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Brief Report for Barltlett Springs fault system, Round Valley section (Class A) No. 29b

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Compiled in cooperation with the California Geological Survey

citation for this record: Bryant, W.A., compiler, 2000, Fault number 29b, Barltlett Springs fault system, Round Valley section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 09/02/2014 12:50 PM.

Synopsis General: The Bartlett Springs fault system is a major northwest-trending zone comprised of discontinuous, steeply dipping dextral strike-slip faults associated with the San Andreas fault system. The Bartlett Springs fault system can be mapped for at least 120 km from the southern side of Round Valley southeast to near Clear Lake. North of Round Valley, Herd (1978 #5234) suggested that the Lake Mountain fault may by the northern continuation of the Bartlett Springs fault system, indicating a total length of about 165 km. Traces of the Bartlett Springs fault zone locally are delineated by geomorphic evidence of latest Pleistocene and Holocene strike-slip displacement, especially in the vicinity of Lake Pillsbury (dePolo and Ohlin, 1984 #5231; Taylor and Swan, 1986 #5245; Swan and Taylor, 1991 #5243; 1991 #5244; Bryant, 1993 #4908). The Lake Pillsbury area also is characterized by a somewhat broad, linear zone of microseismicity with focal-plane solutions that are predominantly dextral strike-slip (Dehlinger and Bolt, 1984 #5230; Castillo and Ellsworth, 1993 #5281). Taylor and Swan (1986 #5245) excavated and logged trenches across traces of the Bartlett Springs fault zone in the Lake Pillsbury area. Here they documented evidence of Holocene displacement. Taylor and Swan (1986 #5245) reported that stratigraphic relations exposed in one of their trenches can be interpreted to represent at least three faulting events. Swan and Taylor (1986 #5245) reported that the most recent fault rupture event occurred from 300 to 1,000 yr ago, based on their observation that the fault affects the modern soil. Taylor and Swan (1986 #5245) reported a Holocene slip rate of 1 to 2 mm/yr for the fault zone near Lake Pillsbury. Lisowski and Prescott (1989 #5238) reported a creep rate of 8 mm/yr for the 1985-1989 period near Round Valley. Evidence of creep elsewhere along the Bartlett Springs fault system has not been reported.

Sections: This fault has 3 sections. dePolo and Ohlin (1984 #5231) designated four segments for the fault system: from north to south, they are the Elk Creek fault (after California Department of Water Resources, 1969 #4910), the Hot Springs shear zone (after Etter, 1979 #5232), the Chalk Mountain segment, and the Wilson fault [35] (after Lawton, 1956 #5237). Taylor and Swan (1986 #5245) divided the Bartlett Springs fault zone into six segments from the area east of Clear Lake, north of the Wilson fault [35], north to Round Valley, based on differences in geomorphic expression of the fault zone. These segments, from north to south include the Elk Creek, Coyote Rocks, McLeod Ridge, Twin Valley, Reister Rock, and Chalk Mountain. The majority of mapping along the Bartlett Springs fault system is reconnaissance in nature and very little data exists to characterize paleoseismic behavior and delineate paleoseismic segments, thus we consider them herein as non-genetic fault sections. For this compilation the Bartlett Springs fault system has been grouped into three principal sections and include, from north to south, the Lake Mountain section [29a] (after Herd, 1978 #5234), the Round Valley section [29b], and the Bartlett Springs section [29c].
County(s) and State(s) HUMBOLDT COUNTY, CALIFORNIA
TRINITY COUNTY, CALIFORNIA
MENDOCINO COUNTY, CALIFORNIA
Physiographic province(s) PACIFIC BORDER
Length (km) This section is 75 km of a total fault length of 160 km.
Average strike N40°W (for section) versus N23°W (for whole fault)
Sense of movement Dextral
Dip Direction N
Historic earthquake
Most recent prehistoric deformation Late Quaternary (<130 ka)
Slip-rate category Between 1.0 and 5.0 mm/yr
Date and Compiler(s) 2000
William A. Bryant, California Geological Survey