Fault Slip Modes Research Articles

Global subduction slow slip events and associated earthquakes.

Three decades of geodetic monitoring have established slow slip events (SSEs) as a common mode of fault slip, sometimes linked with earthquake swarms and in a few cases escalating to major seismic events. However, the connection between SSEs and earthquake hazard has been difficult to quantify and contextualize beyond regional studies. We aggregate a geodetic record of SSEs from subduction zones in the circum-Pacific region. In aggregate, earthquake rates increase up to threefold concurrent with and proximal to SSEs. The relative amplitude of this increase is correlated with the SSE size and, to a lesser extent, their depth and region. The subdued and coincident earthquake response to SSE stress transfer suggests a more limited role of static stress transfer and a very short relaxation timescale for the triggered seismicity. The observed range of behavior does not support a major connection between SSEs and earthquake hazard.

Controls of fault geometry and thermal stress on fault slip modes: Implications for permeability enhancement and injection-induced seismicity

Fluid pressurization within the fault zone generates increasing pore pressure and stress change which is liable to create shear and/or brittle fractures within the reservoir volumes and subsequently generating earthquakes of varying magnitudes. Here, we explored time-dependent fault weakening processes in the fault zone which are dependent on several factors, including the rate of cold-water injection, modes of injection (hydromechanical (HM) and thermo-hydro-mechanical (THM) interactions), and changing fault spatial configurations using data from Niger Delta Basin. The variation in the stability of different fault models in withstanding stresses induced by HM and THM fluid interactions is evident. Fault permeability enhancement and the behaviour of slip event under isothermal and non-isothermal conditions revealed that stress and pore pressure perturbations have a first order control on the rate of fault dilation and compression. It is observed that the progressive cooling of the reservoir induced thermal stress which induced the timing of slip by unloading the fault to earlier seismic rupture in the non-isothermal case, and accelerates the magnitude of the fault reactivation and the accompanied induced seismicity. Owing to increased tendency of shear failure during injection, fracture opening through shear dilation is more enhanced in THM simulation as the fracture permeability is significantly higher than in HM. This effect becomes increasingly more dominant with intermediate fault angle and joint orientation. Certain fault/joint configurations which were resistant to shear failure under isothermal injection had their frictional resistance broken by thermal stress. The results also indicate that there is higher pore pressure build-up in THM than in HM as the injection rate increases and reservoir temperature drops during cold injections.. This study has demonstrated the importance of fully characterizing the fracture geometries and configurations of normal faulting regime in addition to fluid injection conditions when developing fractured reservoirs to mitigate seismic risks and hazards that could result from early fault reactivation.

Acoustic Emission Reveals Multiple Slip Modes on a Frictional Fault

The spectrum of fault slip modes spans a continuum from fast ruptures to slow slip events. The nucleation of a certain slip mode is governed by the frictional heterogeneity of fault interface and the rheological fault stiffness. There is a mounting evidence that a single fault can host multiple slip modes. In laboratory experiments we study acoustic emission (AE) initiated by a sliding frictional fault and focus our attention on gouge-filled faults hosting multiple slip modes. Deformation experiments were performed on a slider model setup with a precise control of mechanical parameters and monitoring the acoustic signal in the frequency range of 20–80 kHz. We have shown that the cumulative AE energy linearly depends on block displacement. Besides that, there is a high inverse correlation (-0.94) between fault friction andb-valueof frequency-amplitude distribution of AE in the performed experiments. Provided that velocity weakening is specific for the fault interface, the self-organization of a gouge-filled fault at the micro scale is the key parameter that controls the frictional behavior of fault hosting multiple slip modes. Resting on a quantitative categorization of AE waveforms, two AE subpopulations have been distinguished. One of them manifests as AEs with harsh onsets. The second one exhibits a gradual amplitude rise and tremor-like waveforms. A longer duration of the intergrain rupture is specific for the second AE subpopulation. During a laboratory seismic cycle, the first AE subpopulation retains parameters, while the second one exhibits a pronounced cyclic recurrence ofb-value. Theb-valueof the second subpopulation gradually decreases before slip events and recovers after them. Two AE subpopulations, probably, point to the coexistence of two dynamic subsystems. The revealed precursory changes of AE subpopulations are common for the entire spectrum of slip modes. We speculate on the unity of underlying mechanisms of different slip modes.

