WEBER, GERALD E., JENNIFER THORNBURG, Earth Sciences Dept., UCSC, Santa Cruz, CA; and JEFFREY M. NOLAN, Nolan Associates, Santa Cruz, CA

Abstract: Recurrence Intervals, Recency of Movement and Holocene Slip Rates Across the San Gregorio Fault Zone at Point Ano Nuevo, San Mateo County, California

Exploratory trenching of the principal faults within the San Gregorio fault zone near Point Ano Nuevo has provided information on the pattern of faulting, and the recency and timing of fault movement. A review of these studies, which were conducted by different parties, indicates the following:

1. The Frijoles fault and Ano Nuevo thrust fault display clear evidence of between 8-13 episodes of recurrent ground rupture on secondary traces of the fault zone during the late Pleistocene and Holocene.
2. Trench exposures along the primary trace of the Frijoles fault indicate a minimum of four episodes of surface ground rupture in the past 6000 to 8000 years. This suggests a minimum recurrence interval of 1500 to 2000 years for earthquakes large enough to produce vertical offsets of approximately one half meter or greater.
3. Truncation of faults by the base of the plowed horizon, and offsets of the modern soil indicate very recent fault movement. Agricultural plowing on the terrace surfaces has significantly altered the original geomorphic expression of the Frijoles fault by destroying the original fault scarps.
4. It appears the majority of the Holocene displacement on the San Gregorio fault zone is occurring on the Frijoles fault. The Coastways fault receives only a portion of the late Holocene slip. However, there is evidence of recurrent Holocene movernent.
5. Both late Holocene and historic sedimentation in coastal stream valleys has obliterated geornorphic evidence of late Holocene surface rupture.

To date we have not been able to accurately determine the late Holocene horizontal slip rate across the San Gregorio fault zone. This reflects our inability (to date) to locate suitable piercing points across the fault from which to measure Holocene slip.

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