Final Environmental Impact Statement - New Federal Courthouse

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Geology, Soils, and Seismic Conditions layers. The thickness of these deposits varies from less than one foot up to 300 feet. Underlying the alluvial deposits is bedrock, consisting of Eocene age marine-deposited sediments of the Eugene Formation. Alternative 1 The Riverfront site lies on an alluvial floodplain located approximately 200 to 300 feet east of the Willamette River. The U.S. Geological Survey (USGS) has mapped a concealed or approximately located fault that runs northwest to southeast across the Willamette River loop bordering Willamalane Island Park. Kelly Butte, a volcanic intrusion, is located about '/4 mile northwest of the site. Alluvial deposits underlie the site, but information regazding the specific types of alluvium and their thickness is unavailable. Alternative 2 The Chiquita (formerly Agripac) site lies on an alluvial floodplain approximately 300 feet west/southwest of the Willamette River. Alluvial deposits dominated by a basal, coarse-grained, silty, sandy gravel were observed in all boreholes drilled during a phase II investigation, with the exception of the southeast perimeter of the site. Very coarse gravel and cobbles were present in this azea to depths of 11 to 17 feet below ground surface. Tan to greenish brown sandstone likely representing the Eugene Formation was encountered beneath the alluvium, reportedly extending to depths exceeding 275 feet below ground surface (Scientech 1999). Seismic Conditions Regional Conditions A major tectonic plate boundary between the Juan de Fuca and North American plates lies just west of the coast of Oregon. This convergent boundary is called the Cascadia subduction zone, and the shallow, dipping zone of contact between the plates is called the Cascadia fault zone. The Juan de Fuca plate moves northeastwazd with respect to the North American plate at a rate of approximately 4 centimeters per year. As it collides with North America, the Juan de Fuca plate slides beneath the continent and sinks slowly into the earth's mantle (USGS 1996). Although seismic activity is clearly associated with converging plate margins in other parts of the world, there is little direct evidence from historical and instrumental seismic records of significant seismic activity attributable to the Cascadia subduction zone. Despite this lack of direct evidence, the cumulative result of extensive geologic reseazch by USGS, the National Earthquake Hazard Reduction Program, the Oregon Department of Geology and Mineral Industries, and various Pacific Northwest universities indicates that western Oregon is nonetheless at risk from subduction earthquakes. As a result, the state of Oregon and the International Conference of Building Officials have increased western Oregon's building code earthquake hazard classification requiring buildings to be far more earthquake resistant. Until 1990, Oregon was ranked in seismic zone 2 on the Uniform Building Code scale of zero (low risk) to 4 (highest risk). In 1990, Oregon was reclassified to seismic zone 2B, and in 1993, western Oregon's ranking was increased to seismic zone 3. Final EIS 40 New Federal Courthouse