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Effects of Pore Pressures on the Brittle-Ductile Transition in Porous Geomaterials: Laboratory Constraints on the Strength of Seismogenic Faults

Taka Kanaya (Brown University), Greg Hirth (Brown University)

Multiscale Mechanics of Particulate Media

Mon 4:20 - 5:40

Sayles 105

We are conducting triaxial compression experiments on porous geomaterials to characterize the Brittle-Ductile Transition at elevated pore pressures. This study aims to test the fundamental assumption commonly made in models of geologic faults that the depths of the Brittle-Ductile Transition in Earth’s Crust, where devastating earthquakes nucleate, increase with pore pressure following Terzaghi’s effective pressure law for brittle failure. A suite of undrained tests are performed through sealing water in a quartz sandstone at the pressure and temperature conditions where ductile flow has been observed under fluid absent conditions. Despite the inferred attainment of an extreme pore pressure throughout the experiments, our stress-strain curves and macro- and micro-structures suggest that the sandstone underwent ductile flow via dislocation creep at all pore-fluid contents. These findings are incompatible with the assumption that an extreme pore pressure promotes brittle failure associated with a near zero stress even at great depths.// A second series of experiments are conducted under a drained condition using Argon as a pore fluid at the pressure and temperature conditions within the Brittle-Ductile transition. At temperatures of ~300 °C fracture strengths obey the effective pressure law, exhibiting a linear relationship between strength and differential pressure (confining pressure minus pore pressure). Conversely, at a temperature of 900 °C we observe a significant reduction in fracture strength and brittleness under a greater pore pressure, but identical differential pressure, incompatible with the effective pressure law assuming a pore-pressure coefficient of unity. The inferred coefficient may be consistent with those predicted from models of effective pressure laws that consider modification in the geometrical configuration of granular solids and pore fluids via thermally-activated processes.