Frictional measurements were made on natural fault gouge at seismic slip rates using a high-speed rotary-shear apparatus to study effects of slip velocity, acceleration, displacement, normal stress, and water content. Thermal-, mechanical-, and fluid-flow-coupled FEM models and microstructure observations were implemented to analyze experimental results. Slightly sheared starting material (Unit 1) and a strongly sheared and foliated gouge (Unit 2) are produced when frictional heating is insignificant and the coefficient of sliding friction is 0.4 to 0.6. A random fabric gouge with rounded prophyroclasts (Unit 3) and an extremely-fine, microfoliated layer (Unit 4) develop when significant frictional heating occurs at greater velocity and normal stress, and the coefficient of sliding friction drops to approximately 0.2. The frictional behavior at coseismic slip can be explained by thermal pressurization and a temperature-dependent constitutive relation, in which the friction coefficient is proportional to 1/T and increases with temperature (temperature-strengthening) at low temperature conditions and decreases with temperature (temperature-weakening) at higher temperature conditions. The friction coefficient, normal stress, pore pressure, and temperature within the gouge layer vary with position (radius) and time, and they depend largely on the frictional heating rate. The critical displacement for dynamic weakening is approximately 10 m or less, and can be understood as the displacement required to form a localized slip zone and achieve a steady-state temperature condition. The temporal and spatial evolution of hydromechanical properties of recovered from the Nankai Trough (IODP NanTroSEIZE Stage 1 Expeditions) have been investigated along different stress paths, which simulate the natural conditions of loading during sedimentation, underthrusting, underplating, overthrusting, and exhumation in subduction systems. Porosity evolution is relatively independent of stress path, and the sediment porosity decreases as the yield surface expands. In contrast, permeability evolution depends on the stress path and the consolidation state, e.g., permeability reduction by shear-enhanced compaction occurs at a greater rate under triaxialcompression relative to uniaxial-strain and isotropic loading. In addition, experimental yielding of sediment is well described by Cam-Clay model of soil mechanics, which is useful to better estimate the in-situ stress, consolidation state, and strength of sediment in nature.

Understanding the mechanics of seismogenesis along major plate boundary fault zones requires not only quantification of physical properties along the plate interface but also the characterization of consolidation properties and stress state in the surrounding material. In subduction margins, fault behavior is influenced by the interplay of accretionary wedge and active dcollement strength and stress state. Similarly, in other tectonic settings, such as major crustal fault zones, the onset of seismogenesis may be due to the evolving frictional or mechanical behavior along the plate interface and immediately adjacent wall rock material. Despite their importance, there are few studies that determine the stress regime from changes in consolidation state of prism sediments and characterize the frictional strength and stability of natural fault gouges at elevated temperatures. Laboratory experimentation is essential in characterizing these properties because it allows for a detailed investigation of deformation along numerous loading paths and frictional behavior at in situ geothermal conditions. In order to examine the variations in consolidation and stress state within the Nankai Trough offshore southwest Japan and to characterize the frictional behavior along the Alpine Fault zone in New Zealand, I evaluated uniaxial consolidation and triaxial deformation experiments on natural prism and fault zone materials. These experiments are designed to 1) provide the first comprehensive assessment of consolidation state across an accretionary complex, 2) explore how the consolidation state can be used as a proxy for past loading history and in situ stress state in the vicinity of a major fault system in the Nankai Trough, and 3) evaluate the importance of temperature on the slip stability of the Alpine Fault zone. Results of uniaxial consolidation experiments on Nankai prism materials, which were collected during numerous Integrated Ocean Drilling Program expeditions, indicate that there is an across-trench variation in consolidation state within the Nankai accretionary complex. The outer accretionary prism sediments near the deformation front (Sites C0006 and C0007) and megasplay fault zone (Sites C0001 and C0004) exhibit the most severe apparent over-consolidation, as documented by large over-consolidation ratios (OCR) ranging from 2.98-4.17 and 0.82-4.45, respectively. Farther landward, the forearc Kumano Basin (Site C0002) and underlying accretionary prism (Sites C0002 and C0009) have lower OCR values that range from 2.26-3.04 and 1.23-1.93, respectively. These results, in combination with shipboard data, suggests the presence of erosive events near the megasplay fault zone, minor cementation in the forearc basin, and an increase in horizontal stress associated with a complex tectonic loading history within the outer and inner accretionary prism. All of these processes will lead to an apparent over-consolidation (i.e. OCR >1) in the one-dimensional consolidation experiments. To further investigate the effects of complex loading on the consolidation state and deformation behavior of Nankai prism materials, I conducted additional uniaxial consolidation and triaxial deformation experiments targeting slope basin sediments near the megasplay fault zone. These samples were collected from a laterally continuous slope apron section that is progressively incorporated into the shear zone of a major out-of-sequence thrust. These results indicate that the slope basin sediments become increasingly over-consolidated with proximity to the megasplay fault zone, with