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
Fault Magnitude of Maximum Credible Earthquake 30-Year Probability (Percent) a
Calaveras 5.8-6.9 a All ruptures: 59
M≥6.7: 11
M≥7.0: 2
M≥7.5: 0
Greenville 6.2-6.9 a All ruptures: 8
M≥6.7: 3
M≥7.0: 1
M≥7.5: 0
Macaama (South) 6.9 b Not Provided
West Napa 6.5 b Not Provided
Sources:
a
Working Group on California Earthquake Probabilities 2003.
b
International Conference of Building Officials 1997.
Secondary Seismic Hazards—Liquefaction and Ground
Failure
Secondary seismic hazards refers to liquefaction and related types of ground
failure, as well as seismically induced landsliding. Liquefaction occurs when the
strength and stiffness of a soil are reduced by earthquake shaking or other rapidly
applied loading, and is of greatest concern in areas where well-sorted sandy
unconsolidated sediments are present in the subsurface and the water table is
comparatively shallow.
State seismic hazards maps have not been issued for the Sears Point 7.5-minute
quadrangle, where the project is located. However, based on criteria developed
by Knudsen et al. (2000) to identify liquefaction susceptibility in the 9-County
Bay Area, areas overlying Bay mud at the project site are considered susceptible
to liquefaction hazard. Groundwater in these areas varies seasonally with rainfall,
runoff, and agricultural pumping activities, but generally exceeds ground surface
elevations in the winter months (Wetlands and Water Resources. 2005b). In
general, Knudsen et al. (2000) assign a very high liquefaction potential to
artificial fill over Bay Mud (afbm) and artificial fill, levee (alf), and a high
liquefaction potential to Holocene San Francisco Bay mud (Qhbm) in areas
where groundwater levels are less than 10 feet below ground surface.
Landslides and Other Slope Stability Hazards
Based on USGS mapping by Wentworth et al. (1997), landslides have
historically occurred in the upland areas primarily north of Highway 37, on steep
slopes underlain by the Petaluma Formation. Landslide-prone areas within the
project boundary are mapped as Quaternary silt, clay, sand, and gravel (Q-cs) and
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.1-9
April 2012