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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.