OCR values of 0.39-1.84 at Site C0008 to 2.31-3.79 and 0.94-3.99 at Sites C0022 and C0004, respectively. The high OCR values at Sites C0022 and C0004 could be explained by the onset horizontal tectonic loading and compression as the megasplay fault zone accommodates tectonic stress in response to plate convergence. To explore this hypothesis, I conducted triaxial experiments on Site C0022 samples to probe the yield envelope along different loading paths. The yield stresses are consistent with a model in which the maximum horizontal stress increases while the effective vertical stress remains constant as the megasplay fault is approached. The change in stress state can explain the gradual increase in the OCR values from normally consolidated sediment at Site C0008 to highly over-consolidated sediment at Sites C0022 and C0004. This is consistent with independent observations of stress state near the megasplay fault zone, which show a strike-slip to thrust faulting stress regime, and suggests that this is a novel approach for interpreting the complex in situ stress state within tectonically loaded sediments. In Chapter 3, I characterize the frictional properties of the Alpine Fault zone, which is a major transpressional fault on the South Island of New Zealand, and explore their implications for the onset of earthquake nucleation and unstable slip at depth. I conducted high-temperature and -pressure shearing experiments of Alpine Fault principal slip zone (PSZ-1) and wall rock material from depths of 111.5-142.9 m obtained by the International Continental Scientific Drilling Program (ICDP) Deep Fault Drilling Project (DFDP). At temperatures 180C, principal slip zone (PSZ-1) is frictionally weaker than the surrounding wall rock material, with friction coefficient () values decreasing from = 0.46 at 23C to 0.35-0.40 at 180C. Following this, the slip zone drastically strengthens to values comparable to that of the wall rock material, with = 0.87-0.90 and = 0.87 at 500C for the PSZ-1 and wall rock material, respectively. This contrast in frictional strength between the fault-core and wall rock material suggests shear localization within the upper ~3-4 km of the Alpine Fault zone. The overall increase in friction strength is accompanied by a transition from stable to unstable behavior at temperatures 180C, as documented by a decrease in the friction rate parameter (a-b) for all material. The frictional results, in combination with constraints on the rheologic critical stiffness (kc), suggests that the highest potential for earthquake nucleation correspond to depths 4 km along the Alpine Fault zone. Collectively, the results of the experiments in this dissertation provide insight into the consolidation and stress state of actively accreting subduction margins and new constraints on the updip limit of seismicity within crustal fault zones. I demonstrate that there is an across-trench variation in consolidation state within the Nankai accretionary prism. This variation can be used as a proxy for the stress state and history of sediments within the accretionary prism, with implications for sediment cementation, erosive events, or enhanced lateral tectonic loading. This work also provides a new technique for interpreting the in situ stress state of tectonically loaded sediment through meticulous analysis of sediment yield behavior and consolidation state. This novel technique can be implemented for future investigations of stress state in other tectonic regions and subduction margins. Additionally, I show that an increase in temperature will result in a concurrent increase in frictional strength and transition to potentially unstable behavior within the Alpine Fault zone. By analyzing the frictional results in the context of a simple stiffness stability criterion, I am able to define the updip limit of seismicity along the Alpine Fault zone. This type of stiffness analysis can be incorporated into future frictional stability studies to provide additional constraints of seismogenesis along other plate boundary fault zones.

Nankai Trough, earthquake mechanics, shear experiments, friction, slow earthquakes, velocity-weakening, frictional healing, borehole observatories, physical properties, pressure monitoring.

The fault gouge, generated by the wear of previous slips, plays a key role in the slip stability and weakening mechanisms of a fault. For a sheared gouge, physicochemical transformations such as rock dissolution or high-temperature fusion can occur. The latter can give rise to infill materials that lodge within the pores of the gouge and strongly impact the mechanical and rheological behavior of the fault. Through this thesis work, three 2D models of sheared gouges were proposed using the Discrete Element Method for a thorough understanding of : (i) how infill materials (matrix or cement) participate in the weakening of the fault during a slip reactivation, (ii) the rheological behavior observed through the formation of shear bands and its link with the physical and mechanical characteristics of the gouges, (iii) the contribution of each of these properties in the breakdown energy and friction laws observed. A first model highlights three types of cemented materials with different rheological behaviors and shearing resistance, depending on the initial surface percentage of cementation. This study also leads to a redrawing of the breakdown energy dissipated by the fault considering the three mechanisms involved in gouge shearing: fault dilation, Coulomb friction, and the cemented bond failure. Then, a material only consisting of the "matrix" element is modeled to highlight the relationship between the intrinsic properties of the matrix and its rheological behavior. The importance of the percentage of matrix present within the gouge is evaluated in a third model. Finally, we detail an energetic method to link the evolution of each deformation band (Riedel bands) with the behavior of the entire gouge. The friction laws resulting from these models can be used in larger-scale dynamic models to study fault seismic and aseismic behavior.

