SEISMIC REFLECTION PROFILES IMAGE THE RODGERS CREEK FAULT AND TRENTON RIDGE BENEATH URBAN SANTA ROSA, CALIFORNIA
WILLIAMS, R.A.,USGS, Golden, CO 80401, email@example.com; LANGENHEIM, V.E., and MCLAUGHLIN, R.J., USGS, Menlo Park, CA 94025; ODUM, J.K., WORLEY, D.M., and STEPHENSON, W.J., USGS, Golden, CO 80401; KENT, R.L.,NEES at University of Texas, Austin, TX 78702; McCULLOUGH, S.M., San Jose, CA; KNEPPRATH, N.E., and LESLIE, S.R., USGS, Menlo Park, CA 94025.
The USGS in collaboration with the NEES group at the University of Texas, Austin, collected 13-km of high-resolution seismic-reflection data in two profiles on the Santa Rosa Plain to image basin structure and stratigraphy in this seismically-active area, and to provide constraints for earthquake hazard assessment. We acquired the data using a P-wave minivib I truck, which swept from 15 to 120 Hz, along city streets and creek-side roads. The common-midpoint spacing of these data is 5 m while nominal fold is 36 traces. The Trenton Ridge is a concealed basement high that may contain an active fault and appears to partition the hydrologic response of the Santa Rosa Plain basin fill. The exact depth of this feature, mapped by gravity data, was not known. The Rodgers Creek fault, a northward extension of the Hayward fault, has never been imaged by seismic reflection data. The north-south trending Piner Creek profile images a basement surface that arches to within about 275 m of the ground surface and deepens to about 800 m and 400 m at the northern and southern ends of the profile, respectively. Overlying the bedrock surface are layered Miocene-Quaternary reflectors in the 50- to 800-m depth range that display increased downwarping with depth north and south of the ridge crest. The east-west trending Santa Rosa Creek profile images several faults including the Rodgers Creek fault as it passes near Doyle Elementary School. West of the Rodgers Creek fault we interpret a sedimentary basin more than 1 km deep that underlies downtown Santa Rosa, which was heavily damaged in the 1906 earthquake. This basin shallows to the west as the profile crosses the southeastern side of Trenton Ridge. The westward edge of the ridge appears to be marked by a steeply-dipping fault that displaces basement and Plio-Quaternary reflectors and may be the northern edge of the Cotati basin.