-situ conditions. Experiments were conducted inside a pressure vessel and loaded at a true-triaxial stress state. The samples exhibit velocity-strengthening behavior and transition to velocity-neutral behavior with increasing slip. We also measure frictional healing and demonstrate that the healing rates are consistent with those measured from quartz-feldspathic-rich rocks. Experiments were conducted under a constant pore-pressure boundary condition where transient changes in pore-volume were compensated by modulating the fluid volume within the fault. Our data show that a step increase in sliding velocity leads to dilatancy and causes an influx of fluid into the fault. In contrast, a step decrease in sliding velocity causes the fault zone to compact and reduces fluid flow into the fault. Data show that changes in flow rate scale systematically with fault slip velocity. Broadly speaking, the frictional and poromechanical data indicate that shallow faults within the DMG should favor aseismic creep as opposed to unstable slip. Our data are consistent with elastic dislocation models of fault slip and InSAR data, indicating that faulting in the DMG is predominantly aseismic. The lack of velocity weakening behavior observed in our samples indicates that heterogenous frictional properties are needed to explain the co-existence of both aseismic and seismic slip in the DMG.

Friction of hornblende and tremolite at hydrothermal conditions with low effective normal stress and high pore pressure

In order to understand the friction properties of hornblende and tremolite as common minerals in deep fault zones, we used natural hornblende and tremolite as the simulated gouge material to perform shearing experiments at temperatures from 303 to 607 °C under an effective normal stress of 20 MPa, a pore pressure of 100 MPa, and displacement rates of 0.0488–1.22 μm/s. The steady-state friction coefficient of hornblende ranges from ~0.76 to ~0.88. The corresponding steady-state rate dependence is velocity-weakening (negative a-b) at temperatures from 303 to 403 °C, which transitions to velocity-strengthening (positive a-b) at temperatures above 500 °C. Steady-state friction coefficients of tremolite exhibited values between 0.70 and 0.76. The corresponding steady-state rate dependence is velocity-strengthening (positive a-b) for all the tested temperatures. The direct rate effect (a value) for hornblende roughly increases with temperature, while the corresponding evolution effect (b value) essentially declines with temperature. The direct rate effect (a value) of tremolite increases with increasing temperature for temperatures above 400 °C, with a quite minor evolution effect (b value) that indicates viscous-like slip behavior at all the tested temperatures. There are a large number of remnants of large particles distributed pervasively over sheared gouges, and significant particle size reduction is only observed in localized shear zones. Pervasive cataclastic flow seems to dominate the shear deformation of hornblende for temperatures above 400 °C and that of tremolite for all the tested temperatures. The velocity-weakening behavior of hornblende at low effective normal stress may apply to amphibole-bearing fault rocks in subduction zones where unstable slip events can occur at a narrow temperature range of 300–400 °C. In contrast, the very strong velocity-strengthening observed for tremolite provides a favorable condition for blocking the nucleation of unstable slip events.

Fabric controls fault stability in serpentinite gouges

SUMMARY Serpentinites are polymineralic rocks distributed almost ubiquitously across the globe in active tectonic regions. Magnetite-rich serpentinites are found in the low-strain domains of serpentinite shear zones, which act as potential sites of nucleation of unstable slip. To assess the potential of earthquake nucleation in these materials, we investigate the link between mechanical properties and fabric of these rocks through a suite of laboratory shear experiments. Our experiments were done at room temperature and cover a range of normal stress and slip velocity from 25 to 100 MPa and 0.3 to 300 µm s−1, respectively. We show that magnetite-rich serpentinites are ideal materials since they display strong sensitivity to the loading rate and are susceptible to nucleation of unstable slip, especially at low forcing slip velocities. We also aim at the integration of mechanical and microstructural results to describe the underlying mechanisms that produce the macroscopic behaviour. We show that mineralogical composition and mineral structure dictates the coexistence of two deformation mechanisms leading to stable and unstable slip. The weakness of phyllosilicates allows for creep during the interseismic phase of the laboratory seismic cycle while favouring the restoration of a load-bearing granular framework, responsible of the nucleation of unstable events. During dynamic slip, fault zone shear fabric determines the mode of slip, producing either asymmetric or Gaussian slip time functions for either fast or slow events. We report rate/state friction parameters and integrate our mechanical data with microstructural observations to shed light on the mechanisms dictating the complexity of laboratory earthquakes. We show that mineralogical and fabric heterogeneities control fault slip behaviour.

Scaling of Repeating Earthquakes at the Transition From Aseismic to Seismic Slip

AbstractSome observations of repeating earthquakes show an unusual, non‐self‐similar scaling between seismic moment and corner frequency, a source property related to rupture size. These observations have been mostly reported in regions at the transition from stable to unstable slip, in geothermal reservoirs and subduction zones. What controls the non self‐similarity of these ruptures and how this is linked to the frictional stability of the interface are still open questions. Here we develop seismic cycle simulations of a single unstable slipping patch to investigate the mechanisms underlying this behavior. We show that temporal changes of normal stress on a fault can produce ruptures that exhibit the observed anomalous scaling. Our results highlight the role of fault zone fluid pressure in modulating the effective normal stress and contributing to the sliding stability of the fault.

