Cross-effects of roughness and shear cycle on frictional behavior and damage characteristics of faults in granite

Rupture speed plays a critical role in earthquake dynamics, seismic energy release, and ground shaking characteristics. While variations in rupture speed of earthquake fault slip from fast to slow are well-documented in nature and in the lab, the responsible mechanisms are not fully understood. Here we address the physical mechanisms for variations in rupture speed using an array of strain gage rosettes in direct shear experiments to estimate the rupture speed of stick-slip instabilities. The experiments were conducted with applied normal stresses spanning one order of magnitude, ranging from 2 to 20 MPa. High-speed records of shear strains at 13 equidistant locations along 15-cm-long granite faults were analyzed to understand the effects of normal stress on rupture dynamics. Our data follow the expectation that higher normal stress generally promotes faster rupture speeds, consistent with observations from natural fault systems.Our analysis reveals the interplay between stress conditions, stored elastic energy, and fault behavior. The experiments provide insights into how changes in normal stress affect the propagation of frictional rupture along a simulated fault surface with a thin layer of moisturized quartz gouge (Min-U-Sil, 40). A concise relation between normal stress and rupture speed based on linear elastic fracture mechanics is derived to explain our observations.Fracture energy scales linearly with normal stress, which tends to reduce rupture speed as normal stress increases. However, the greater difference between peak and residual strength at higher normal stresses allows for more energy to be released during fault slip. Thus, as normal stress increases, the energy release rate, which scales quadratically with normal stress, outpaces the linear increase in fracture energy, leading to higher rupture speeds.Our results provide important information for seismic hazard assessment and the development of more accurate rupture models for earthquake forecasting. By clarifying the role of normal stress in modulating rupture speed, our work illuminates the complex interactions between stress conditions and earthquake rupture dynamics. Overall, our data underscore the significance of considering normal stress variations in seismological methods to improve earthquake estimations and hazard assessments.

Cyclic shearing behavior, dynamic characteristics, and post-cyclic volume change of a peat sublayer from the Port Lands area of Toronto (Ontario, Canada) are investigated in this study. Laboratory specimens are trimmed from block samples collected from a depth of about 4.0 to 4.5 m. Constant-volume cyclic direct simple shear tests indicate an initial reduction of effective stress with number of stress cycles. However, the corresponding excess pore pressure ratios do not exceed 60%, indicating a cyclic mobility behavior in the peat specimens. Maximum shear moduli of the peat samples are also determined from shear wave velocity measurements. Post-cyclic volumetric strain, as well as the variations of secant modulus, modulus reduction, and damping ratio of the peat, are presented in terms of cyclic shear strain and compared with other studies. Empirical relationships are proposed for characterizing the shear modulus and damping ratio of Toronto peat.

The impact of long‐term CO2‐brine‐rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO2 storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre‐existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO2‐charged formation water and fault gouges (1–1000 years residence time, i.e. 10–106 pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σneff = 50 MPa; Pf = 25 MPa; V = 0.2‐100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay‐rich caprocks overlying potential CO2 storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault‐gouge mineralogy imposed a stronger control on clay‐rich fault behavior than the extent of CO2‐brine‐rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO2‐brine‐rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain). © 2018 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons, Ltd.

Brick masonry in earth mortar is a traditional construction technique still used in South and Southeast Asia. Timber beams and columns are often taken advantage of to improve the seismic performance of such structures. Nonetheless, the contribution of these timber elements has not been fully understood yet partially due to the complex friction behaviour between timber and masonry. In the present research, a cyclic loading triplet test is performed on two specimen types. In one type, units are fired solid bricks while in the other they are timber blocks. Joints are composed of earth mortar. Results between the two types are compared, paying attention to friction coefficients and damage patterns. Then, numerical analysis is performed to reproduce a previously performed pull-out test, taking advantage of the results obtained from the triplet test. Though the analysis, frictional behaviour between a timber beam and masonry wall in earth mortar was closely observed in terms of normal and shear stress distributions in the contact surfaces and the deformation of earth mortar. The test showed the contrasts of the frictional performance between brick–mortar and timber-mortar contact surfaces. The analysis suggested that it was effective to physically discretise earth mortar. In this way, frictional behaviour between a timber element and masonry wall was properly reproduced. This paper sheds light on the interaction between timber and brick masonry in earth mortar under lateral loading such as earthquakes.

