Dynamic friction during earthquakes, sliding at seismic slip velocities, is interpreted to be low (<0.1). Thermal pressurization of pore fluids (or simply TP) is widely expected to be activated and dominant during earthquakes, though due to the intricate boundary conditions of TP it is rarely observed and studied in controlled laboratory conditions. This thesis explores the controls and limitations of TP through laboratory experiments and theoretical calculations. TP is activated in velocity-step experiments on water-saturated, low-permeability (<10-19 m^2) Frederik diabase saw-cut faults. The diabase samples are thermally-cracked and their permeability is measured before the friction experiments. Dynamic frictional weakening is short-lived in bare surface faults but is more persistent in wider, gouge-filled faults. Friction decreases by up to 52% of its initial value at relatively modest fault slip velocities of 2.5 mm/s, which suggest that TP may be activated in the early stages of fault slip in natural fault zones. Friction transitions during these experiments from weakening to strengthening and over successive TP events, implying that the hydraulic diffusivity of the fault rocks increase during TP. Stress calculations and measurements show that tensile stresses are induced during TP in the experimental faults, which promote microcrack dilation, which correlates with an increase in hydraulic diffusivity. Theoretical calculations demonstrate that key parameters that control TP evolve considerably during TP, as temperatures and effective stress change in the fault zone. Specifically, the pressurization factor, which controls fluid pressurization, is highly sensitive to temperature, where at high temperatures (>350℃) it decreases continuously due to an increase in fluid …</0.1).>
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