A Delphi consensus study on the treatment of slipped capital femoral epiphysis: Considerable consensus in mild and moderate slips and limited consensus in severe slips.

The aim of this expert consensus study was to establish consensus on the treatment of different types of slipped capital femoral epiphysis and on the use of prophylactic screw fixation of the contralateral unaffected side. In this study, a four-round Delphi method was used. Questionnaires including all possible theoretical slip scenarios were sent online to 14 participants, experienced in the field of children's orthopedics and in the treatment of hip disorders in children. In-situ fixation was considered to be the first treatment choice in all types of mild slip scenarios and in moderate, stable ones. Performing in-situ fixation was not favored in moderate, unstable, and in all severe slip scenarios. In moderate to severe, unstable slip scenarios, there was consensus on the use of gentle closed or open reduction and internal fixation. Any consensus was not established in the optimal treatment of severe, stable slips. There was also consensus on the use of prophylactic screw fixation of the contralateral side in case of co-existing endocrine disorder and younger age. The establishment of consensus on the treatment of all types of slipped capital femoral epiphysis even among the experienced surgeons does not seem to be possible. The severity of the slip and stability of the slip are the primary and secondary determinants of the surgeons' treatment choices, respectively. In-situ fixation is still the preferred treatment option in several slip types. Gentle capital realignment by closed or open means is recommended in displaced, unstable slips. Prophylactic screw fixation of the contralateral side is indicated under certain circumstances. level V.

Non‐Precursory Accelerating Aseismic Slip During Rupture Nucleation

AbstractAccelerating aseismic slip events have been commonly observed during the rupture nucleation processes of the earthquake. While that accelerating aseismic slip is usually considered strong evidence for precursory activity, it remains unclear whether all accelerating aseismic slip events are precursory to an incoming earthquake. Two contrasting nucleation models have been introduced to explain the observations associated with the nucleation of unstable slip: the pre‐slip and cascade nucleation models. Each of these two‐end members, however, has its own limitations. In this study, we employ Discrete Element Method simulations of a 2‐D strike‐slip fault to simulate various rupture nucleation and triggering processes. Our simulation results manifest that the final seismic event is a product contributed by multiple pre‐slip nucleation sites, which may interact, causing clock advance or cascade nucleation rupture processes. We also introduce a strengthening perturbation zone to investigate the role of a single nucleation site in an imminent seismic event. The simulation results reveal a new type of non‐precursory aseismic slip, representing the region favoring the generation of the precursory slip process but not correlating to the incoming main event, which differs from the previous interpretation of precursory slip. Furthermore, we include weakening perturbation zones in some simulations to demonstrate how small earthquakes may or may not trigger a nucleation site depending on spatial and temporal conditions. Our simulation results imply that such non‐precursory but accelerating aseismic slip events may suggest a fault segment that appears weakly coupled but possesses the potential to be triggered seismically.

Effects of phyllosilicate content on the slip behavior of fault gouge: Insights from room-temperature friction experiments on quartz–talc mixtures

Shallow slow slip events (SSEs) occur on the shallowest reaches of subduction megathrust faults. To investigate how phyllosilicate fraction in quartzo-feldspathic sediments influences the style of fault slip behavior, we conducted frictional sliding experiments on various binary mixtures of quartz and talc using a biaxial testing apparatus at room temperature, room humidity, sliding velocities of 2 or 0.66 μm/s, and a normal stress of 10 MPa. With increasing talc content, the steady state coefficient of friction decreases from 0.7 to 0.2 in association with a transition from velocity-weakening (VW) to velocity-strengthening (VS) behavior. For the gouges with 0–10 wt% talc, fault slip behavior, following velocity steps, transitions from regular stick slips via sustained oscillations to attenuated oscillations with talc content. The oscillations emerge only in a narrow range of talc contents (i.e., 4–10 wt%) at near velocity-neutral conditions. The observed transition is attributed to an increase in the stiffness ratio K = k/kc, where k is the elastic stiffness of the system and kc is the critical stiffness of the fault, although K is larger than 1 for both stick-slip and sustained-oscillation behaviors, and thereby the critical stiffness criterion is not satisfied. We also find that for unstable slip events, the smaller the stress drop magnitude, the slower the peak slip velocity. Our results suggest that mixed gouges containing VW and VS materials show velocity-neutral behavior and facilitate the nucleation of shallow SSEs, although finely tuned rate-and-state friction parameters are required to allow spontaneous slow slip.

