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Brief Report for West Napa fault, Browns Valley section (Class A) No. 36a

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Compiled in cooperation with the California Geological Survey

citation for this record: Bryant, W.A., compiler, 2000, Fault number 36a, West Napa fault, Browns Valley section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website,, accessed 10/01/2014 02:10 AM.

Synopsis General: Late Pleistocene and Holocene active dextral strike-slip fault generally located along the western side of Napa Valley. Detailed reconnaissance-level mapping exits for most of fault, based on geologic and geomorphic data from Weaver (1949 #5317), Fox and others (1973 #5253), Helley and Herd (1977 #509), Pampeyan (1979 #1245), and Bryant (1982 #5313). Galehouse (1991 #5314) reported possible evidence of creep along the West Napa fault zone following the 1989 Ms 7.1 Loma Prieta earthquake, but the result of 18.5 years of monitoring the fault zone for creep has shown a lack of detectable displacement (Galehouse, 1999 #5500). The West Napa fault is divided into two sections in this compilation. Holocene slip rate and recurrence interval data have not been determined for this fault. Several site-specific studies in compliance with Alquist-Priolo Act (Hart and Bryant, 1997 #4856) have documented location and approximate age of most recent faulting, but detailed paleoseismic investigations have not been done.

Sections: This fault has 2 sections. There is insufficient data to delineate seismogenic segments and there has been no previous attempts at defining sections. However, for this compilation the West Napa fault zone is divided into two sections. The northern section, named here the Browns Valley section, is delineated by a zone of north-northwest-striking late Pleistocene faults that generally lack geomorphic evidence of Holocene displacement (Bryant, 1982 #5313). The southern section, named here the Napa County Airport section, is delineated by northwest-striking dextral slip faults that exhibit geomorphic evidence of Holocene displacement (Helley and Herd, 1977 #509; Bryant, 1982 #5313).
County(s) and State(s) NAPA COUNTY, CALIFORNIA
Physiographic province(s) PACIFIC BORDER
Length (km) This section is 33 km of a total fault length of 43 km.
Average strike N18°W (for section) versus N20°W (for whole fault)
Sense of movement "D, DN, DR"
Dip Direction E
Historic earthquake
Most recent prehistoric deformation Late Quaternary (<130 ka)
Slip-rate category Between 0.2 and 1.0 mm/yr
Date and Compiler(s) 2000
William A. Bryant, California Geological Survey