Tectonic faults fail in a spectrum of slip modes ranging from aseismic creep to fast elastodynamic ruptures. In the laboratory, these slip modes and fault frictional stability can be quantified by second-order changes in friction, and modeled using experimentally-derived designer friction laws known as rate-and-state friction (RSF). Even though RSF has been utilized to study fault slip and stability for many decades, the parameters constituting RSF and their relationship to the underlying grain-scale frictional contact mechanics, particularly in the context of slow and fast ruptures, are poorly constrained. While light intensity-based imaging techniques provide some insights into the evolution of microscopic frictional contacts during shear, their utility is limited in the case of opaque geologic media such as sheared rock and granular fault gouge. Motivated by the successful application of ultrasonic wave monitoring for imaging rock joints and fractures, I use ultrasonic acoustic monitoring for a range of fault slip behaviors in the laboratory, to constrain the micromechanical behavior of deforming load-bearing asperities that make up tectonic faults. In this dissertation, I ask fundamental questions surrounding the deformation of microscopic load-bearing asperity populations that make up frictional interfaces and granular fault gouge assemblages. I dissect the various parameters that make up the RSF constitutive framework, and ask what frictional state and the critical slip distance represent in the context of creeping tectonic faults. I also strive to answer whether the microphysical mechanisms operating across the spectrum of slip behaviors, from stable sliding to fast ruptures, are similar or fundamentally different. I examine the role of normal stress and velocity perturbations on experimental rate-state faults, particularly in the context of contact-scale processes, and use these insights to constrain the potential origins of shallow slow earthquakes, both frictional and mineralogical, at the Hikurangi subduction margin. I start this dissertation by introducing the problem statement broadly and providing some context for the known and unknown aspects of interfacial contact-scale friction in Chapter 1. In Chapter 2, I probe an extended RSF formulation, incorporating the role of normal stress and velocity variations on frictional state, and its application to rough, planar faults using ultrasonic wave amplitudes. In chapters 3-5, I generate a range of slow and fast slip modes on mature faults with simulated wear and jointly characterize precursory creep and ultrasonic wave properties in the context of frictional state evolution. Chapter 3 demonstrates that ultrasonic wave amplitudes have a long, temporal precursory signal strongly related preseismic fault acceleration for the full spectrum of unstable slip modes. I quantify the sensitivities of ultrasonic wave amplitudes and velocities on stress and slip rate in Chapter 4, and demonstrate how they can be used as long- and short-term precursors respectively to seismicity in the lab and, perhaps occasionally, in crustal faults. Chapter 5 leverages results from the previous chapters to provide a framework for laboratory earthquake forecasting using machine learning on the continuous evolution of ultrasonic wave properties over multiple slow and fast stick-slip cycles. Finally, I introduce shallow slow earthquakes in the Hikurangi subduction margin in Chapter 6. I perform RSF experiments and continuous ultrasonic monitoring on input material to the plate interface obtained during an ocean drilling expedition in mid-2018 in order to better constrain the frictional and hydrologic regime facilitating shallow slow slip in this region. This dissertation provides fundamental insights into the microscopic processes that govern fault friction at the laboratory and crustal scales over a range of slip modes. I demonstrate the underlying similarities between these slip modes and provide insights into the microphysical mechanisms that could modulate fault slip behavior. Finally, I introduce time-lapse monitoring of seismic amplitudes and velocities as a viable method to probe transient fault zone processes over multiple scales.

Faults are primary focuses of both fluid migration and deformation in the upper crust. The recognition that faults are typically heterogeneous zones of deformed material, not simple discrete fractures, has fundamental implications for the way geoscientists predict fluid migration in fault zones, as well as leading to new concepts in understanding seismic/aseismic strain accommodation. This book captures current research into understanding the complexities of fault-zone internal structure, and their control on mechanical and fluid-flow properties of the upper crust. A wide variety of approaches are presented, from geological field studies and laboratory analyses of fault-zone and fault-rock properties to numerical fluid-flow modelling, and from seismological data analyses to coupled hydraulic and rheological modelling. The publication aims to illustrate the importance of understanding fault-zone complexity by integrating such diverse approaches, and its impact on the rheological and fluid-flow behaviour of fault zones in different contexts.

