Environmental Impact Statement - Sonoma Land Trust

California Department of Fish and Game
U.S. Fish and Wildlife Service
Section 3.1. Geology, Soils, and Paleontology
Diked Baylands
The bay muds in the diked baylands are composed of Reyes silty clay, 0 to 2
percent slopes, an acidic (pH approx. 4.5), poorly drained soil. These soils
originated from estuarine mudflats and marshes formed during the Holocene and,
once disconnected from the tides following dike construction in the late 19 th and
20 th century, were subject to draining, oxidation, compaction, grazing, and oathay
farming. Over time, the former wetland soils have gradually dried and
decomposed, causing the land to subside by several feet below historic
elevations. This soil type exhibits high compressibility, low shear strength, and
generally low permeability but is underlain by much stronger and less
compressible soils. Runoff and erosion is typically slight. Shrink-swell potential
is high (Miller 1972).
Geologic Hazards
Primary Seismic Hazards—Surface Fault Rupture and
Groundshaking
The project site is not located within any Earthquake Fault Zone delineated by
the State of California pursuant to the Alquist-Priolo Earthquake Fault Zoning
Act (Hart and Bryant 1997), nor is it traversed by any faults recognized by the
Uniform Building Code as active (International Conference of Building Officials
1997). The project site is thus considered unlikely to undergo surface fault
rupture.
However, Sonoma County, including the Project site, is located in the Uniform
Building Code’s Seismic Zone 4, the most seismically active of the four seismic
zones in the United States. A number of active faults are close enough that the
site and surrounding area are likely to experience substantial seismic
groundshaking during the lifespan of the Project. Faults recognized as active by
the State of California and zoned pursuant to the Alquist-Priolo Earthquake Fault
Zoning Act include, from east to west, the Concord Green Valley fault, 16 miles
to the east; the Calaveras fault, 30 miles to the south; the Greenville fault, 28
miles to the southeast; the Hayward-Rodgers Creek fault, 3 miles to the
northeast; the Maacama fault, 33 miles to the northwest; and the San Andreas
fault, 18 miles to the west. The West Napa fault, which is approximately 10 miles
northeast of the site, is not zoned by the state but is classified as a Type B seismic
source 3 , as defined in the 1997 UBC (International Conference of Building
Officials 1997).
3 Type A seismic sources are faults that are capable of producing large magnitude (M≥7.0) events that have a high
rate of seismic activity. Type B seismic sources are faults other than types A and C. Type C seismic sources are
faults that are not capable of producing large magnitude earthquakes and that have a relatively low rate of seismic
activity.
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.1-7
April 2012