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<br />TABLE II <br />SEISMOTECTONIC PROVINCES OF COLORADO <br />(Kirkham and Rogers, 1981) <br /> <br />PROVINCE MAXIMUM CREDIBLE <br />EARTHQUAKE <br />Rio Grande Rift <br />Southern Subprovioce M~ = 6.5 to 7.5 <br />Northern Subprovince M~ = 6.0 to 7.0 <br />Eastern Mountain Province M = 6.0 to 6.75 <br />Western Mountain Province M~ = 6.0 to 6.5 <br />Plains Province M~ = 5.5 to 6.0 <br />Uinta-Elkhead Province ML = 5.5 to 6.5 <br />Colorado Plateau Province M~ = 5.5 to 6.5 <br />More recently, Risk Engineering (1994), as part of studies for the <br />former Rocky Flats Nucleaz Weapons Plant redefined seismotectonic <br />provinces and azeal source zones. Based on this work, we adopted five <br />azeal source zones for use in the current PSHA. In the following <br />sections, we discuss each source zone and associated maximum earth- <br />quake magnitudes. Earthquake recurrence relations are presented on <br />Figures 3 through 7. <br />Denver Basin -Source 1 <br />The Denver Basin is a lazge, asymmetrical, north-trending structural <br />downwarp extending from southern Wyoming to south of Colorado <br />Springs. Uplift of the Colorado Front Range in late Cretaceous time <br />(65 mybp) was accompanied by subsidence of the Denver basin to the <br />east. The deepest portion of the basin lies along a synclinal axis <br />directly adjacent and pazallel to the range front of the Rocky <br />Mountains. The geometry of the Denver basin on Figure 1 is based on <br />the Petroleum Information Corp. Oil and Gas map of the United States <br />(1983). <br />The level of seismicity with the Denver Basin source has been low <br />during historical time. The majority of seismic activity within the <br />Denver Basin occurs in the Denver area and diffusely along the eastern <br />Front Range. Most of the earthquake activity in the Denver azea is <br />30 <br />