Demian Saffer & Chris Marone, The Pennsylvania State University Harold Tobin, The University of Wisconsin-Madison
One of the primary goals of EarthScope, and the SAFOD experiment in particular, is to improve our understanding of faulting and earthquakes. Through sampling, down-hole measurements, and long-term monitoring, the SAFOD experiment will provide data to test hypotheses regarding long-term fault strength, earthquake nucleation and recurrence, and the role of fluids in faulting. Laboratory investigations of the frictional, elastic, and fluid transport properties of fault and wall rocks are a key part of achieving these goals. This collaborative project focuses on laboratory measurements of frictional, permeability, and elastic properties for both the active San Andreas Fault (SAF) zone sampled during SAFOD Phase 3 drilling, and for outcrop samples of lithologies that represent the host rock for the cored SAF material. The PI's are applying their experimental results to understand processes governing (1) the strength, sliding stability, and healing of major faults, (2) the hydraulic behavior of faults - both locally as related to long-term and dynamic weakening mechanisms and regionally as elements within crustal scale fluid flow systems, and (3) the rock properties and in situ conditions that cause signatures observed remotely by geophysical surveys. The friction and permeability measurements are being carried out in the Penn State rock and sediment mechanics laboratory, under a range of stress conditions in both a uniaxial loading system and a true triaxial system. The friction experiments include measurements of shear strength and rate-and state parameters relevant to sliding stability, on both powdered gouge and intact "wafers" of material. Permeability measurements are being conducted using flow-through and transient techniques under a range of uniaxial strain and triaxial stress boundary conditions. The elastic property component of the study includes ultrasonic wave velocity and attenuation measurements on intact mini-cores conducted in a pressure vessel at the University of Wisconsin-Madison, and measurements on shearing layers as part of friction experiments at Penn State. Ultimately, this ongoing research will provide experimental constraints on rock properties for the San Andreas Fault system that relate to its mechanical strength, sliding stability, potential role as a barrier to regional fluid flow, and the interpretation of in situ conditions from geophysical data.
