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Geotechnical Investigation Report Detroit Lake Water Reservoir Detroit, Oregon C.3 SEISMICITY C.3.1 Historic Seismicity Information on the historical record of Oregon earthquakes dates back to approximately 1841. Prior to 1900, approximately 30 earthquakes had been recorded. Several hundred earthquakes have been recorded in the state since 1900, especially since the 1980s when the University of Washington established a recording station. Catalogues of earthquake events are available from Berg and Baker (1963) and Johnson et al. (1994). Wong et al. (2000) also provided a summary of Oregon earthquakes. Oregon is a region of low to medium historical seismicity. Clusters of earthquakes are recorded in the Klamath Falls region (M = 6.0), northeast Oregon (M= 5.0 Umatilla, M= 6.5 Milton Freewater), and the Portland – Northern Willamette Valley (M= 5.6 Mt. Angel). Figure C-1 shows historic earthquakes in the site area (within 50-mile radius). C.3.2 Seismic Sources There are several types of seismic sources in the Pacific Northwest that are related to the presence of the Cascadia Subduction Zone (CSZ; Wong et al., 2000). Volcanic sources beneath the Cascade Range, generally to the east from the site, are not considered further in this study as volcanic earthquakes rarely exceed about Mw=5 in size and, thus, do not pose a significant ground-shaking hazard except in their immediate vicinity. C.3.3 Cascadia Subduction Zone – Interface Earthquakes The CSZ megathrust represents the boundary between the subducting Juan de Fuca and the overriding North American plates. The CSZ interface earthquake occurs at a recurrence interval of approximately 400 years. Geologic evidence suggests that the most recent earthquake occurred in January 1700, probably ruptured much of the length of CSZ, and was estimated at M=7.0 to 9.0. The OSSC recommends use of an M=8.5 which likely corresponds to a 10 percent chance of being exceeded in 50 years. This study considers an M=8.5 earthquake. The distance from the edge of the CSZ megathrust to the City of Detroit is approximately 160 kilometers (Wong & Silva, 2000). We have used a conservative distance of 120 km with a depth of 50 km and a magnitude M=8.5 for our analysis. The peak bedrock acceleration using Young’s et al. (1988) attenuation relationship is calculated to be 0.11 g. C.3.4 Intraslab Earthquakes A number of researchers have noted the complete absence of intraslab seismicity in Western Oregon (Ludwin et al., 1991; Rogers et al., 1996). With the possible exception of 1873 Richter Magnitude 6.75 Crescent City Earthquake, no moderate to large intraslab earthquakes have occurred in the CSZ from south of Puget Sound to Cape Mendocino. These earthquakes are postulated to have a deep focus of 40 to 80 km in the subducted Juan de Fuca Plate, and theoretical magnitudes of up to 7.8. For our analysis, we used a M=7.0 earthquake as suggested by the OSSC. These earthquakes are expected to have epicenters for 50 to 70 km from the site. The peak bedrock acceleration using Young’s et al. (1988) attenuation relationship is calculated to be 0.11 to 0.14g. We have considered a peak ground acceleration of 0.14 g for our analysis. November 23, 2009 Project No. 72852.000 C-2