Friction experiments under in-situ stress reveal unexpected velocity-weakening in Nankai accretionary prism samples

The Nankai Trough hosts diverse fault slip modes, ranging from slow slip events to megathrust earthquake ruptures. We performed laboratory friction experiments on samples collected by the Integrated Ocean Drilling Program offshore Kii Peninsula, Japan. This study systematically investigates the effect of effective normal stress on frictional strength and the velocity-dependence of friction for natural fault zone and wall rock samples collected from depths of 270 to ∼450 meters below seafloor (mbsf), and over a range of shearing velocity spanning from 0.01–30 μm/s. In addition, cohesive strength was determined before and after each velocity step experiment while the sample was unloaded. We tested both powdered and intact specimens at estimated in-situ effective stresses, as well as at higher stresses representing deeper portions of the megasplay fault (Sites C0004, C0010) and the frontal thrust zone (Sites C0007, C0006). The apparent coefficient of sliding friction μs varies between 0.22–0.53 and correlates inversely with clay content. Direct measurements of cohesive strength show that on average 11% of the residual shear strength, and up to ∼30% for some specimens, can be attributed to cohesion. Friction coefficient slightly decreases as a function of increasing effective normal stress, attributed to a decreasing proportion of cohesive strength. The lowest μs values are observed for samples from the frontal thrust zone Site C0007 and the megasplay fault Site C0010. All samples show a combination of velocity-strengthening and velocity-weakening behavior, but intact samples tested under in-situ effective normal stress and low shearing velocities (<∼1 μm/s) exhibit consistently large velocity-weakening frictional behavior. The observed velocity-weakening behavior is related to the induration state of the material, which affects the real area of contact along shear surfaces. This is supported by direct measurements of cohesive strength, showing that higher cohesion values in intact samples correspond with a more pronounced evolutionary effect in velocity step tests that, in turn, map to lower values of the rate-dependent friction parameter a−b. We propose that fault zones in the Nankai subduction zone are likely to be velocity-weakening, and thus potentially able to host the nucleation of unstable slip, from seismogenic depths to the seafloor. We also find that testing disaggregated fault zone samples and employing effective stresses exceeding those in-situ lead to overestimation of a−b, emphasizing the importance of testing intact samples under in-situ conditions in laboratory friction studies.

Laboratory study on the effects of fault waviness on granodiorite stick-slip instabilities

SUMMARY The effects of fault waviness on the fault slip modes are unclear. Laboratory study on the effects of the centimetre-scale fault contact distribution, which is mainly controlled by the fault waviness, on granodiorite stick-slip instabilities may help to unveil some aspects of the problem. The fast and slow stick-slip motions were separately generated in two granodiorite samples of the same roughness but different fault contact distributions in the centimetre scale in the laboratory. The experimental results show the following: (1) the fault with the small contact area and heterogeneous contact distribution generates fast stick-slip instabilities, while the fault with the large contact area and homogeneous contact distribution produces slow stick-slip events; (2) the nucleation processes of the fast stick-slip events are characterized by abrupt changes once the nucleation zones expand to the critical nucleation length that is observed to be shorter than the fault length, while the slow stick-slip events appear as a gradual evolution of the nucleation zones leading to total fault sliding. These indicate that, unlike the micron-scale fault contact distribution controlled by roughness, which depends mainly on the grain size of the abrasives used for lapping the fault surface, the centimetre-scale fault contact distribution, which depends mainly on the waviness of the fault surface profile, also plays an important role in the fault slip modes. In addition, the effects of the fault waviness on the fault friction properties are preliminarily analysed based on the rate- and state-dependent friction law.

The relationship between fault zone structure and frictional heterogeneity, insight from faults in the High Zagros

Frictional heterogeneities within fault zones depend strongly on lithology as well as shear fabric and strain localization structures. Here we investigate the impact of fault rock composition and shear fabric on friction constitutive properties for mature High Zagros (Iran) fault rocks. We present field observations along with results from friction experiments on intact and powdered fault rock from carbonate, quartzofeldspathic and schist. We measured frictional strength and rate/state dependence over normal stresses from 25 to 100 MPa and shear slip velocities from 3 to 300 μm/s. The sliding friction coefficients of intact fault rocks are lower than their powdered equivalents. The foliated quartzofeldspathic and lensoidal carbonate rocks from the Main Zagros Reverse Fault (MZRF) exhibit velocity strengthening friction and stable sliding. In contrast, the cataclasite carbonate rocks from the younger, Main Recent Fault (MRF) exhibit potentially unstable, velocity-weakening frictional behavior, consistent with regional seismicity. Our results suggest that solution seams, cleavage processes, and development of stylolite and calcite veins in the carbonate fault rocks of the MZRF continually rejuvenate and rework shear structures, which leads to dominantly aseismic shear. In contrast, younger fault rocks of the MRF form highly localized shear zones and shear fabric that result in velocity-weakening frictional behavior and seismic slip. Our field observations and laboratory data provide insights for using friction data and fault zone structure to build predictive models of the mode of fault slip in zones of tectonic collision.

Evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode

Laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation, when fault slip velocity is slow (<0.001 cm/s), and dynamic rupture when fault slip rates exceed centimeters per second. The application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting. Here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding ( v ∼0.001 cm/s) to dynamic stick slip ( v > 1 cm/s). The full range of fault slip modes was achieved by controlling the ratio K = k / k c , where k is the elastic loading stiffness and k c is the fault zone critical rheologic stiffness. We show that K controls the transition from slow-and-silent slip ( K > 0.9) to fast-and-audible ( K v = 3 cm/s, slip duration 0.003 s) slip events. Microstructural observations show that with accumulated strain, deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains, which favor the development of frictional instabilities. Once this fabric is established, fault fabric does not change significantly with slip velocity, and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness.

A study of different fault slip modes governed by the gouge material composition in laboratory experiments

A study of different fault slip modes governed by the gouge material composition in laboratory experiments

Study of fault slip modes

We present the data of the laboratory experiments on studying the regularities of gradual transition from the stick-slip behavior to aseismic creeping on the interblock boundary. The experiments show that small variations in the material composition in the principal slip zones of the faults may cause a significant change in the fraction of seismic energy radiated during the dynamic unloading of the adjacent segment of the rock mass. The experiments simulate interblock sliding regimes with the values of the scaled kinetic energy differing by a few orders of magnitude and relatively small distinctions in the strength of the contacts and in the amplitude of the released shear stresses. The results of the experiments show that the slip mode and the fraction of the deformation energy that goes into the seismic radiation are determined by the ratio of two parameters—the stiffness of the fault and the stiffness of the enclosing rock mass. An important implication of the study for solving the engineering tasks is that for bringing a stressed segment of a fault or a crack into a slip mode with low-intensity radiation of seismic energy, the anthropogenic impact should be aimed at diminishing the stiffness of the fault zone rather than at releasing the excessive stresses.

A new finite element model of buried steel pipelines crossing strike-slip faults considering equivalent boundary springs

A new finite element model considering equivalent boundary springs was proposed to analyze the displacement and strain of the pipeline under strike-slip faults. The purpose of introducing equivalent boundary springs was to simulate the interaction of the pipeline-soil more sufficiently, because the dimension of current models was far smaller than the actual one of the interaction of the pipeline-soil. Subsequently, a closed-form solution was derived to obtain the stiffness coefficient of equivalent boundary springs. Besides, a contact model was used in the new finite element model. The proposed model was verified through the comparison of the obtained solutions to the results of a referred model with an exact solution, with minor deviations which do not exceed about 4.34%. Meanwhile, the effects of fault displacements, kinetic friction coefficients, fault slip modes and constraint conditions of pipeline ends on the displacement and strain of pipelines were investigated. The results show that local buckling occurs more easily under smaller fault displacements, while the tensile failure tends to occur under larger fault displacements. Also, larger kinetic friction coefficients are beneficial for pipeline safety, and constraint conditions of pipeline ends have a large effect on the displacement and strain of pipelines.

A microphysical interpretation of rate‐ and state‐dependent friction for fault gouge

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.

The role of fluid pressure in induced vs. triggered seismicity: insights from rock deformation experiments on carbonates.

Fluid overpressure is one of the primary mechanisms for tectonic fault slip, because fluids lubricate the fault and fluid pressure reduces the effective normal stress that holds the fault in place. However, current models of earthquake nucleation, based on rate- and state- friction laws, imply that stable sliding is favoured by the increase of pore fluid pressure. Despite this controversy, currently, there are only a few studies on the role of fluid pressure under controlled, laboratory conditions. Here, we use laboratory experiments, to show that the rate- and state- friction parameters do change with increasing fluid pressure. We tested carbonate gouges from sub hydrostatic to near lithostatic fluid pressure conditions, and show that the friction rate parameter (a − b) evolves from velocity strengthening to velocity neutral behaviour. Furthermore, the critical slip distance, Dc, decreases from about 90 to 10 μm. Our data suggest that fluid overpressure plays an important role in controlling the mode of fault slip. Since fault rheology and fault stability parameters change with fluid pressure, we suggest that a comprehensive characterization of these parameters is fundamental for better assessing the role of fluid pressure in natural and human induced earthquakes.

Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes.

Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed.

Healing and sliding stability of simulated anhydrite fault gouge: Effects of water, temperature and CO2

Anhydrite-bearing faults are currently of interest to 1) CO2-storage sites capped by anhydrite caprocks (such as those found in the North Sea) and 2) seismically active faults in evaporite formations (such as the Italian Apennines). In order to assess the likelihood of fault reactivation, the mode of fault slip and/or fault leakage, it is important to understand the evolution of frictional strength during periods of no slip and upon reloading (healing and relaxation behavior) and of the velocity dependence of friction of anhydrite fault gouge. Therefore, we performed slide–hold–slide experiments combined with a velocity-stepping sequence using simulated anhydrite fault gouge (>95wt.% CaSO4). Vacuum-dry and water-wet experiments were performed at temperatures ranging from 20 to 150°C, and at an effective normal stress of 25MPa. We also performed tests using dry CO2, water-wetted CO2 and CO2-saturated water as pore fluid, but only at 120°C. If pore fluid was present, a fluid pressure of 15MPa was present. Vacuum-dry samples exhibit similar frictional healing to samples containing lab-air, but healing is significantly enhanced in wet samples. Dry samples exhibit velocity-weakening behavior at T ≥120°C, and wet samples exhibit velocity-strengthening behavior over the full temperature range. The presence of CO2 does not influence the healing behavior or the velocity-dependence of friction. Samples containing water-wetted CO2 exhibit behavior similar to wet samples. We infer that the healing in dry samples is controlled by plastic asperity creep (Dieterich-type), possibly through dislocation creep and/or twinning. In wet samples healing is inferred to be controlled by increases in contact area and cohesion by pressure solution. Using a pressure solution rate model to extrapolate healing by contact area growth indicates that the maximum re-strengthening through such a mechanism will only take days to tens of days.

Lithological control on the deformation mechanism and the mode of fault slip on the Longitudinal Valley Fault, Taiwan

The Longitudinal Valley Fault (LVF) in Taiwan is creeping at shallow depth along its southern half, where it is bounded by the Lichi Mélange. By contrast, the northern segment of the LVF is locked where it is bounded by forearc sedimentary and volcanoclastic formations. Structural and petrographic investigations show that the Lichi Mélange most probably formed as a result of internal deformation of the forearc when the continental shelf of South China collided with the Luzon arc as a result of the subduction of the South China Sea beneath the Philippine Sea Plate. The forearc formations constitute the protolith of the Lichi Mélange. It seems improbable that the mechanical properties of the minerals of the matrix (illite, chorite, kaolinite) in themselves explain the aseismic behavior of the LVF. Microstructural investigations show that deformation within the fault zone must have resulted from a combination of frictional sliding at grain boundaries, cataclasis (responsible for grain size comminution) and pressure solution creep (responsible for the development of the scaly foliation and favored by the mixing of soluble and insoluble minerals). The microstructure of the gouge formed in the Lichi Mélange favors effective pressure solution creep, which inhibits strain-weakening brittle mechanisms and is probably responsible for the dominantly aseismic mode of fault slip. Since the Lichi Mélange is analogous to any unlithified subduction mélanges, this study sheds light on the mechanisms which favor aseismic creep on subduction megathrust.

On the relation between fault strength and frictional stability

A fundamental problem in fault mechanics is whether slip instability associated with earthquake nucleation depends on absolute fault strength. We present laboratory experimental evidence for a systematic relationship between frictional strength and friction rate dependence, one of the key parameters controlling stability, for a wide range of constituent minerals relevant to natural faults. All of the frictionally weak gouges (coefficient of sliding friction, μ < 0.5) are composed of phyllosilicate minerals and exhibit increased friction with slip velocity, known as velocity-strengthening behavior, which suppresses frictional instability. In contrast, fault gouges with higher frictional strength exhibit both velocity-weakening and velocity-strengthening frictional behavior. These materials are dominantly quartzofeldspathic in composition, but in some cases include certain phyllosilicate-rich gouges with high friction coefficients. We also find that frictional velocity dependence evolves systematically with shear strain, such that a critical shear strain is required to allow slip instability. As applied to tectonic faults, our results suggest that seismic behavior and the mode of fault slip may evolve predictably as a function of accumulated offset.