Collaborative Research: Influence of Structure, Composition and Fluid-Rock Chemistry on Mode of Slip in the San Andreas Fault Zone at SAFOD

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In collaboration, Judith Chester and Frederick Chester of Texas A&M University, James Evans of Utah State University, and David Kirschner of Saint Louis University are analyzing the cores taken during phase 3 sidetrack drilling at SAFOD to characterize the composition, structure, and deformation mechanisms of the San Andreas fault zone. The work is addressing key questions regarding mineralogic transformations, fluid-rock interactions, physical properties, and mechanics of the fault. This study is addressing four general hypotheses that relate to the 1) architecture of the fault zone, 2) processes involved in seismic and aseismic deformation, 3) absolute strength of the fault, and 4) energy budget of earthquakes. A number of techniques are being used to test the hypotheses. Mapping the mesoscopic structure and lithology of the core is providing information on the fault-zone architecture, which is being conveyed to the scientific community in order to guide the ongoing investigations and sampling by other research groups. Optical and electron microscopy, XRF, XRD, and stable isotope analyses are being used to characterize the microstructures, mineral reactions, and fluid-rock interactions that are important in the deformation of seismic and aseismic faults in the San Andreas fault zone. Mesoscopic and microscopic fabric analyses of faults, fractures, and veins are delimiting the average orientations of principal stresses relative to the fault zone, and thus place constraints on the long-term strength of the fault zone. Optical and electron microscopy is being used to characterize the reduction in grain size and formation of new fracture surfaces in seismically active portions of the fault zone, and thus will help place constraints on the energy budget of seismic events. This research project, in combination with the results of other SAFOD-related projects, is working to document at seismogenic depths the physical and chemical processes that are most important in generating earthquakes, and thus to reduce the devastating loss of life and property damage that earthquakes cause in the U.S. and world. The project is contributing to broader research programs and activities of national interest including the National Earthquake Hazard Reduction Program (NEHRP) and the Southern California Earthquake Center (SCEC). At least three graduate and two undergraduate students are participating in the project as part of their thesis research, and are gaining valuable experience in scientific drilling and numerous laboratory techniques. The PIs are continuing to educate the general public on the EarthScope SAFOD project and earthquakes through dedicated web presentations, media interviews and presentations to lay audiences.