Sticky, slippery, creepy, crackly crust: geologic, experimental, and analytical approaches to fault slip.

Abstract:

Geologic exposures of tectonic faults and shear zones exhibit a wide variety of structures, compositions, and pressure-temperature histories. Near-surface faults from systems as varied as relatively quiet low-angle detachment faults and seismically hazardous plate-boundary systems are filled with granular fault gouges. Materials from larger depths have mixtures of granular materials, veins, and products of diagenetic-to-metamorphic, diffusive processes. Materials exhumed from beneath orogens and deeper subduction-zone complexes are dynamically recrystallized, but also have syn-tectonic fractures and veins. How many of the geological observations are consistent with a mechanism for slow-slip events (SSEs) via velocity-dependent frictional changes, effective stress variations (via fluid pressure fluctuations), and overall weak-fault regimes? And can the geologic record help us evaluate some of the unexplained temporal and spatial patterns of SSEs relative to large earthquakes, microseismicity, and tremor? Physical experiments and analytical solutions derived from first-principles have been invaluable in addressing these questions. Experiments illustrate that magnitude and periodicity of stick-slip events are sensitive to force-chain dynamics, which are in turn sensitive to packing density and boundary conditions for strain (i.e. dilation vs. constant volume shear). At higher temperatures, as materials are less and less granular and behave rather as continuous materials with a bulk viscosity, fractures can still propagate into shear zones. These semi-brittle regimes are well-described by analytical solutions for a forced damped oscillator, which predicts different magnitudes and time scales of creep events for different shear-zone widths and resistance to fracture. Extending this approach across different micromechanical regimes (including variable frictional and effective stress regimes) can maximize our use of the geologic record as both source of information and ground truth for geophysical deduction.