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<br />6. SEISMICITY <br /> <br />The historic seismicity of Colorado has been described by a number of <br />workers including Hadsell (1968), Simon (1969, 1970, 1972), <br />Presgrave (1979), Kirkham and Rogers (1981, 1985, 1986, in prepara- <br />tion), Stover and others (1984), Oaks and Kirkham (1986), and Risk <br />Engineering (1994). Little doubt exists that Colorado is an area of <br />relatively low historic seismicity. The largest events felt or instnunen- <br />tally recorded over the 116+ year earthquake history of the state <br />include Modified Mercalli (MM) intensity VII and Richter magnitude <br />M,,=5.3-5.4 events, respectively. Moreover, no earthquakes have been <br />unequivocally associated with known geologic structures; and no <br />surface fault rupture has been detected anywhere in the state as the <br />result of historic earthquake activity. The appazent discrepancy <br />between contemporary seismicity and known tectonic structures, <br />including the Rio Grande rift, is not unusual. Long recurrence inter- <br />vals on the order of several hundred to several thousand years may <br />explain the appazent low level of contemporazy seismicity and poor <br />spatial relationship to known Quaternary faults in Colorado. <br />6.1 REGIONAL SEISMICITY <br />Earthquake activity in the Trout Creek dam site area and, indeed, most <br />of Colorado has been infrequent and of relatively low level. Conse- <br />quently, few seismograph stations have been established in the region. <br />Current instrumental detection and location capability for events of <br />magnitude 3.0 to 3.5 or greater is on the order of several tens of <br />kilometers. Prior to 1960, events were located from felt reports. <br />A computer seazch of the Eastern, Central, and Mountain States <br />Catalog, National Earthquake Information Center (USGS) files for <br />Colorado indicates 275 seismic events greater than a threshold magni- <br />tude of 2.0 occurred in the 116 yeazs between November 1882 and <br />January 1998. Maximum MM intensity and body wave magnitude for <br />the 116 yeaz period of record aze VII (estimated M~ 6.2) and mb=5.3, <br />respectively. The unedited earthquake catalog is attached as Appendix <br />I. It should be noted that many earthquakes tabulated in Appendix I are <br />16 <br />