Collaborative Research: Brittle fracture, deformability, permeability, and seismic anisotropy adjacent to and inside the San Andreas Fault Zone through SAFOD

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

A collaborative research project is underway at the University of Wisconsin-Madison and the State University of New York-Stony Brook, the principal objective of which is to investigate brittle fracture, deformability, permeability and seismic anisotropy adjacent to and inside the San Andreas Fault Zone, as part of the EarthScope science program. This laboratory study is being carried out on core extracted from the San Andreas Fault Observatory at Depth (SAFOD) hole. The geophysical properties that are characterized through this study are fundamental in efforts to understand the mechanisms that bring about earthquakes along the San Andreas Fault. A unique true-triaxial apparatus available in the Madison laboratory is used to obtain empirical strength criteria for the rocks in and adjacent to the San Andreas Fault, information that is crucial to assessing the tectonic stress conditions when used in conjunction with the dimensions of borehole breakouts logged along the hole. The true triaxial tests also provide necessary information on mechanical properties like the elastic moduli, volumetric strain, dilatancy onset, and brittle fracture of the exhumed fault and country rocks. The study seeks to answer questions such as how these properties vary across the fault zone and beyond; how they depend on environmental factors like situ stress, mineralogy, pore pressure, and temperature; how mechanical properties measured on core samples compare with properties inferred from borehole logs and surface-based geophysical observations. Permeability and seismic anisotropy measurements are conducted in the Stony Brook laboratory. The retrieval of core samples in the vicinity of the fault zone and the systematic measurement of permeability as a function of stress provide significant background data on the San Andreas Fault architecture, as well as critical constraints on whether certain mechanisms for pore pressure excess and fault weakening can be operative. No previous systematic studies of seismic anisotropy on fault cores have been conducted. While fairly sophisticated theoretical models relating stress-induced damage and velocity anisotropy are available, laboratory measurements of such velocities under controlled stress and pore pressure conditions as well as quantitative characterization of the damage are necessary to elucidate the mechanics of such phenomena. This comprehensive study of some mechanical and hydraulic properties of the fault material and country rock in the San Andreas Fault Zone is a prerequisite in accomplishing the SAFOD general goal of understanding fault zone processes and earthquake physics.