Understanding the stick-slip behavior of fault is crucial for studying earthquake nucleation and disaster prevention. However, experimental research on fault slip under true three-dimensional stress condition is relatively rare, particularly those focusing on the effect of lateral stress (σp) parallel to the fault plane on stick-slip characteristics. In this study, direct shear experiments under true three-dimensional stress condition were conducted to investigate the influence of σp on the mechanical and acoustic emission (AE) properties of stick-slip in granite fault. The results show that increasing σp enhances the stick-slip initiation strength (τslip) and the corresponding friction coefficient of fault, leading to an elevation in peak slip velocity and fault slip seismic parameters. The strengthening effect of σp also increases the proportion of high-energy stick-slip events, accompanied by a rise in the proportion of shear cracks during fault slip, further influencing τslip. Additionally, higher σp results in an elevated average AE b-value, indicating a gradual decrease in AE magnitudes, corresponding to the enhancement in τslip. Furthermore, b-value decline prior to stress drop can serve as a key indicator for predicting stick-slip events. These findings provide valuable theoretical insights for fault activation-induced hazards and the assessment of such hazards.

We analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature (T) and pore water pressure (Pp) on the sliding behavior of faults. Rate‐stepping sliding tests were performed on laboratory faults in granite containing “gouge” (granite powder), both dry at 23° to 845°C [Lockner et al., 1986], and wet (Pp = 100 MPa) at 23° to 600°C [Blanpied et al., 1991, 1995]. Imposed slip velocities (V) ranged from 0.01 to 5.5 μm/s, and effective normal stresses were near 400 MPa. For dried granite at all temperatures, and wet granite below ∼300°C, the coefficient of friction (μ) shows low sensitivity to V, T, and Pp. For wet granite above ∼350°, μ drops rapidly with increasing T and shows a strong, positive rate dependence and protracted strength transients following steps in V, presumably reflecting the activity of a water‐aided deformation process. By inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate‐ and state‐dependent constitutive law. One or two state variables were used to represent slip history effects. Each velocity step yielded an independent set of values for the nominal friction level, five constitutive parameters (transient parameters a, b1, and b2 and characteristic displacements Dc1 and Dc2), and the velocity dependence of steady state friction ∂μss/∂ ln V = a‐b1−b2. Below 250°, data from dry and most wet tests are adequately modeled by using the “slip law” [Ruina, 1983] and one state variable (a = 0.003 to 0.018, b = 0.001 to +0.018, Dc ≈ 1 to 20 μm). Dried tests above 250° can also be fitted with one state variable. In contrast, wet tests above 350° require higher direct rate dependence (a = 0.03 to 0.12), plus a second state variable with large, negative amplitude (b2 = −0.03 to −0.14) and large characteristic displacement (Dc2 = 300 to >4000 μm). Thus the parameters a, b1, and b2 for wet granite show a pronounced change in their temperature dependence in the range 270° to 350°C, which may reflect a change in underlying deformation mechanism. We quantify the trends in parameter values from 25° to 600°C by piecewise linear regressions, which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip. The modeling results suggest that the succeptibility for unstable (stick‐slip) sliding is maximized between 90° and 360°C, in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust.

The slip behavior of crustal faults is known to be controlled by the mineralogic composition of the fault gouge. The exact properties determining the frictional behavior of geologic materials, including diverse remains an important question. Here, we use a geochemical approach considering the role of water-rock interactions. As a mechanism, we suspect that the mineral surface charge allows attractive and repulsive forces (Van Der Waals type), and that those forces may influence the static mechanical behavior of clays (cohesion, static friction).  On the other hand, we suspect that the water bound to the mineral surfaces may play a role during shearing.  To address these ideas, we measured the cation exchange capacity (CEC) of 10 different rock and mineral types, including non-clays and a range of phyllosilicate minerals, using CEC as a proxy for the mineral surface charge and the ability to bind water to the mineral surfaces.  For these materials, we conducted laboratory shearing experiments measuring the pre-shear cohesion, peak friction coefficient, residual friction coefficient, post-shear cohesion, and velocity-dependent friction parameters under 10 MPa effective normal stress.  Our results show that low CEC materials (< 3 mEq/100g) tend to exhibit high friction, low cohesion, and show velocity-weakening frictional behavior. The phyllosilicate minerals exhibit larger CEC values up to 78 mEq/100g and correspondingly lower friction coefficients, higher cohesion, and velocity-strengthening frictional behavior. Zeolite exhibits a relatively high CEC value typical of phyllosilicates, but its strength and frictional characteristics are that of a non-clay with low CEC. This suggests that grain shape and contact asperity size may be more important for non-phyllosilicates. For phyllosilicates, we suggest that the systematic patterns in strength and frictional behavior as a function of CEC could be explained by water bound to the mineral surfaces, creating bridges of hydrogen or van der Waals bonds when the particles are in contact. Such bonding explains the large cohesion values for high-CEC materials under zero effective stress, whereas surface-bound water trapped between the particles under load explains low friction.  Beyond the results of this study, CEC appears to be a controlling factor for other properties such as permeability and even the amount of bound DNA in sediments. 

