Seismic-Reflection Profiling Across the Urban Area of Santa Clara Valley, California: Images of the Northeastern Margin of the Cupertino Basin

An 8-km-long, P-wave, vibroseis seismic-reflection profile provides new information regarding the Miocene to Pliocene-Quaternary (P-Q) sedimentary structure in the Cupertino Basin on the southwestern side of the Santa Clara Valley, California. We acquired the profile using as many as 177 channels with 10-m receiver and source intervals. A prominent southward dipping reflection package, believed to be the Miocene Monterey Fm. (MF), unconformably underlies 500-600 m of generally flat, well-layered P-Q section. The unconformity, which is only clear along the southern 2 km of the profile, has as much as 40 m of relief over lateral distances of about 250 m. Within the MF a wedge of clear reflections thicken to the south of Creekside Park in Cupertino, with the wedge base approaching 1.2 km depth below Rainbow Dr. These relationships are a striking confirmation of the previously proposed buried Cupertino Basin filled largely with MF. Wedge reflections onlap to the north against a strong reflection that dips about 20 degrees south. This south-dipping reflector separates a highly reflective zone above from a relatively transparent zone below and to the north, although there are hints of south-dipping reflections at 2 km depth below Rainbow Dr. The profile provides no evidence for fault offset greater than 20 m within the 300-m depth range of water wells from which the Cascade and Santa Clara Faults were proposed 30 years ago. There is evidence, however, of a south-dipping, south-side-up fault near the north end of the profile. This fault terminates at a depth of about 550 m, above which strata are slightly warped up to a depth of about 300 m. Faults with smaller offsets may be present but are not detected because of the 8 to 10 m vertical resolution limit of the data. The seismic profile crossed five busy highways, severely reducing the subsurface coverage, thus possibly missing some faults. However, prominent reflectors on either side of these roads are at similar depths, which indicates that faults having vertical offsets greater than 20 m are not likely beneath the roads.