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Geophysical and geological studies provide evidence for cyclic changes in fault‐zone pore fluid pressure that synchronize with or at least modulate slip events. A hypothesized explanation is fault valving arising from temporal changes in fault zone permeability. In our study, we investigate how the coupled dynamics of rate and state friction, along‐fault fluid flow, and permeability evolution can produce slow slip events. Permeability decreases with time, and increases with slip. Linear stability analysis shows that steady slip with constant fluid flow along the fault zone is unstable to perturbations, even for velocity‐strengthening friction with no state evolution, if the background flow is sufficiently high. We refer to this instability as the "fault valve instability." The propagation speed of the fluid pressure and slip pulse, which scales with permeability enhancement, can be much higher than expected from linear pressure diffusion. Two‐dimensional simulations with spatially uniform properties show that the fault valve instability develops into slow slip events, in the form of aseismic slip pulses that propagate in the direction of fluid flow. We also perform earthquake sequence simulations on a megathrust fault, taking into account depth‐dependent frictional and hydrological properties. The simulations produce quasi‐periodic slow slip events from the fault valve instability below the seismogenic zone, in both velocity‐weakening and velocity‐strengthening regions, for a wide range of effective normal stresses. A separation of slow slip events from the seismogenic zone, which is observed in some subduction zones, is reproduced when assuming a fluid sink around the mantle wedge corner. Plain Language Summary: Slow slip events are observed in subduction zones worldwide. Their mechanism is not well understood, but geophysical and geological research suggests a relation with recurring changes in fluid pressure within the fault zone. Here we explore the fault valve mechanism for slow slip events using mathematical and computational models that couple fluid flow through fault zones with frictional slip on faults. The fault valve mechanism (arising from cyclic changes in the permeability or resistance to fluid flow) produces pulses of high fluid pressure, accompanied by slow slip, that advance along the fault in the direction of fluid flow. We quantify the conditions under which this occurs as well as observable properties like the propagation speed and rate of occurrence of slow slip events. We also perform simulations of subduction zone slow slip events using fault zone and frictional properties that vary with depth in a realistic manner. The simulations show that the fault valve mechanism can produce slow slip events with approximately the observed rate of occurrence, while also highlighting some discrepancies with observations that must be addressed in future work. Key Points: We analyze the dynamics of fault slip with fault‐zone fluid flow and fault‐parallel permeability enhancement with slip and sealing with timeFault‐valve instability produces unidirectional aseismic slip and pore pressure pulses even with velocity‐strengthening frictionSubduction zone earthquake cycle simulations show that the fault‐valve instability can produce slow slip events below the seismogenic zone [ABSTRACT FROM AUTHOR]

Fluids influence fault zone strength and the occurrence of earthquakes, slow slip events, and aseismic slip. We introduce an earthquake sequence model with fault zone fluid transport, accounting for elastic, viscous, and plastic porosity evolution, with permeability having a power‐law dependence on porosity. Fluids, sourced at a constant rate below the seismogenic zone, ascend along the fault. While the modeling is done for a vertical strike‐slip fault with 2D antiplane shear deformation, the general behavior and processes are anticipated to apply also to subduction zones. The model produces large earthquakes in the seismogenic zone, whose recurrence interval is controlled in part by compaction‐driven pressurization and weakening. The model also produces a complex sequence of slow slip events (SSEs) beneath the seismogenic zone. The SSEs are initiated by compaction‐driven pressurization and weakening and stalled by dilatant suctions. Modeled SSE sequences include long‐term events lasting from a few months to years and very rapid short‐term events lasting for only a few days; slip is ∼1–10 cm. Despite ∼1–10 MPa pore pressure changes, porosity and permeability changes are small and hence fluid flux is relatively constant except in the immediate vicinity of slip fronts. This contrasts with alternative fault valving models that feature much larger changes in permeability from the evolution of pore connectivity. Our model demonstrates the important role that compaction and dilatancy have on fluid pressure and fault slip, with possible relevance to slow slip events in subduction zones and elsewhere. Plain Language Summary: Water in the crust plays an important role in controlling the strength of fault zones and frictional sliding, which manifest as earthquakes and slow slip events that do not produce ground shaking. In this study, we perform computer modeling of earthquake sequences that are coupled to the evolution of fluid pressure and rock properties. In particular, compaction or dilation of the water‐filled pore space in rock drives changes in fluid pressure and influences the fault's frictional resistance to slip. The model quantifies the effects of compaction and dilation on both large earthquakes and slow slip events, providing specific predictions regarding slow slip event properties, pressure changes, and changes in fluid flow that might be testable with geophysical, geologic, and geochemical data. Key Points: Fluid pressure changes from pore compaction and dilatancy influence slow slip events in our modelModeled slow slip events span long‐term events lasting for months to years to rapid short‐term events for a few daysEarthquake recurrence interval is controlled in part by compaction‐driven pressurization and weakening [ABSTRACT FROM AUTHOR]

It is widely recognized that fluid injection can trigger aseismic fault slip. However, the processes by which the fluid‐rock interactions facilitate or inhibit slip are poorly understood and some are oversimplified in most models of injection‐induced slip. In this study, we perform a 2D anti‐plane shear investigation of aseismic slip that occurs in response to fluid injection into a permeable fault governed by rate‐and‐state friction. We account for porosity and permeability changes that accompany slip, including dilatancy, and quantify how these processes affect pore pressure diffusion, which couples to aseismic slip. Fault response to injection has two phases. In the first phase, slip is negligible and pore pressure closely follows the standard linear diffusion model. Pressurization eventually triggers aseismic slip close to the injection site. In the second phase, aseismic slip front expands outward and dilatancy causes pore pressure to depart from the linear diffusion model. We quantify how prestress, injection rate, permeability and other fluid transport properties affect the slip front migration rate, finding rates ranging from 10 to 1,000 m/day for typical parameters. The migration rate is strongly influenced by the fault's closeness to failure and injection rate. The total slip on the fault, on the other hand, is primarily determined by the injected volume, with minimal sensitivity to injection rate. Additionally, we show that when dilatancy is neglected, slip front migration rate and total slip can be several times higher. Our modeling demonstrates that porosity and permeability evolution, especially dilatancy, fundamentally alters how faults respond to fluid injection. Plain Language Summary: The underground injection of fluids during wastewater disposal, geothermal operations, and other energy‐production activities has been linked to the occurrence of earthquakes. In addition to earthquakes, fluid injection can also trigger aseismic slip on faults, that is, frictional sliding that occurs so slowly that seismic waves and ground shaking are not produced. Here, we perform computer modeling of fluid injection and aseismic slip, exploring how the injection rate and fluid transport properties influence the aseismic slip response. We speculate that additional complexity in frictional properties and other conditions would cause aseismic slip to be accompanied by numerous, small earthquakes (microseismicity), as is often observed during injection. We quantify the rate at which aseismic slip migrates outward from the injection site and compare predicted migration rates to observed microseismicity patterns. Our model also predicts fluid pressure changes, slip, rock deformation surrounding the fault, and fluid flow paths that might be measurable and used to validate the modeling. Key Points: Modeling of constant rate fluid injection into a fault predicts steadily propagating aseismic slip frontMigration rate of aseismic slip front increases with injection rate and ranges from 10 to 1,000 m/day for typical parametersDilatancy and permeability enhancement alter system response as compared to linear pore pressure diffusion [ABSTRACT FROM AUTHOR]