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Brief Report for San Gregorio fault zone, San Gregorio section (Class A) No. 60a

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Compiled in cooperation with the California Geological Survey

citation for this record: Bryant, W.A., and Cluett, S.E., compilers, 1999, Fault number 60a, San Gregorio fault zone, San Gregorio section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 07/24/2014 02:24 AM.

Synopsis General: Holocene active, structurally complex transpressional fault zone as much as 5 km wide. The fault zone is mainly located offshore, west of San Francisco Bay and Monterey Bay, with onshore locations at promontories, such as Moss Beach, Pillar Point, Pescadero Point, and Point A?o Nuevo. Cumulative dextral displacement may total about 155 km (Dickinson, 1996 #5397), but a component of west-vergent reverse displacement also characterizes the fault zone (Lewis, 1994 #5405). Simpson and others (1998 #5414) reported a late Pleistocene dextral slip rate of 3.5-4.5 mm/yr along the Seal Cove fault [60a, San Gregorio section], based on displaced paleochannel deposits at Moss Beach. This is a partial slip rate because faults offshore to the west are also considered active and may contribute an unknown amount of dextral slip. Weber (1994 #5420) calculated a late Quaternary slip-rate of about 10 mm/yr, based on correlation of dextrally offset marine terrace deposits at Point A?o Nuevo. Conversely, dextrally offset stream channels near Point A?o Nuevo indicate a late Pleistocene slip-rate of 4-10 mm/yr (Weber, 1994 #5420). Alternatively, Sedlock, (1999 #5411) argues that piercing points are not well constrained across dextrally offset stream channels and suggests that a lesser dextral slip rate of 1-3 mm/yr better characterizes the San Gregorio fault zone. Clark and Rosenberg (1999 #5394) estimated late Quaternary and Holocene dextral slip rates of between 0.4 and 3.5 mm/yr, based on offset streams, shoreline angles, and colluvial deposits. The most recent earthquake along the San Gregorio fault zone occurred after 1270 AD to 1400 AD, but prior to the arrival of Spanish missionaries in 1775 AD (Simpson and others, 1997 #5413).

Sections: This fault has 2 sections. There is insufficient data to document seismogenic segments. Petersen and others (1996 #4860) and Working Group on Northern California Earthquake Potential (1996 #1216) modeled the fault zone with two segments. Their segment boundary was placed in the north-central part of Monterey Bay. Fault segments designated by Petersen and others and Working Group on Northern California Earthquake Potential herein are considered as sections.
County(s) and State(s) SANTA CRUZ COUNTY, CALIFORNIA
SAN MATEO COUNTY, CALIFORNIA
Physiographic province(s) PACIFIC BORDER
Length (km) This section is 131 km of a total fault length of 241 km.
Average strike N23°W (for section) versus N26°W (for whole fault)
Sense of movement Dextral
Dip Direction V; E
Historic earthquake
Most recent prehistoric deformation Latest Quaternary (<15 ka)
Slip-rate category Greater than 5.0 mm/yr
Date and Compiler(s) 1999
William A. Bryant, California Geological Survey
Sereyna E. Cluett, California Geological Survey