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Creators/Authors contains: "Allison, Kali L."

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  1. Abstract

    Fault-zone fluids control effective normal stress and fault strength. While most earthquake models assume a fixed pore fluid pressure distribution, geologists have documented fault valving behavior, that is, cyclic changes in pressure and unsteady fluid migration along faults. Here we quantify fault valving through 2-D antiplane shear simulations of earthquake sequences on a strike-slip fault with rate-and-state friction, upward Darcy flow along a permeable fault zone, and permeability evolution. Fluid overpressure develops during the interseismic period, when healing/sealing reduces fault permeability, and is released after earthquakes enhance permeability. Coupling between fluid flow, permeability and pressure evolution, and slip produces fluid-driven aseismic slip near the base of the seismogenic zone and earthquake swarms within the seismogenic zone, as ascending fluids pressurize and weaken the fault. This model might explain observations of late interseismic fault unlocking, slow slip and creep transients, swarm seismicity, and rapid pressure/stress transmission in induced seismicity sequences.

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  2. Abstract

    Localized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.

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  3. Abstract

    We quantify sliding stability and rupture styles for a horizontal interface between an elastic layer and stiffer elastic half‐space with a free surface on top and rate‐and‐state friction on the interface. This geometry includes shallowly dipping subduction zones, landslides, and ice streams. Specific motivation comes from quasiperiodic slow slip events on the Whillans Ice Plain in West Antarctica. We quantify the influence of layer thickness on sliding stability, specifically whether a steadily loaded system produces steady sliding or stick‐slip sequences. We do this using both linear stability analysis and nonlinear earthquake sequence simulations. We restrict our attention to the 2‐D antiplane shear problem but anticipate that our findings generalize to more complex 2‐D in‐plane and 3‐D problems. Steady sliding with velocity‐weakening rate‐and‐state friction is linearly unstable to Fourier mode perturbations having wavelengths greater than a critical wavelength (λc). We quantify the dependence ofλcon the rate‐and‐state friction parameters, elastic properties, loading, and the layer thickness (H). Confirming previous studies, we find thatλc ∝ H1/2for smallHand is independent ofHfor largeH. The linear stability analysis provides insight into nonlinear earthquake sequence dynamics of a nominally velocity‐strengthening interface containing a velocity‐weakening region of widthW. Sequence simulations reveal a transition from steady sliding at smallWto stick‐slip events whenWexceeds a critical width (Wcr), withWcr ∝ H1/2for smallH. Overall, this study demonstrates that the reduced stiffness of thin layers promotes instability, with implications for sliding dynamics in thin layer geometries.

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