HR: 0800h
AN: T21A-0441    [Abstracts]
TI: Upper-Crustal Reflectivity of the Central California Coast Range Near the San Andreas Fault Observatory at Depth (SAFOD), USA
AU: * Ryberg, T
EM: trond@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Fuis, G S
EM: fuis@usgs.gov
AF: USGS, 345 Middlefield MS 977, Menlo Park, CA 94025 United States
AU: Bauer, K
EM: klaus@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Hole, J A
EM: hole@vt.edu
AF: Dept. of Geosciences, Virginia Tech, 4044 Derring Hall (MC 0420), Blacksburg, VA 24061 United States
AU: Bleibinhaus, F
EM: bleibi@vt.edu
AF: Dept. of Geosciences, Virginia Tech, 4044 Derring Hall (MC 0420), Blacksburg, VA 24061 United States
AB: We describe a new method of extracting seismic reflections that are visually evident in shot gathers but which may or may not come into focus in an image processed using conventional CDP reflection processing. This method has proven extremely useful in the central California Coast Range, near the San Andreas Fault Observatory at Depth (SAFOD), where conventional CDP processing has thus far produced an image that has few to no clear reflections, although special processing has imaged a couple of steeply dipping reflectors. The image described here includes both gently and steeply dipping reflections that combine to produce an interpretable image of the subsurface.
Our data was recorded along a 46-km profile centered on SAFOD and perpendicular to the San Andreas Fault (SAF), with 62 shots and 912 recorders (shot spacing 0.5 to 1 km; receiver spacing 25 to 50 m). Although conventional CDP processing produced an image with few to no clear reflections, reflections are definitely visible in shot gathers. Using our new method, coherent energy (reflections and other phases) are picked on shot gathers and converted automatically to line drawings, and then the line drawings are migrated in a tomographic velocity model. The final image has clear reflectivity, including both gently and steeply dipping events. We see subhorizontal to gently west-dipping reflective bands within the granitic Salinian block at depths of 6 to 14 km, beginning at approximately the SAF. Within the Franciscan melange east of the fault, we see diffuse gently west-dipping reflectivity at depths of 4 to 10 km. Near the Coast Range (or here Waltham Canyon) fault (CRF), we see a sharp, steeply east-dipping reflector that begins approximately 2 km below the surface and 2 km west of the surface trace of the CRF. At approximately 4 km depth this reflector bends to become gently east dipping. A short but clear zone of west-dipping reflectors connects the top of this curved reflector to the surface trace of the CRF. The reflective zone within the Franciscan block to the west of this system of sharp reflectors terminates at this system. A preliminary interpretation of the CRF region is as follows: the Great Valley sequence to the east of the Franciscan, consisting of folded Mesozoic and Cenozoic sedimentary rocks, is disposed as a blunt wedge that indents the Franciscan block, with the back thrust, or the west-dipping reflectors at the top of this interpreted wedge, now exposed at the surface as the CRF.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005