Electromagnetic Imaging of Fluids in the San Andreas Fault: Collaborative Research with University of Washington and Oregon State University

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9614411 Egbert This is a collaborative project between Oregon State University and the University of Washington to acquire and interpret high resolution magnetotelluric (MT) data across two segments of the San Andreas Fault (SAF). In previous work the P.Is. have used continuous MT profiling data to obtain high resolution images of electrical resistivity to depths of 4 km across the Parkfield segment, revealing a narrow zone of anomalously low resistivity in the core of the fault. In the first phase of this project, they will continue work at Parkfield, collecting additional profiles across the San Andreas Fault near Middle Mountain, to obtain a three-dimensional image of electrical resistivity in this area. To extend the image to seismogenic depths they will take MT data at distances of up to 20 km from the fault, and will employ controlled source exploration methods. In the second phase, a three-dimensional high resolution survey of the central creeping segment of the San Andreas fault will be conducted near Hollister, California. Because the resistivity of crustal rocks is largely controlled by fluid content, these surveys will allow constraints to be placed on the distribution and connectivity of fluids within the fault zone, and to show hmw these properties vary between segments with different mechanical behaviors. These results are expected to ultimately provide important information on the role of fluids in the earthquake process. ***