The Interaction Between Frictional Slip and Viscous Fault Root Produces Slow Slip Events

AbstractWe consider a model where an unstable frictional region, governed by rate and state friction, interacts with a viscous zone with Newtonian rheology. The system is loaded at distance with a constant velocity. Pore pressure variations are considered and we show that the model of Segall and Rice (1995, https://doi.org/10.1029/95jb02403) relating porosity changes to variations of the state variable could be derived considering viscoplastic deformation of a population of identical asperities. We perform a linear stability analysis in the case of a constant pore pressure in agreement with the full numerical results. For a given value of the viscosity of the viscous region, stable slip is promoted at low normal stress and unstable slip at high normal stress. Near the transition from stable to unstable slip, modest acceleration of slip, resembling slow slip events (SSE) are observed. We show that our model can reproduce real SSE sequences in the Guerrero subduction zone which are the largest worldwide. The best fit parameters suggest that SSEs happen in areas of low effective normal stress (for the frictional region) and low viscosity (for the viscous region). In our model, SSEs happen in a regime where the viscous region is able to counteract the instability of the frictional one. We show that considering a rate strengthening rheology or a non newtonian one leads to the same linear stability results. Our work shows that a simple model with homogeneous spatial properties can lead to complex dynamics, covering a wide range of observed sliding modes, from steady‐state creep to seismic slip and SSEs.

Repeating Low Frequency Icequakes in the Mont‐Blanc Massif Triggered by Snowfalls

AbstractDeformation mechanisms of glaciers are highly sensitive to basal temperature; the motion of temperate glaciers is dominated by basal slip while cold‐based glaciers deform mainly by internal creep. While basal slip is usually aseismic, unstable slip sometimes occurs and can be detected by seismometers. I have detected clusters of repeating low‐frequency icequakes (LFIs) in the Mont‐Blanc massif. Some properties of LFIs are similar to the high‐frequency icequakes (HFIs) located at the base of Argentière glacier (Helmstetter, Nicolas, et al., 2015, https://doi.org/10.1002/2014jf003288). Both HFIs and LFIs occur as bursts of tens to several thousand events lasting for days or weeks, with typical inter‐event times of several minutes during bursts. Unlike HFIs that have a broad spectra, LFIs have a characteristic frequency of about 5 Hz at all stations, suggesting a rupture length of about 100 m. Seismic amplitudes and seismic waveforms of LFIs progressively evolve with time within each cluster, suggesting changes in either rupture length or rupture velocity. Most LFIs are detected during snowfall episodes while HFIs are not correlated with snowfall episodes. In this study, I used all available seismic stations within or around the Mont‐Blanc massif between 2017 and 2022. I found LFIs located all over the massif but mainly above 3,000 m. Some clusters are clearly associated with cold basal ice (near Mont‐Blanc summit) while others below 2,700 m a.s.l. are likely located under temperate glaciers and two clusters could be associated with landslides. This observation of LFIs on cold glaciers is consistent with laboratory friction experiments suggesting that cold ice promotes unstable slip (McCarthy et al., 2017, https://doi.org/10.1098/rsta.2015.0348; Saltiel et al., 2021, https://doi.org/10.1785/0220200480; Zoet et al., 2013, 2020, https://doi.org/10.1002/jgrf.20052, https://doi.org/10.1029/2020gl088964).

Roles of Normal Stress, Roughness, and Slip Displacement in the Stability of Laboratory Fault in a Sandstone

Unstable slip of a fault block is considered to be the main cause of shallow earthquakes. However, the underlying mechanism of the stability-to-instability transition of faults has not been fully understood. Here, we used the stiffness ratio, which is the ratio between the shear stiffness of the fault subjected to direct shear and the critical stiffness to evaluate the fault stability degree from stable to unstable slip, and examined the effects of normal stress, roughness, and slip displacement on the fault stability. Our experimental results show that with the increase in slip displacement, the shear stiffness change in stable slip mainly includes four stages, namely “rapid increase–keep unchanged–slow increase–rapid decrease”, and unstable slip tends to occur in the last two stages. This process of shear stiffness change is accelerated by the increase in normal stress and the decrease in fault roughness. Our study reveals that fault stability over slip is mutually dictated by asperity interlocking and wear-induced gouge. Asperity interlocking controls fault stability when the gouge amount is low, whereas the fault gouge prevails with the increased wear of the fault surface since the gouge generated during slip can participate in the subsequent friction process. Thus, we infer that the stable–unstable transition of fault over slip is a spontaneous process due to the interplay of asperity interlocking and wear-induced gouge lubrication.

Implications of slow fault slip for hydraulic-fracturing-induced seismicity

Since the discovery of slow-slip phenomena, scientific understanding of the behavior of active fault systems has been transformed significantly. It is now recognized that tectonic fault systems are characterized by a spectrum of slip behavior, from “regular” (stick-slip) earthquakes that radiate elastic wave energy and occur on a timescale of seconds, to slow-slip events with durations ranging from minutes to years. More recently, slow-slip phenomena have been observed and modeled in association with injection-induced seismicity. This includes evidence for predominantly slow fault slip during injection that triggered dynamic rupture elsewhere on a fault. In the case of hydraulic fracturing, slow-slip behavior is consistent with the frictional characteristics of faults in clay-rich rocks. A change in pore pressure or slip rate can cause a fault to transition from slow to unstable slip. Through real-time monitoring of slip-slip processes and, potentially, the development of operational adjustments to reduce the hazard of damaging ground motions, a better understanding of slow-slip processes could contribute to improved risk mitigation for induced earthquakes. Effective tools for direct observation of slow-slip processes include tiltmeters, strainmeters, global navigation satellite systems, interferometric synthetic aperture radar, and distributed fiber-optic sensing.