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3.7 Soils, Geology, and Seismicity

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3.7 Soils, Geology, and Seismicity BURBANK AVENUE ANNEXATION DRAFT SUBSEQUENT EIR SOILS, GEOLOGY, AND SEISMICITY 3.7 SOILS, GEOLOGY, AND SEISMICITY Consideration of soils, geology and seismicity conditions is an important aspect of all development in the nine counties surrounding San Francisco Bay (the Bay Area). This includes areas like the Santa Rosa Plain where the Project is located adjacent to creeks that drain to the Llano de Santa Rosa and the Russian River. Although most projects, including the proposed Project analyzed in this SEIR, have little or no effect on the geology of the Bay Area, any project involving construction will have some effect on soils and topography; and all will be affected by certain geologic events, such as earthquakes, landslides or erosion. The purposes of reviewing the soils, geology and seismicity information of any projects are: (1) to identify potentially hazardous conditions; (2) to identify potential impacts of the proposed project; and (3) to provide techniques to reduce, eliminate or avoid these conditions and impacts. 3.7-1 SETTING Regional Setting Regional Geology The regional geologic framework of the Bay Area (Figure 3.7-1), and Sonoma County in particular, can be understood through the theory of plate tectonics. The Earth’s mantle is composed of several large plates that move relative to each other. The San Andreas fault zone is at the junction of two such plates. The Pacific plate, on the west side of the zone, is at the junction of two such plates, and is moving north relative to the North American plate on the east side. One of the results of this movement is the regional rock deformation that is expressed in the general northwest trend of valleys and ridges in Sonoma County. This is visible, for example, in the orientation of the Rodgers Creek fault and the Sonoma Mountains, northeast of the Llano de Santa Rosa. Another result of plate movement is the regional seismicity that Sonoma County has in common with the rest of the Bay Area. All of the geologic formations in the Santa Rosa area are on the North American plate. The oldest bedrock formations exposed in the area are the Petaluma claystone/mudstone and the Sonoma Volcanics (lava flows and volcanic ash), both of the Tertiary period (deposited between 7 million and 1.6 million years ago). Gravels, sands, silts and clays of the major alluvial fans in the northwest-trending valleys are younger, of the Quaternary in age (the last 1.6 million years). Fluvial sand, silt and clay were deposited in local streams cut into the alluvial fans during the latest several thousand years of this period. AUGUST 30, 2005 PARSONS PAGE 3.7-1 Date: 10/2002 Rodgers Cr Project Site San Santa Rosa Andr eek 1 101 eas Napa Gr Sonoma W est Napa een 80 Petaluma V T olay alley - Novato Vallejo Concord San 680 Rafael Hayward Walnut Creek San Gregorio San Francisco LEGEND 101 Highway Burbank Avenue Annexation Site Holocene Fault (active in last 10 thousand years) Quaternary Fault (active in last 2 million years) MILES 0 10 20 Source: Modified from USGS OFR-99-517 City of Santa Rosa Parsons Burbank Avenue Annexation EIR Regional Fault Map Figure 3.7-1 BURBANK AVENUE ANNEXATION DRAFT SUBSEQUENT EIR SOILS, GEOLOGY, AND SEISMICITY Regional Seismicity The regional seismic context is important because the forces that will affect the Project are regional in nature: that is, they probably will be generated outside the Project area, outside the City, and possibly, outside the County. However, the effects of these forces must be accommodated within new development in compliance with regulations and guidelines established by the State and the City. Table 3.7-1 contains the maximum parameters for known faults affecting the Southwest Santa Rosa vicinity. Table 3.7-1 Estimated Maximum Earthquake Magnitudes Peak Horizontal Approximate Duration of Maximum Accelerations in Rock Distance from Moment Strong Shaking Intensity and Stiff Soil (Gravity)4 Project to fault Fault Magnitude1 (seconds)2 (MMI)3 (miles) Napa 6.7 18-30 VII-VIII 0.3 – 0.4 20E Rodgers Creek 7.1 18-30 IX-X > .60 2E San Andreas 7.1 30-60 V-VI 0.2 – 0.3 17W Hayward 7.3 30-60 VII-VIII 0.3 – 0.4 31W Source:EIP Associates 2000 1 For the purposes of describing the size of the design (or scenario) earthquake of a particular fault segment, moment magnitude (Mw) of the characteristic earthquake for that segment has replaced the concept of a maximum credible earthquake of a particular Richter magnitude. This has become necessary because the Richter Scale “saturates” at the higher magnitudes; that is, the Richter scale has difficulty differentiating the size of earthquakes above magnitude 7.5. The Mw scale is proportional to the area of the fault surface that has slipped, and thus is directly related to the length of the fault segment. Although the numbers appear lower than the traditional Richter magnitudes, they convey more precise (and more useable) information to geologic and structural engineers. 