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I I I I I I I I I I I I I I I I I I I A.6. SEISMOLOGICAL CONSIDERATION. 1. General. A general review of published literature was conducted to provide a basis for determining the seismicity of the Reservoir No. 6 site. The pertinent data Is summarized In the following section. This data Is current only up to 1976 but Is recent enough to provide a sound basis for determining the site seismicity. 2. Historical Data. Colorado Is considered a region of minor earthquake activity (I,2). Eastern Colorado Is ne9rly aseismic; whereas, the northwest and southwest portions of the state and the south central areas of the state are considered Inactive areas where no earthquakes have occurred hlstorlcaliy. Most of the recorded earth Quake shocks have been centered west of the Rocky Mountain Front Range. Epicenters of historical earthquakes larger than Modified Mercalll Intensity II are shown In Table No. 1 (2,3,4). The data tabulated shows no confirmed earthquakes larger than Intensity VII within 200 miles of the site. Twenty-six (26) earthquakes of Intensity V to VII have been recorded within 100 miles of the site. The nearest known earthquake was recorded In the Greeley area on May 26, 1969 with a Modified Mercalll Intensity IV. The most recent earthquake which has produced damage was on August 9, 1967 located In the Denver area and lIsted as an Intensity VII earthquake. This earthquake was one of a series of earthquakes centered In the Denver area which are believed to have occurred due to Injection of watlte fluids In a deep well at the Rocky Mountain Arsenal. 3. PublIshed Seismic Risk Zones. The seismic risk zones published by the Uniform Building Code as shown on Figure 3 Indicates Colorado Is In the Zone 1 category which Indicates minor seismic risk corresponding to Modified Mercalll Intensities V to VI. Seismicity maps produced for the U.S. Geological Survey (6) shows the site to be located In a zone which has a maximum potential Intensity VII. The Applied Technology Council presented peak acceleration coefficient maps and seismic hazard concepts which place the site In a zone with an assigned horizontal acceleration at the bedrock surface equivalent to 0.06 g. 4. Ground Motion. Based on the Intensity IV earthquake recorded In the Greeley area and considering the attenulltlon to the site as proposed by Gupta & Nuttll (7), a MM Intensity III would be felt at the site. Based on the Intensity VII earthquake recorded In Denver In 1967 and considering the attenuation to the site, a MM Intensity V would be felt at the dam site. Using correlations of Trlfunac & Brady (8), a MM Intensity V would cause a maximum horizontal ground acceleration at the site of 0.0378 g. 6. Desl~n Coefficient. During the final design, the stability of the Reservoir No. 6 dam during an earthquake will be determined using a O.lg horizontal seismic coefficient. This coefficient IS conservative based on the lIterature review and the Information provided In the previous sections. The recommended seismic coefficient for this area for ptleudo-statlc stability analysis of earth dams by the U.S. Army Corps of Engineers (6) Is 0.06 g. This coefficient was Increased to provide what Is considered to be a more conservative approach PAGE 8 Chapter V - W~. 8 l'eulbUlty