Final Environmental Impact Statement - New Federal Courthouse

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Geology, Soils, and Seismic Conditions Locat Conditions The Oregon Department of Geology and Mineral Industries (2000) recently compiled seismic information for a comprehensive report on the Eugene/Springfield area. The Oregon azea is susceptible to three types of earthquake: crustal, intraplate, and subduction zone earthquakes. Crustal earthquakes occur on faults that aze between 5 and 15 miles deep. Most of the earthquakes recorded in the Eugene area are crustal events, such as the 1993 Scotts Mills earthquake (magnitude of 5.5 to 6.0 on the Richter scale) and Klamath Falls earthquake (magnitude 5.9 to 6.0 on the Richter scale) (Figure 7). Intraplate earthquakes occur about 25 to 40 miles deep within the Juan de Fuca plate; this type of earthquake could potentially occur 25 to 30 miles directly below Eugene. The 1949 and 1965 Puget Sound intraplate earthquakes (magnitudes of 7.1 and 6.5 on the Richter scale, respectively) were centered approximately 110 to 150 miles north of Portland. Subduction zone earthquakes occur along a boundary between two converging tectonic plates. The largest earthquake of this type anticipated in the Northwest, where substantial stress appears to build up along the junction between the Juan de Fuca and ,North American plates, has an expected magnitude of 8.0 to 9.0 on the Richter scale. Evidence indicates that subduction zone earthquakes occur off of the coast of Oregon an average of every 300 to 500 years (USGS 1996). The USGS has mapped two visible faults and two concealed or approximately-located faults in the metropolitan azea of Eugene. One visible fault is located northeast of Spencer Butte in south Eugene, and the other is located south of the Oak Hill School site in southeast Eugene. Of the two concealed or approximately located faults, one runs northwest to southeast across the Willamette River loop bordering Willamalane Island Park, adjacent to the Alternative 1 site. The other is located in west Eugene, south of the crossing of the Southern Pacific Railroad and West 11 ~' Avenue. Although there is no recorded history associated with these faults, the possibility of movement during an earthquake cannot be ruled out (Lane Council of Governments 1989). Both Eugene and Portland aze located along aoohwest-trending fault that horizontally offsets the Cascades. Portland experienced an earthquake in 1962 caused by horizontal movement of the fault. Both of the 1993 earthquakes, Scott-Mills and Klamath Falls, were felt in the Eugene area, but neither caused any reported damage (Black 1999 personal communication). The Oregon Department of Geology and Mineral Industries seismic hazard assessment is based on three potential earthquake hazards: slope instability, liquefaction (i.e., water-saturated soils behaving like liquids during earthquakes), and ground shaking amplification (i.e., the tendency for ground shaking to be increased by the soils and rocks near the surface). There is no zone A designation (an area of greatest potential hazard during an earthquake) within the area mapped for Eugene/Springfield. Both of the alternative sites fall within the azea designated as zone D, lowest hazard. The relative slope instability hazard is mapped as no hazard for both sites and the relative amplification hazard also is mapped as the lowest hazard. Ground shaking amplification is a greater hazard for azeas underlain by Holocene gravel channel deposits (younger alluvium) than those areas underlain by Pleistocene gravels (older alluvium). New Federal Courthouse 41 Final EIS