--> Abstract: Slip Rate and Earthquake History of the Northern San Gregorio Fault, Half Moon Bay to Moss Beach, CA, by G. D. Simpson and W. R. Lettis; #90920 (1999).

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SIMPSON, GARY D., SHN Consultants, Eureka, CA; and WILLIAM R. LETTIS, William Lettis & Associates, Inc., Walnut Creek, CA

Abstract: Slip Rate and Earthquake History of the Northern San Gregorio Fault, Half Moon Bay to Moss Beach, CA

Between Half Moon Bay and Moss Beach, the on-land portion of the San Gregorio fault forms an east-facing escarpment up to 30 rn high. This on-land fault trace appears to represent the eastern member of a subparallel pair of faults. The western member of this pair lies offshore, and has been imaged in marine geophysical surveys. These faults bound an uplifted fault block, the remnants of which form the Seal Cove Bluffs from Pillar Point on the south to the mouth of San Vicente Creek on the north. This uplifted block is crossed at Seal Cove by an ancestral San Vicente Creek paleochannel that appears to have in-filled immediately following the stage 5a sea level high stand (83 ka). Geomorphically, this paleochannel is expressed as a 200-m-wide windgap across the uplifted block. Across the marine terrace surface northeast of the fault, the continuation of this in-filled paleochannel forms a low ridge that intersects the fault southeast of Seal Cove. The displacement of this paleochannel represents the cumulative postlate Pleistocene offset along the on-land trace of the San Gregorio fault. Borehole transects and interpretation of existing borehole and trench data suggest the paleochannel has been offset between 300 and 360 m. Based on an inferred paleochannel age of 80 to 85 ka, the late Pleistocene slip rate of the on-land fault trace is 3.5 to 4.5 mm/yr. The slip rate of the offshore strand is not known, but may be comparable.

Trench studies across the fault zone near Seal Cove document two earthquakes since 670 A.D. A recent trench along the modern course of San Vicente Creek exposed a broad drag fold at the northeast edge of the fault zone, adjacent to the tip of a northeastvergent thrust fault. The fold records nearly 6 m of apparent vertical relief in alluvium radiocarbon dated at 14.7 ka.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California