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Abstract
Rice conducted an onshore-offshore seismic survey across the central California transform margin. Refraction velocity modeling of shot and receiver gathers has generated two models with similar near surface features. Model synthetics produce excellent fits to first arrivals and good matches for later arrivals in the data. CMP stacking of nearer offset traces imaged an event interpreted as reflections from the top of a dipping lower crustal layer. Model raytracing indicates layer flattening landward and seaward of the coast. High amplitude late arrivals seen at long offset on receiver gathers are modeled by imbricating the lower crust beneath the coast. This feature is interpreted as a result of overthrusting of the continental and Farallon plates onto the Pacific plate. The models differ in the middle crust, where one includes a laterally discontinuous low velocity zone. No direct indication of this LVZ exists in the data or in offshore reflection profiles.
