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<br />. <br /> <br />'~'';;. ~ <br />~~:..i' <br /> <br />4. <br /> <br />Seismicity <br /> <br />The conduit area lies in a portion of Colorado where only minor <br />earthquake events are considered likely and a rare higher <br />intensity earthquake is only probable. (Scott, 1970) <br /> <br />Colorado is considered a region of minor seismicity, as can be <br />seen on the seismic zone map of the United States, Figure 111-2, <br />which shows intensity of earthquakes as measured on the Mercalli <br />scale. A description of the Mercalli scale and its relationship <br />to other scales used to measure magnitude and acceleration of <br />earthquakes can be seen in Figure 111-3. No earthquake destructive <br />enough to cause small landslides has occurred in Colorado within <br />recorded history. Hundreds of minor events with intensities <br />of IV or less occur annually from epicenters which are indicated <br />on Figure 111-4. <br /> <br />. <br /> <br />Criteria used by the Bureau of Reclamation for evaluation of <br />seismicity of project areas includes both the historical data and <br />the geologic structural framework of the area. Seismic ~ctivity <br />is associated with faulting; where faults are present, a <br />"credible" possibility of seismic activity exists. If there has <br />been movement along faults within the past 500,000 years, the <br />probability of a destructive earthquake is accepted since an <br />intensity greater than IV is considered necessary to cause such <br />movement, and the fault is considered active. <br /> <br />Historic data for the Colorado Springs and Pueblo epicenters at <br />the two end points of the conduit location, and for two other <br />epicenters to the east shows the maximum seismic activity to be <br />an event with intensity VI which occurred northwest of Pueblo in <br />1870, and one of intensity IV north of Pueblo in 1963. Historic <br />aata 1.8 ava11able only since 1810. Geolugically, maJuL '"lud \!lluUL <br />faults are present within a 50-mile radius of the conduit, and <br />movement wi~hin the past 500,000 years along some of these has been <br />identified by Scott (1970). This is the basis for allowing the <br />probability of a destructive earthquake. A Bureau of Reclamation <br />evaluation of the maximum credible earthquake at Pueblo Dam <br />placed intensity at 6.5 (Richter) with impact on points ,5 to 10 <br />miles away from the epicenters. (Allen, 1973) This can be <br />accepted as maximum for the project strip at points which are ,.,ithin <br />this distance from the identified epicenters in Figure 111-4. <br /> <br />. <br /> <br />III-6 <br />