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Geotechnical Investigation Report Detroit Lake Water Reservoir <br />Detroit, Oregon <br />C.3.5 Crustal Earthquakes and Faults <br />Due to their proximity, the crustal faults are possibly the most significant seismic sources in <br />the Detroit Lake City region. Studies (Yelin & Patton, 1991) of small earthquakes in the <br />region indicate that most crustal earthquake activity is occurring at depths of 10 to 20 km. <br />The location of mapped Quaternary age faults within a 50-mile radius from the site is shown <br />on Figure C-2. The distance of the four closest faults or fault zones is listed in the table <br />below: <br />Table C-2: Faults within the site Vicinity <br />Proximity to Site <br />Fault Zone <br />(Surface projection in km) <br />Clackamas River Fault Zone22 <br />Warm Springs Fault Zones35 <br />Canby-Molalla Fault43 <br />Metolius Fault Zone, Green Ridge Section43 <br />The closest potentially active fault zone is the Clackamas River fault zone, located <br />approximately 22 km from the site with a length of 29 km. This fault zone is interpreted as a <br />broad zone of mostly northwest-striking normal and right-lateral strike-slip faults that offset <br />early Pleistocene, Pliocene and Miocene volcanic rocks in the Cascade Range thus the fault <br />is classified as Quaternary (<1.6 million years) with a slip rate category of <2 mm/year <br />(USGS, 2007). <br />C.3.6 Earthquake Shaking Estimates <br />We have reviewed estimates of earthquake shaking in the area of the site from potential <br />fault sources for an earthquake source from the Clackamas River fault zone and the <br />Cascadia Subduction Zone. PGAs are estimated by the US Geological Survey – National <br />Mapping Program and Joyner and Boore (1993). These estimates are provided in Table C-3 <br />below. <br />Table C-3: Estimates of Earthquake Shaking <br />U.S. Geological Survey – National Mapping Program <br />10% PE in 50 Years2% PE in 50 Years <br />PGA <br />500-Year Event2,500-Year Event <br />0.10 g0.23 g <br />Joyner & Boore (1993) Crustal Fault at 10 km Depth <br />M = 6.00.21 g <br />M = 6.50.27 g <br />C.3.7 Input Earthquakes for Site Response Analysis <br />The design earthquake models used in our analyses and peak horizontal ground <br />acceleration (PHGA) on rock at the recording stations prior to modification in our model are <br />provided below <br />November 23, 2009 <br />Project No. 72852.000 <br />C-3 <br />