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2003 Abstract Information

   

Abstract Title
Definition of the Silver Creek Fault and Evergreen Basin from Active-
Source Seismic Reflection Imaging, San Jose, California


Abstract Author
W.J. Stephenson, R.A. Williams, J.K. Odum, C.M. Wentworth, R.T. Hanson, and R.C. Jachens

Abstract Text
Preliminary interpretation of 20 km of P-wave seismic reflection data provides new information on the configuration of the basement surface, the nature of the sedimentary basin fill and the location of the Silver Creek Fault (SCF) adjacent to and within the elongate, northwest-trending Evergreen Basin (EB) located in San Jose, California. These data, which were acquired as part of a larger project to understand seismic hazards in the Santa Clara Valley, were focused on determining fault locations, basin shape, and seismic velocity structure that could affect earthquake ground motions. The 40-km long by 8-km wide EB has been defined previously by gravity modeling and seismic tomography. We acquired two seismic profiles using a 240-channel recording system with 5-m receiver and 10-m source intervals. Profile 1, which follows the Guadalupe River northwestward just west of the western edge of the EB, reveals a moderately undulating basement surface overlain by about 400 m of well-layered Pleistocene and possibly Pliocene sedimentary deposits. Basement paleotopography is indicated by undulations of up to 50 m of relief over about 200 m lateral distance, with overlying beds truncated against the basement highs.

Profile 2 trends northeastward and crosses the EB. A 2-km-long, and as deep as 450-m basement reflection on the western end of this profile shows 100 m of local relief and dips gently eastward before appearing to terminate abruptly in the vicinity of the previously inferred trace of the SCF. A steep gravity gradient and a groundwater boundary inferred from InSAR are the only previous constraints on the location of the SCF here as the fault has no instrumentally-recorded seismicity. We interpret this basement reflection termination to be the location of the SCF. To the east, the sedimentary fill appears to thicken abruptly as indicated by the generally flat-lying layered reflections extending to at least 1.5 km depth. The SCF is poorly constrained but appears to dip steeply to the east, as indicated by the series of westward terminations of reflections just east of the fault. Bedding in a 500-m wide zone above the easternmost basement reflection is tilted and deformed, relative to reflections outside the SCF zone, but the presence of faulting is unclear. The trace of the InSAR boundary directly overlies the eastern tip of the basement reflection termination, and it also overlies the zone of more concentrated deformation, but it is not clearly associated with faulted near-surface sediments at this preliminary stage of analysis.