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CHAPTER 10. EARTHQUAKE <br />10-5 <br />Table 10-2. National Earthquake Hazard Reduction Program (NEHRP) Soil Classification System <br />NEHRP <br />Soil Type Description Mean Shear Velocity to <br />30 m (m/s) <br />A Hard Rock 1,500 <br />B Firm to Hard Rock 760-1,500 <br />C Dense Soil/Soft Rock 360-760 <br />D Stiff Soil 180-360 <br />E Soft Clays < 180 <br />F Special Study Soils (liquefiable soils, sensitive clays, organic <br />soils, soft clays >36 m thick) <br /> <br />10.2. HAZARD PROFILE <br />Earthquakes can last from a few seconds to over five minutes; they may also occur as a series of tremors <br />over several days. The actual movement of the ground in an earthquake is seldom the direct cause of injury <br />or death. Casualties generally result from falling objects and debris, because the shocks shake, damage or <br />demolish buildings and other structures. Disruption of communications, electrical power supplies and gas, <br />sewer and water lines should be expected. Earthquakes may trigger fires, dam failures, landslides or releases <br />of hazardous material, compounding their disastrous effects. <br />Small, local faults produce lower magnitude quakes, but ground shaking can be strong and damage can be <br />significant in areas close to the fault. In contrast, large regional faults can generate earthquakes of great <br />magnitudes but, because of their distance and depth, they may result in only moderate shaking in an area. <br />10.2.1 Past Events <br />According to Washington State Department of Natural Resources, over 1,000 earthquakes occur annually <br />in the State. This is an average of approximately 3 per day though most go unfelt and do not cause damage. <br />Larger magnitude earthquakes, which result in damage, occur less frequently in the state. Table 10-3 lists <br />past seismic events that have impacted the planning area with a magnitude of 3.0 or greater since 1971, <br />according to the Pacific Northwest Seismic Network. <br />Table 10-3. Historical Earthquakes Impacting Kittitas County <br />Date Latitude Longitude Depth (km) Magnitude <br />08/18/1971 47.6488 -120.1457 13.23 3.20 <br />06/15/1976 47.6247 -120.3268 0.75 3.0 <br />07/13/1977 47.0902 -120.9840 3.26 3.90 <br />06/27/1978 46.8767 -120.9717 12.38 3.60