Cyclic shear behavior of en-echelon joints under constant normal load

Effect of dynamic cyclic shear on frictional strength weakening of a plane joint

Despite considerable progress in monitoring natural subduction zones, key aspects of megathrust seismicity remain puzzling, mainly due to the temporally incomplete and spatially fragmented available record. Scaled seismotectonic models yield valuable insights by spontaneously creating multiple stick-slip cycles in controlled, downscaled three-dimensional laboratory replicas. Here we report recent progress in analog modeling of the megathrust seismicity, particularly focusing on a meters-scale elasto-plastic model featuring a frictionally segmented, granular fault that mimics the subduction channel at natural subduction zones. We showcase how by employing analog materials under low-stress conditions, the potentialities of monitoring can be maximized using three diverse techniques: 1)  Precise monitoring of surface spatial deformation over time is achieved through digital image correlation techniques, mirroring a uniformly distributed dense geodetic network spanning land to trench in real subduction zones. 2) A Micro-Electro-Mechanical (MEMS) accelerometric network, emulating a seismic network, captures seismic wave propagation at the model surface. 3) Embedded piezoelectric sensors within the granular analog fault capture near-field acoustic signatures of frictional instabilities. These diverse monitoring techniques allow for investigating the consistency between continuous seismic activity and surface deformation data, offering insight into both micro and macroscopic features of analog seismic cycles. At the macroscopic level, the models' frictional behavior can be numerically reproduced via rate and state numerical simulations, considering earthquake fault slip as a nonlinear dynamical process dominated by a single slip plane. At smaller scales, the model accounts for complexities in fault slip emerging from grain interactions, reflecting nonlinearities that arise when considering faults as distributed three-dimensional volumes. These fundamental attributes, coupled with their capacity to create extensive catalogs of small labquakes, make scaled seismotectonic models exceptional apparati for employing Machine Learning (ML) in comprehending multi-scale spatiotemporal seismic processes. Cutting-edge Deep Learning methods are employed to predict the spatiotemporal evolution of surface deformation, where regression algorithms not only forecast timing but also the propagation and magnitude of analog earthquakes across diverse spatiotemporal scales. Given that one of the monitoring systems used in seismotectonic analog models mimics a geodetic-like network in nature (GNSS data-Global Navigation Satellite Systems), an attempt to generalize the promising outcomes achieved in the laboratory to natural subduction faults is proposed.  Such promising avenues emphasize the potential for ML to bridge the gap between laboratory experiments and real-world seismic events. These initial findings, combined with advancements in the instrumentation of fault laboratories in nature and expanding data reservoirs, reinforce the belief that ML can significantly augment our understanding of the multiscale behaviors of natural faults.

Slow slip events (SSEs) are the slowest type of discrete slip within the full spectrum of fault-slip behaviors and have been confirmed by both geodetic (e.g., Dragert et al., 2001) and laboratory data (e.g., Ikari, 2019). They have attracted considerable attention due to their mutual interaction with earthquake processes, and multiple approaches have been employed to investigate different aspects of SSEs. Here, we present a study that combines laboratory friction experiments and numerical modeling to explore the mechanisms of SSEs observed through geodetic and borehole data.We conducted velocity-stepping friction experiments on intact core samples retrieved from the major reverse fault zones of the Nankai Trough, southwest Japan. These experiments were performed under both in-situ effective stress conditions and at 10 MPa, with slip velocities ranging from 1.6 nm/s (plate tectonic driving rates) to 30 μm/s. Our results reveal that fault zone samples transition from velocity-weakening to velocity-strengthening behavior as slip velocities increase, and some rate-and-state friction (RSF) parameters exhibit a dependence on sliding velocity. Numerical models (Zhang and Ikari, 2024) using velocity-dependent RSF parameters, constrained by our experimental data, successfully replicate SSEs comparable to those observed in the Nankai Trough (Araki et al., 2017; Yokota and Ishikawa, 2020) by assuming fault patches at depth ranges and sizes consistent with observational data. In contrast, models based on non-transitional frictional behavior (constant RSF parameters) or near-neutral stability (constant RSF parameters with extremely small velocity weakening) generate slip events that are several orders of magnitude faster than observed SSEs. We therefore propose that the transitional frictional behavior with increasing slip velocity is a key mechanism of shallow SSEs in the Nankai Trough.Our study demonstrates that laboratory data obtained from centimeter-scale samples can be used to predict the frictional behavior of real faults on the scale of tens of kilometers. By integrating methodologies from multiple disciplines, we can achieve a more comprehensive understanding of the dynamics governing fault slip behavior.