2 Duration of ground motion at 0.5g within 10 miles of the fault. Estimates based on relationships developed by Bolt, 1973. 3 Estimated Modified Mercalli Intensity damage level based on relationships developed by Perkins and Boatwright, 1995, or Richter 1958 (San Andreas fault only). 4 Estimates based on relationships developed by Seed and Idriss, 1972, Joyner and Boore, 1981, Campbell and Sadigh, 1983. Sonoma County, together with the rest of the Bay Area, is in one of the most active seismic regions in the United States. Each year, low and moderate magnitude earthquakes occurring within or near the Bay Area are felt by residents of the County. Since the mid-nineteenth century, about 150 local earthquakes have been felt in the County. About 10 of these temblors caused some damage in the County; those of 1906 and 1969 being the most destructive. The April 1906 earthquake on the San Andreas fault has been estimated at about Magnitude (M) 8.3 on the Richter scale and practically destroyed the business district of the City of Santa Rosa. The October 1969 earthquakes on the Healdsburg fault registered M5.6 and M5.7. No deaths occurred; several million dollars damage was done, including numerous breaks in water system pipes. More recently, the M4.9 earthquake along the Hayward fault (26 January 1986) was felt in Sonoma County, but no major damage was reported to utilities or facilities. The M7.1 AUGUST 30, 2005 PARSONS PAGE 3.7-3 BURBANK AVENUE ANNEXATION DRAFT SUBSEQUENT EIR SOILS, GEOLOGY, AND SEISMICITY Loma Prieta earthquake of October 17, 1989 on the San Andreas Fault caused severe damage throughout the Bay Area, but not extensively in the vicinity of Santa Rosa. The major fault zones of the San Andreas Fault System were the sources of all these earthquakes and are expected to be the sources of future earthquakes. Even though no known active fault traces pass through the Southwest Plan Area, it is necessary to design structures and facilities to withstand the anticipated effects of seismic vibration from distant, as well as nearby, sources. The City and County General Plan Safety Elements specifically identify the Rodgers Creek Fault, approximately two miles east of the Project area, as a potential source of seismic activity that must be taken into consideration during the planning of development in the City and County. Following the Loma Prieta earthquake, the U.S. Geological Survey estimated the probability of at least one large earthquake (Magnitude 7 or greater) in the San Francisco Bay region within the 30-year period between 1990 and 2020 at about 67 percent. On the Rodgers Creek fault and the Northern East Bay segment of the Hayward fault, the probabilities are estimated at about 22 and 28 percent, respectively, that a large earthquake would occur in this time-frame. Earthquakes of this magnitude are sufficient to create ground accelerations in bedrock and in stiff unconsolidated sediments severe enough to cause major damage to structures and foundations not designed specifically to resist the lateral forces generated by earthquakes, and to underground utility lines not designed with sufficient flexibility to accommodate expected seismic ground motion. There are several potentially active fault zones that could affect future development projects in the Project area. These include faults that are historically active (during the last 200 years), those that have been active in the geologically recent past (about the last 10,000 years, usually referred to as the Holocene epoch), and those that have been active at some time during the Quaternary geologic period (the last 1.6 million years). The San Andreas, Hayward, Napa, Healdsburg, and Rodgers Creek fault zones are all, at least partially, historically active. Parts of each of the major fault zones have been classified as Holocene or Quaternary depending on the age of the evidence of the most recent movement. The Rodgers Creek fault, about two miles to the east, is the closest known active fault to the Project area. Other nearby traces occur in the faults listed in Table 3.7-1. A characteristic earthquake (Mw 7.3) on the Hayward fault is the largest that would affect the Project area, however, a characteristic earthquake (Mw 7.1) on the Rodgers Creek fault would be so much closer to Santa Rosa that its effects would be more severe. The characteristic earthquake of Mw 7.1 on the Rodgers Creek fault is capable of generating intensities of Modified Mercalli Intensity (MMI) VIII and IX in the Project area. Groundshaking intensities of MMI IX generally will not damage specially designed structures, but can cause moderate damage in structures of good workmanship, and heavy damage in ordinarily substantial buildings, foundations and underground utility lines.
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