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.