Interpretation of Continuous-Offset Seismic Data in Central California Margin

Nathaniel E. Putzig, Alan R. Levander

As part of the Pacific Gas & Electric deep crustal geophysical survey, 154 seismic group recorders were deployed in a linear, fixed receiver array to record three one-ton land dynamite shots and 406 10,280-in.³ air gun pops. Receivers were deployed every 350 m in a 50-km line extending to the northeast from the coast at Morro Bay, California. The receivers cross the Sur-Obispo terrane, the Nacimiento and Rinconada fault zones, and part of the sediment-covered Salinian block. A seismic vessel fired the air gun array from 2.4 to 65.5 km offshore at 155-m intervals in line with the onshore receiver spread. The sources extended across the Hosgri fault zone and the Santa Maria basin.

We have produced a reflection image at 5.5 to 6.0 sec two-way traveltime by CMP stacking traces with offsets less than 20 km. We believe this event, which is at about 14-km depth and dips landward at 10°-15°, is the top of the lower crust. An event that appears on common shot and receiver gathers, we interpret as a wide-angle reflection from the base of this dipping lower crustal layer. Ray tracing of receiver gather refraction records indicates that the layer flattens seaward of the coast. We believe that this feature is a remnant of oceanic crust that had been subducting beneath the Mesozoic-Cenozoic California margin and is tectonically correlative with a bright continuous event found on nearby offshore reflection profiles at about 6 sec two-way traveltime.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.