Three Windows in Stress from the Geology of Faults

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Knowledge of the absolute stress on faults is as essential to understanding earthquakes as it has proven elusive to measure. The goal of this project is to estimate absolute stress by measuring the response to a stress perturbation caused by fault surface geometry. Geologic features surrounding faults record this stress perturbation in outcrop and include intense cracking in the host rock (damage zones), striations on fault surfaces, and melted rock. Recently, significant progress in generalizing the interrelationships among total fault displacement, surface roughness and damage. In this project surface geometry and fault features are directly linked through detailed observations of along-strike variations. These variations provide measures of how much the surface irregularities perturbed the stress field around the fault. More specifically, the researchers will: (1) measure tensile fracture patterns surrounding rough and smooth faults or sections of faults and use this measurement to calculate the near-field off-fault stress field; (2) measure the deflection of striations away from bumps on fault surfaces and use this measurement to calculate the background stress relative to the perturbing stress; and (3) measure the along-strike variation in pseudotachylyte in relation to fault geometry and use this measurement to calculate a bound on friction.Understanding the stress on faults is critical for understanding earthquakes at the level necessary to calculate risk from a physics-based model. This project is designed to fill in this knowledge gap by focusing on the evidence recorded in geological faults. This combined modeling and fieldwork strategy promises results that can be transferable to other fields of earthquake physics.