7 Contributions from Geophysics to Better Understanding the Structure and Tectonics of the Western United States

Monday, 11 May 2009
1:00 PM - 5:00 PM

INTEGRATION OF GEOLOGY AND BOREHOLE GEOPHYSICS TO CHARACTERIZE ROCK PROPERTIES AT THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH (SAFOD) SITE, NEAR PARKFIELD, CA

Paper number: 7-1
Program: Theme Sessions
Time: 1:00 PM – 1:20 PM

Authors

Abstract

Relationships between geologic observations and geophysical logging data within the San Andreas Fault Observatory at Depth (SAFOD) borehole, near Parkfield, CA are examined to better understand the link between material properties and fault zone behavior at seismogenic depths. We correlate measurements of composition, alteration, and deformation textures observed at the meter to sub-meter scale to variations in geophysical parameters using cuttings, core samples, and borehole logging data from 10,000 to 11,000 ft MD within the SAFOD Mainhole. At least two active slip surfaces were identified in the geophysical logs at 10,480 and 10,830 ft MD that correspond to creeping movement along the San Andreas Fault (www.earthscope.org/data/safod).

Three-dimensional plots of geophysical parameters, including density, electrical resistivity, and velocity, show a distinct spatial clustering of data. Each data cluster corresponds to a unique range of values and an associated depth interval. Our results indicate that the materials between the two active slip surfaces can be defined by a particular range of parameter values and/or data cluster. Geologic evidence coupled with the geophysical interpretations, imply that the fine-grained sedimentary sequences and serpentinite-rich rocks within the borehole strongly influence fault zone deformation behavior.