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Monterey Bay Studies

Monterey Bay Studies


Monterey Bay National Marine Sanctuary Project, 1999

Location and Structure of the San Gregorio Fault across the Northern Monterey Bay Shelf

by Andrew J. Stevenson, H. Gary Greene, and Roberto Anima


Recently collected single channel high resolution seismic reflection profiles across the San Gregorio Fault Zone reveal a wealth of detail regarding the structure, morphology and recent activity associated with this tectonic element where it crosses the northern Monterey Bay shelf. The data was collected using a 475 Joule boomer as a sound source, with a central frequency near 1 Khz. 2 receivers were used; a 20 element streamer for deep reflections and a 3 element streamer for shallower structures. Lines were run across the fault zone with a spacing of 150m (inner shelf) to 300m (outer shelf), and cover the fault from Ano Nuevo Cove (~ 5m water depth) to the outer shelf seaward of Santa Cruz (~250m water depth). The most striking feature visible in the seismic records is the large amount of compressional deformation associated with the fault, which has been interpreted as primarily a strike-slip feature. This compressional deformation in places affects the youngest sediments imaged in our profiles, suggesting it is ongoing at present. Further evidence of active compression can be seen where the wave planed bedrock surface formed during Quaternary sea level lowstands has been warped and folded, and where anticlinal fold axes adjacent to the fault extend above the surrounding recent sediment blanket, forming linear bedrock outcrops parallel to the adjacent fault.

This abstract is for a poster presented at the 6th Annual Monterey Bay National Marine Sanctuary Research Symposium, Monterey, California, March 9, 1996.

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