ABSTRACT: Epidote and chlorite are both low-grade metamorphic minerals that are widely distributed in deep granite geothermal reservoirs. Hot fluid circulation promotes the precipitation of epidote and chlorite coatings on natural faults and fractures and this can in turn exert control on fault/fracture frictional stability. We use simulated epidote/chlorite gouges and conduct fault shear experiments at conditions typifying the ∼4 km depth typical of deep geothermal reservoirs to explore the effect of epidote/chlorite content on frictional strength and stability. Results indicate that the frictional characteristics of epidote and chlorite gouges vastly differ. Epidote gouge is frictionally strong, with a coefficient of friction of ∼0.73, nearly double that of the chlorite gouge (∼0.35). In addition, the epidote gouge exhibits strong velocity-weakening and unstable frictional behavior under the test conditions, while the chlorite gouge is velocity-strengthening and stable. Fault frictional strength and velocity-weakening behavior are all enhanced for epidote-granite mixed gouges as epidote content increases. However, chlorite-granite mixed gouges show the opposite trend with increasing chlorite content. Our results have important implications in understanding the frictional stability of epidote/chlorite-filled granite faults and its influence on seismicity in deep geothermal reservoirs inhabiting the shallow crust. 1. INTRODUCTION The moment magnitude (Mw) 5.5 Pohang earthquake that occurred in November 2017 has been demonstrated as one of the largest and most damaging earthquakes on the Korean peninsula since the last century (Grigoli et al., 2018; Kim et al., 2018). This event has attracted widespread attention not only for the significant resulting hazard and damage but also due to its connection with the Pohang Enhanced Geothermal System (EGS) project in South Korea. This earthquake (Lee et al., 2019a) directly injured >100 people and resulted in >US$300 million in economic loss, and it is also the largest known injection-induced earthquake at an EGS site. Currently, it has been confirmed that an unmapped pre-existing critically-stressed fault was reactivated at a depth of ∼4.0 km by the fluid injection during EGS stimulation and finally trigger the seismicity (Lee et al., 2019a).

The shaft capacity of piles in sand subjected to cyclic (wave) loading has been observed to decrease significantly with loading cycles (Poulos, 1989). A number of researchers (Boulon and Foray, 1986; Tabucanon et al., 1995; Shahrour et al., 1999) have replicated the characteristics of the load transfer degradation behavior in the laboratory through cyclic interface shear testing with a constant normal stiffness confinement condition (Vesic, 1972). However, no consensus currently exists as to the primary microscale mechanisms that govern cyclic interface shear behavior and load transfer degradation. A research program was undertaken to quantify the contribution of soil properties, cementation, confinement condition, and displacement mode, in load transfer degradation. Monotonic and cyclic interface shear tests were performed using a modified interface direct shear device with a Perspex side window. The specimen particle displacement fields were quantified during selected cycles by capturing high resolution digital images (1600 x 1200 pixels) and using Particle Image Velocimetry (White et al., 2001a). Results indicate that the confinement condition, which is intended to replicate the elastic response of the far-field soil, is of primary importance as it allows for normal stress relaxation with soil contraction adjacent to the interface. The displacement magnitude, particle characteristics, and particle-particle cementation were also observed to affect the magnitude and rate of degradation. It is anticipated that these findings will provide a fundamental rationale to identify field conditions where shear stress degradation is likely to occur and a basis from which more rigorous models may be developed.

Heterogeneous mineralogical composition and fault behaviour: A systematic study in ternary fault rock compositions

In order to understand the earthquake nucleation process, we need to understand the effective frictional behavior of faults with complex geometry and fault gouge zones. One important aspect of this is the interaction between the friction law governing the behavior of the fault on the microscopic level and the resulting macroscopic behavior of the fault zone. Numerical simulations offer a possibility to investigate the behavior of faults on many different scales and thus provide a means to gain insight into fault zone dynamics on scales which are not accessible to laboratory experiments. Numerical experiments have been performed to investigate the influence of the geometric configuration of faults with a rate-and state-dependent friction at the particle contacts on the effective frictional behavior of these faults. The numerical experiments are designed to be similar to laboratory experiments by DIETERICFI and KILGORE (1994) in which a slide-hold-slide cycle was performed between two blocks of material and the resulting peak friction was plotted vs. holding time. Simulations with a flat fault without a fault gouge have been performed to verify the implementation. These have shown close agreement with comparable laboratory experiments. The simulations performed with a fault containing fault gouge have demonstrated a strong dependence of the critical slip distance D c , on the roughness of the fault surfaces and are in qualitative agreement with laboratory experiments.