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<br /> <br />6.1.3 Local Seismicity <br />Minor ground shaking due to earthquakes probably has been felt in the <br />Trout Creek azea during historic time. The strongest historic ground <br />motion at the site probably resulted from the November 15, 1901, MM <br />intensity VI-VII earthquake near Buena Vista. Although the location <br />of this earthquake is imprecise, a map of Colorado seismicity from <br />1867 through 1996 (Colorado Geological Survey) indicates this event <br />occurred on the west side of the Upper Arkansas Valley within about <br />6 miles (10 km) of the Trout Creek dam site. Aside from the possible <br />association of this earthquake with the Sawatch fault, insufficient <br />recorded earthquake activity, however, exists in the project azea and <br />indeed elsewhere in Colorado to determine seismogenic potential of <br />specific faults. Determination of seismogenic potential of individual <br />faults, therefore, rests entirely on geologic evidence. <br />6.2 REGIONAL ZONATION <br />In 1969, Algermissen attempted to define earthquake hazazds in the <br />United States by development of a seismic zonation map based on <br />historic earthquake data. Colorado was placed in Seismic Zone 1, an <br />area of expected minor damage corresponding to Modified Mercalli <br />Intensity VI ground shaking. Subsequently, Algermissen and Perkins <br />(1976) reevaluated historic seismicity on a probabilistic basis. They <br />defined four source zones in Colorado with maximum Modified <br />Mercalli intensities and Richter magnitudes ranging from V to VII and <br />4.3 to 5.5, respectively. Horizontal accelerations in Colorado, includ- <br />ing the project area, were estimated to range from to 0.05 g to 0.07 g <br />with a 90% probability of not being exceeded in fifty years (return <br />period = 475 yeazs). Geologic data (i.e. fault activity), however, were <br />not used in arriving at these estimates. <br />In an effort to integrate geologic data into regional hazard assessment, <br />the USGS (Thenhaus, 1983) sponsored a series of regional workshops <br />on seismic zonation. Colorado was divided into several seismic source <br />zones based on historic seismicity and fault activity. Hypothetical <br />earthquakes assigned to these zones ranged from Richter magnitude <br />M~=6.5 to 7.5. The largest hypothetical earthquakes were associated <br />with the axial graben of the Rio Grande rift in central and south-central <br />21 <br />