The currently favored explanation for the low friction slip along the San Andreas Fault Zone (SAFZ) is the presence of fluids under very high pressure. Since fluid transport is subject to wide variation in both time and space, the in situ measurement of pressure gradients and permeability in a fault zone drill hole will establish the current conditions but provide little indication of pressure/transport patterns over the time interval for pressurization cycles of the fault (decades). This study conducted by researchers from the Lamont-Doherty Earth Observatory of Columbia University tests a methodology to determine the flow of fluids into/from the fault zone using the distribution of rare gas concentrations in pore fluids recovered from cores obtained at depth in the SAFZ. Uranium and Thorium decay-series-elements, and Potassium in rocks produce 4He and 40Ar as well as nucleogenic (21,22Ne) and fissiogenic (132,134,136Xe) rare gases. Fluid 'ages' derived from rare gas concentrations depend on the production rates and the release rates into the pore fluids. Vertical and horizontal profiles of these gases in pore fluids can be modeled to determine fluid flow velocities and directions. Samples for these analyses are being obtained by sub-sampling fresh drill core recovered in the framework of the EARTHSCOPE/SAFOD project from various depths in the SAFZ. Because rare gas loss from the rock occurs over a timescales of hours to days, drill core is sub-sampled to remove the outer rind (which has already lost rare gases) and the internal sections are then placed in high vacuum containers in the field. The rare gases released to the headspace over a period of weeks to months are determined by mass spectrometry and the total pore fluid by weight. The entire suite of measured noble gas isotopes (3,4He, 20, 21, 22Ne, 36, 38, 40Ar, 84,86Kr, and 129,131, 132, 134, 136Xe) is used to separate the non-atmospheric (produced in the rocks and accumulated in pore fluids) from the atmospheric noble gas components. The study also investigates the concentrations of rare gases in the rocks and of the elements producing them to determine the source function for excess rare gas abundances in matrix pore fluids. Some of the approaches described above have seen numerous applications in studies of shallow aquifers and in a few cases of deep drilling, but it has never been used in its entirety in the context of deep scientific drilling. This study will establish the methodology providing key information for the source, direction and magnitude of fluid flow that underlie the hypothesis that superhydrostatic fluid pressures are responsible for low friction slip along the SAFZ on timescales of decades. This has wide ranging implications for the understanding the SAFZ and earth quake prediction, affecting millions of people in California. Broader impacts of the study include the training of a graduate student, and the integration of results of this study in the Earth Science curriculum at Columbia University. This work is also integrated in the research and educational activities of Columbia's Center for Hazards and Risk Research (CHRR) whose mission it is to advance the predictive science of natural and environmental hazards and the integration of science with hazard risk assessment and management.
