Fountaingrove Environmental Impact Report - City of Santa Rosa ...

3.6 GEOLOGY, SOILS, AND SEISMICITY
The Project site is situated east of the Santa Rosa Plain, in the lower foothills of the Sonoma
Mountains. The site lies within an area of gently to moderately rolling terrain on a narrow
strip of undeveloped land located between Thomas Lake Harris Road on the west and a golf
course fairway on the east. Maximum site relief is on the order of 100 feet, with ground
surface elevations ranging from about 400 to 500 feet above mean sea level. The site
generally slopes downward in a west-southwest direction. East of the site, the ground
surface rises to higher elevations of the Sonoma Mountains. West of the site, a series of
relatively steep, southwest-facing slopes descends to the Santa Rosa Plain.
Local drainage features within the Project vicinity site include subdued drainage swales,
closed or nearly closed drainage depressions, and abrupt changes in stream direction and
gradients (Giblin, 2006a). A seasonal, west-flowing creek is located a few hundred feet north
of the Project site. A second creek, Piner Creek, flows across the south end of the site. Flows
in Piner Creek are controlled by the Fountaingrove Lake dams, located approximately 800
feet east of the site.
Geology
Geologic Setting. The California Coast Range physiographic province is located along the
complex boundary margin between two tectonic plates: the North American Plate and the
Pacific Plate. Geologic conditions in the Project area have been and continue to be primarily
controlled by the interaction of these two massive blocks of the earth’s crust. Under the
current tectonic regime, the Pacific Plate moves northwestward relative to the North
American Plate at a rate of about 4 centimeters per year (WGCEP, 2003). Although relative
motion between the two plates is predominantly lateral (strike-slip), an increase in
convergent motion along the plate boundary within the past few million years has resulted
in the formation of mountain ranges and structural valleys of the Coast Ranges province.
See Figure 3.6-1 for a regional geology map.
At the latitude of the Project site, the San Andreas fault system comprises several major
faults, including the San Andreas, Rodgers Creek-Healdsburg, Maacama, and
Concord-Green Valley Faults. In addition to these, many other named and unnamed faults
within the region accommodate relative motion between major faults and relieve
compressional stresses that also act along the plate boundary. See Figure 3.6-2 for the
regional fault map.
The nearest fault to the Project is the Rodgers Creek-Healdsburg fault, the main trace of
which has been identified within several hundred feet of the site. The slip rate on the
Rodgers Creek fault is estimated to be approximately 9 mm/year with a recurrence interval
for large earthquakes estimated at approximately 222 years (Giblin, 2006a; Appendix K).
Faults can be described by one of two activity classes: active and potentially active, as
defined by the CDMG (CDMG, 1992). Active describes Historic and Holocene faults that
have had surface displacement within about the last 11,000 years. Potentially active
describes faults showing evidence of surface displacement during Quaternary time (the past
1.6 million years). A third class of faults, pre-Quaternary age faults, are classified as inactive.
This classification is not meant to imply that inactive fault traces will not rupture, only that
they have not been shown to have ruptured within the past 1.6 million years and that the
probability of fault rupture is low.
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