COLLABORATIVE RESEARCH: In Situ and Laboratory Measurement of Seismic Velocity Across an Exposed Brittle Fault Gouge Zone

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Hole 9909376 In crustal faults such as the San Andreas and subduction zone megathrusts, anomalous seismic velocity, seismic reflectivity, and electrical conductivity have been interpreted as signatures of fluid content, pore pressure, fault gouge presence or composition, and damage zone effects. However, the relative contribution of these factors to detectable fault zone properties is poorly known. The PIs will conduct a combined field and laboratory study of seismic velocity and electrical conductivity across an unweathered exposure of the active Seal Cove strand of the San Gregorio Fault in Moss Beach, California. Compressional and shear wave velocity and anisotropy and electrical conductivity will be measured in the field through very high-resolution seismic and resistivity surveys, and in the laboratory on representative samples at elevated pressure. Field measurements will document long length-scale effects that cannot be sampled in the laboratory, bridging the gap between laboratory and geophysical field data, while the experimental data will document pore pressure and stress effects at depth and the relationship of physical properties to lithology and structural geology. The results will be used along with existing lithologic and structural data to create an integrated model of fault zone physical properties.