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<br />Two other earthquakes in the province aze worth mention, even though <br />the structures they occtuted on are neither well known nor defined by <br />seismicity. These two events caused ground shaking at Aspen, <br />Colorado: MM Intensity III-IV on February 3, 1941, and MM Intensity <br />V on October 17, 1960. <br />6.1.2 Rocky Mountain Arsenal <br />From April of 1962 through the early 1970s, literally hundreds of <br />earthquakes were appazently induced by injection of fluid wastes at the <br />Rocky Mountain Arsenal northeast of Denver (Hollister and Weimer, <br />1968). The lazgest of these events occurred in 1967 after fluid injection <br />had ceased and had body-wave magnitudes of m6=5.0 (April 10, <br />1967); m6=5.3 (August 9, 1967); and mb=5.2 (November 27, 1967) and <br />mb=5.3 (August 9, 1967). These events, located about 100 miles (160 <br />km) from the project site, were accompanied by ground shaking of <br />MM intensity V to VI. <br />In 1966, the USGS established a dense net of seismometers azound the <br />azsenal well enabling location of earthquake epicenters to within +l <br />kilometer. Data from this net defined anorthwest-trending, elliptical <br />zone of epicenters. According to Kirkham and Rogers (1981), the <br />distribution of epicenters within this elliptical zone is related to a <br />northwest-striking basement fault zone subparallel to the predominant <br />structural grain of Precambrian rocks in the Front Range. The <br />existence of this fault, however, is problematical (Risk Engineering, <br />1994) because no surface expression or displacement of <br />pre-Quaternary bedrock has been documented. <br />Appazent relationships between fluid injection at the Rocky Mountain <br />Arsenal, the Denver earthquakes and estimates of seismic risk in the <br />Front Range area were reviewed by Major (1981). Three points are <br />noteworthy. First, the magnitude distribution of the azsenal earth- <br />quakes is normal (6=0.84). Induced earthquakes might be expected to <br />show abnormal b-values. Second, the energy budgets of pumping and <br />earthquake occurrence do not correlate well in time. And finally, a <br />magnitude 4.2 event occurred on April 2, 1981, long after fluid injec- <br />tion had ceased. Accordingly, seismicity associated with the <br />northwest-trending Rocky Mountain Arsenal zone results from sponta- <br />neous release of crustal strain energy with or without fluid injection. <br />20 <br />