HR: 09:15h
AN: T11F-06 INVITED     [Abstracts]
TI: Characterization of fault zone structure at the SAFOD site with magnetotelluric exploration
AU: * Unsworth, M
EM: unsworth@phys.UAlberta.ca
AF: University of Alberta, Department of Physics , Edmonton, AB T6G 0B9 Canada
AU: Bedrosian, P A
EM: bedros@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, D-14473 Germany
AB: Magnetotelluric exploration has played an important role in site characterization at the SAFOD site. An initial magnetotelluric (MT) survey in 1994 revealed a fault zone conductor (FZC) extending to a depth of around 2-3 km. A more detailed MT survey in 1997 showed that this feature was typical of the Parkfield segment of the San Andreas Fault. Other MT surveys have shown that similar zones of low resistivity are present worldwide on other major strike-slip faults. These features are generally attributed to a zone of enhanced fracturing and elevated fluid contents, although clay mineralization can also decrease the resistivity. Some segments of major strike-slip faults also exhibit zones of low resistivity in the mid-crust, which may be directly related to crustal flow processes (e.g. creeping segment of the San Andreas Fault and the North Anatolian Fault at Izmit). Since the MT data collection at Parkfield in 1994-7, additional inversion and modeling studies have been undertaken and constrain the depth extent of the low resistivity wedge to around 1.5 to 2 km. High- resolution earthquake locations now reveal that seismicity begins at the base of the low resistivity wedge. The \textit{in situ} resistivity imaged with MT shows a significant correlation with velocities derived from seismic tomography. While the shallow geoelectric structure at the SAFOD site is relatively well understood, the deeper structure at seismogenic depths is less well constained. Synthetic inversion studies show that a longer MT profile could image features in the mid-crust that may be associated with crustal flow and deformation processes. It is possible that the width of a shear zone in the lower crust could be determined by a carefully executed MT survey.
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting