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2. STUDY AREA CHARACTERISTICS Subduction Zone (CSZ) source mechanism. The CSZ originates off the coast of Oregon and Washington and subducts beneath both states. The two source mechanisms associated with the CSZ currently are thought to be capable of producing moment magnitudes of approximately 9.0 and 7.5, respectively (Geomatrix, 1995). Earthquakes caused by movements along shallow crustal faults, generally in the upper 10 to 15 miles, result in the third source mechanism. In Oregon, these movements occur on the crust of the North American tectonic plate when built-up stresses near the surface are released. According to the Oregon Department of Geology and Mineral Industries (DOGAMI), the two largest earthquakes in recent years in Oregon, Scotts Mills, (magnitude 5.6) and Klamath Falls main shocks (magnitude 5.9 and magnitude 6.0) of 1993 were crustal earthquakes (DOGAMI, 2004). Historical Seismicity and Earthquake Risk and Probability Two earthquake databases managed by the U.S. Geological Survey (USGS) National Earthquake Information Center were searched to identify magnitude and location of historical seismic events that have occurred within 60 miles of the Project site (USGS, 200la). The databases searched were, "USGS/NEIC 1973-Present," and "Significant U.S. Earthquakes (1568-1989)." These searches identified 31 seismic events of all magnitudes and intensities that occurred between 1892 and 2004 (the records are more complete for the time period from 1987 to present). Table 2.2.3-1 identifies only those seismic events that meet the following criteria: · Magnitude and/or intensity data are available. · The magnitude of the event is 3.0 or higher. · The intensity of the event using the Modified Mercalli (MM) Intensity Scale (Table 2.2.3- 2) is III or higher, or the event was actually "felt." For reference, an intensity of MM III is associated with shaking that is "felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake" (USGS, 2002). In comparison, an event with an intensity of MM VII would produce the following effects: "Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars"(USGS, 2002). · The seismic event was not an aftershock associated with a larger quake at the same location. MWMC_2.0_REV23.DOC 2-7