Initial Study MND - Sonoma County Agricultural Preservation and ...

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Taylor MountainINITIAL STUDY/MITIGATED NEGATIVE DECLARATIONB.3.6 Geology, Soils, and MineralsGEOLOGY, SOILS, AND MINERALSWould the project:a. Expose people or structures to potential substantial adverseeffects, including the risk of loss, injury, or death involving:PotentiallySignificantImpactLess thanSignificantWithMitigationIncorporatedLess thanSignificantImpactNo Impacti) Rupture of a known earthquake fault, as delineated on themost recent Alquist-Priolo Earthquake Fault Zoning Mapissued by the State Geologist for the area or based on othersubstantial evidence of a known fault? Refer to Division ofMines and Geology Special Publication 42.ii) Strong seismic ground shaking?iii) Seismic-related ground failure, including liquefaction?iv) Landslides?b. Result in substantial soil erosion or the loss of topsoil?c. Be located on geologic units or soil that is unstable, or that wouldbecome unstable as a result of the project, and potentially resultin on- or off-site landslide, lateral spreading, subsidence, liquefaction,or collapse?d. Be located on expansive soil, as defined in Table 18-1-B of theUniform Building Code (1994), creating substantial risks to life orproperty?e. Have soils incapable of adequately supporting the use of septictanks or alternative wastewater disposal systems where sewersare not available for the disposal of wastewater?f. Would the project result in the loss of availability of a knownmineral resource that would be of value to the region and theresidents of the State?g. Would the project result in the loss of availability of a locallyimportant mineral resource recovery site delineated on a localgeneral plan, specific plan, or other land use plan?Significance criteria established by CEQA Guidelines, Appendix G.B.3.6.1 SettingGeologic SettingTaylor Mountain is the northern extent of the Sonoma Mountains. It is underlain largely bylayered lava flows composed of andesitic and basaltic rocks of the Sonoma Volcanics unit(Grayner et al., 2007). These flows were laid down approximately 5 to 10 million years ago inthe Miocene-Pliocene age. Subsequent to their deposition, the lava flows have been uplifted,tilted to the northeast, and faulted.SoilsThe soils on Taylor Mountain consist primarily of the Goulding and Toomes series, which arewell-drained clay loams composed of weathered volcanic material that are considered suitablefor non-irrigated land uses (NRCS, 2010). These soils are commonly on mountainous uplandswith slopes ranging from five to 50 percent. Runoff of Goulding soil is generally rapid and theFinal B-70 September 2012
Taylor MountainINITIAL STUDY/MITIGATED NEGATIVE DECLARATIONhazard of erosion is high. Grazing is common on Goulding soils. Other soils on the project siteinclude the Raynor series (NRCS, 2010), which is commonly found on slopes of nine to 15percent. These soils are well-drained clays and are found on rolling hills. Sheep and cattlegrazing commonly occur on Raynor soils. The soil types vary in location by slope, depth tobedrock, and amount of clays, loams, and gravelly/cobbly material present. Bedrock outcropsoccur along ridgelines and in scattered hillslope locations. Sporadic intrusions of ultramaficrocks and associated serpentine soils also occur at the site.B.3.6.2 Environmental Impacts and Mitigation Measuresa. SeismicityLESS THAN SIGNIFICANT. The Bay Area, in general, is considered seismically active (SeismicZone 4). The faults in Sonoma County are part of the San Andreas Fault system. A recentlyactive fault, the Rodgers Creek Fault, runs through the northeastern section of the propertyalong a northwest/southeast trend. The Rodgers Creek Fault, (Maximum Credible Earthquake7.0) is thought to be a northern extension of the Hayward Fault and is responsible for the 1969Santa Rosa earthquakes (magnitudes 5.6 and 5.7) (Blake et al., 2000). The epicenter of the1969 earthquake was located on the southwestern slopes of Taylor Mountain (City of SantaRosa, 2002). The San Andreas Fault (Maximum Credible Earthquake 8) is located about 20 milesto the southwest (Sonoma County, 2008).Fault Rupture. The Rodgers Creek and San Andreas faults are the two principally active BayArea “strike-slip” faults and have both experienced movement within the last 150 years. Strikeslipfaults are those that primarily exhibit displacement in a horizontal direction (City of SantaRosa, 2002). The area along the Rodgers Creek Fault through the property is designated as anAlquist-Priolo zone. The Alquist-Priolo Earthquake Fault Zoning Act (P.R.C Section 2621 et seq.)prohibits the location of most types of structures for human occupancy across the active tracesof faults in earthquake fault zones. Portions of trails in the eastern area of the park would crossthe Rodgers Creek Fault Zone, but no structures are proposed within the Alquist-Priolo zone.Groundshaking. Taylor Mountain is very likely to be affected by future earthquakes and strongground-shaking can be expected on the project site (Miller Pacific, 2010). The intensity ofshaking depends on distance to fault rupture zone, earthquake magnitude, earthquakeduration, and the specific geology of the site. The Sonoma County General Plan Public SafetyElement identifies the slopes of Taylor Mountain as being subject to Very Strong (XIII) andViolent (IX) ground-shaking in the event of an earthquake on the Rodgers Creek Fault. Visitorsto the Preserve would be subject to violent ground-shaking during an earthquake. However,for the most part, visitors would be on open land, resulting in minor exposure to adverseseismic conditions.Although strong ground shaking could potentially damage structures on the site and pose apotential hazard to the public, these impacts would be reduced to the extent possible throughappropriate engineering and construction. Project facilities would be engineered to withstandexpected ground shaking without substantial adverse impacts. Structural design for allowed usesSeptember 2012 B-71 Final
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