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Brief Report for Maacama fault zone, southern section (Class A) No. 30b

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Compiled in cooperation with the California Geological Survey

citation for this record: Hart, E.W., and Bryant, W.A., compilers, 2001, Fault number 30b, Maacama fault zone, southern section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website,, accessed 04/19/2014 06:33 AM.

Synopsis General: This is a major dextral fault that extends from near Laytonville in Mendocino County nearly to Mark West Creek in Sonoma County. It has been interpreted as a right-stepping northern extension of the Rogers Creek fault [32] and is defined mainly by geomorphic features mapped by Herd and others (1977 #4858), U.S. Army Corps of Engineers (1978 #4867), Huffman and Armstrong (1980 #4862), Pampeyan and others (1981 #1250), Smith (1981 #4863; 1981 #4864; 1981 #4865; 1982 #4866), Bryant (1982 #4851) and Upp (1982 #4868; 1989 #4869). Based on Holocene surface traces, the California Division of Mines and Geology established it as a regulatory Earthquake Fault Zone under the Alquist-Priolo Act (Hart and others, 1983 #4857). McLaughlin (1981 #4859) inferred about 20 km of dextral offset during the Quaternary. Fault creep measured near Ukiah and Willits shows about 5.6 mm/yr, and 7.6 mm/yr, respectively, of dextral slip (Galehouse, 1995 #4853). The fault is fairly well defined by seismicity (Goter and others, 1994 #4855). A paleoseismic study indicates that the most recent event is prehistoric and occurred between 1520 A.D. and 1650 A.D. (Sickler and others, 1999 #4861) whereas other trench investigations indicate Holocene rupture in several places near Ukiah and Willits. Sickler and others (1999 #4861) estimated a preliminary maximum dextral slip rate of 11-14 mm/yr, based on a dextrally offset terrace riser of middle to early Holocene age

Sections: This fault has 2 sections. Jennings (1994 #2878) identifies two Maacama fault sections--a northern and central one (his number 114) and a southern one (his number 141). His twofold sectioning (which is adopted herein) is largely based on surface traces of Holocene faults and is similar that zoned under the Alquist-Priolo Act by the California Division of Mines and Geology (Hart and others, 1983 #4857; 1997 #4856). The two sections vary in strike by about 15 degrees. Working Group on California Earthquake Probabilities (1996 #1216) divided the fault into three sections--north, central and south (their H6, H5, and H4 faults). Petersen and others (1996 #4860) adopted the Working Group on California Earthquake Probabilities sectioning scheme. The northern endpoint of the fault used by Working Group on California Earthquake Probabilities and Petersen extends much farther north than that of Jennings (1994 #2878) or Hart and others (1983 #4857) and appears to be somewhat arbitrary and not based on geomorphic expression. Upp (1989 #4869) recognized 4 "sub zones" for the northern and central sections of others and describes 10 named discontinuous faults. The basis for sectioning and the selection of endpoints used by Working Group on California Earthquake Probabilities, Petersen and others, and Upp is partly unclear.
Physiographic province(s) PACIFIC BORDER
Length (km) This section is 53 km of a total fault length of 160 km.
Average strike N37°W (for section) versus N28°W (for whole fault)
Sense of movement Dextral
Dip Direction V
Historic earthquake
Most recent prehistoric deformation Latest Quaternary (<15 ka)
Slip-rate category Greater than 5.0 mm/yr
Date and Compiler(s) 2001
Earl W. Hart, California Geological Survey
William A. Bryant, California Geological Survey