Fluid-rock interactions have long been recognized as crucial drivers in earthquakes and slow slip events. In the context of induced seismicity, the injection of high-pressure fluid underground during wastewater disposal, hydrothermal energy production or hydraulic fracturing operations have triggered earthquakes in geologically stable regions that previously had minimal detected seismicity. Many hypotheses about how these earthquakes were triggered have been proposed, including pore pressure diffusion, long-range poroelastic stressing, and fault loading and reactivation by aseismic slip. The injection of fluid into a fault not only alters pore pressure and triggers slip, but also changes properties of the fault zone that in turn impact fluid flow, pressure diffusion, and fault slip behavior. The most relevant properties here are porosity and permeability. Many experiments, in both the laboratory and in situ, show that dilatancy (the expansion of pores and the fluids within them) accompanies shear deformation of fault zone rocks. In the absence of fluid flow (i.e., undrained conditions), dilatancy reduces pore pressure, increasing the effective normal stress and strengthening the fault. Porosity changes also alter permeability. As pores dilate and more porous space becomes connected, permeability is enhanced. This facilitates fluid flow and enables pore pressure perturbations to reach greater distances along the fault in a shorter period of time. It is certainly evident that the evolution of porosity and permeability, while complex, can fundamentally influence fluid flow and fault slip behavior, and therefore needs to be taken into account in fault models with hydromechanical coupling. In the context of tectonic earthquakes and episodic slow slip events, rock porosity and permeability changes over the earthquake cycle also dictate the nature of the slip that occurs. During the coseismic period, rapid slip cracks open pore space and causes dilatancy, which strengthens the fault and prevents it from slipping further. Permeability is also enhanced as the porosity increases, which may act to weaken further parts of the fault as the fluid migrates. Over the interseismic period, the fault heals from mechanical compaction, and is also gradually sealed by ductile compaction mechanisms such as pressure solution, which involves dissolving minerals at stressed contact points and depositing them in pores. This closing of pores and permeability reduction increases the pore fluid pressure, which will weaken the fault and cause slip again, and this cycle continues. Understanding how the interplay of dilatancy, compaction produces and arrests fault slip is important in characterizing where and how slow slip events occur, and when that might give rise to earthquakes. In this thesis, I investigate the fault response to pore pressure changes coupled to porosity and permeability evolution using 2D numerical simulations of a strike-slip fault governed by rate-and-state friction. The first part of the thesis investigates aseismic slip triggered by fluid injection in the context of induced seismicity. The goal of this study is to evaluate the controlling factors for the initiation and propagation of aseismic slip, and to make testable predictions of potentially observable quantities like the migration rate of the aseismic slip front, as a function of prestress, permeability, injection rate, and frictional parameters. We showcase comparisons for different prestress conditions, permeability values, injection rates, initial state variables, and frictional properties, evaluating their relative importance in determining slip behavior. We also highlight how neglecting porosity and permeability evolution can drastically change the nature of fault slip, and connect our simulations with a limited set of observations to emphasize the important role of hydromechanical coupling in characterizing fault response to fluid injection. Furthermore, we calibrated our model and fit the results to InSAR observations of aseismic slip in the Delaware Basin that is caused by the injection of oilfield water. This shows the applicability of the numerical model to field data and potentially the monitoring of induced seismicity. The second part of the thesis focuses on earthquake cycle simulations in the tectonic context. We explore pore pressure, porosity and permeability evolution over the earthquake cycle and how they impact the occurrences of slow slip events and earthquake ruptures. The first model builds on the study of injection-induced aseismic slip and adds viscous compaction to porosity evolution to study slow slip events. We show that the slow slip events are driven by the interaction between pore compaction which raises fluid pressure and weakens the fault, as well as pore dilation which decreases fluid pressure and limits the slip instability. Cyclic behaviors of these events can range from long-term events lasting from a few months to years to very rapid short-term events lasting for only a few days. The accumulated slip for each event is on the order of centimeters, and the stress drop is generally less than 10 MPa. The second model ignores porosity evolution and only considers permeability evolution that is coupled to effective normal stress, fault slip and a characteristic healing time over which the fault heals interseismically. We demonstrate the viability of fault valving in an earthquake sequence model that accounts for permeability evolution and fault zone fluid transport. Predicted changes in fault strength from cyclic variations in pore pressure are substantial ($\sim$10-20 MPa) and perhaps even larger than those from changes in friction coefficient. We also show how fluids facilitate the propagation of aseismic slip fronts and transmission of pore pressure changes at relatively fast rates. The modeling framework we introduce here can be applied to a wide range of problems, including tectonic earthquake sequences, slow slip and creep transients, earthquake swarms, and induced seismicity.