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1998 <br />• 983-2348.135 <br />Witkind (1976) and Kirkham and Rogers (1981) have identified a number of late <br />Cenozoic (i.e., displacement sometime in the past 23 million years) faults in the region of <br />the site. These late Cenozoic faults include several with documented historic and/or <br />Holocene age displacement (i.e., displacement in the past 10,000 years), that would be <br />classified as active, and a number with documented, or interpreted late Pleistocene age <br />displacement (i.e., displacement in the past 50,000 to 100,000 yeazs), that would be <br />termed potentially active. The closest fault with historical or Holocene activity is the <br />north- to northwest-striking Oil Creek fault that is located about 8 km (5 mi) northeast of <br />the site at its closest approach (Figure 1). Although Kirkham and Rogers (1981) <br />considered the age of most recent displacement on the Oil Creek fault to be about <br />middle to late Pleistocene (about 80,000 to 200,000 years ago), the 1979 magnitude MI 2.9 <br />Divide earthquake was assigned to the Oil Creek fault (Butler and Nicholl,1986) thus <br />classifying it as an active fault. The next closest active (i.e., Holocene-age) fault appears <br />to be the Ute Pass fault zone located about 25 km (15 mi) northeast, neaz Colorado <br />Springs, Colorado (Figure 1). <br />Although there aze a number of mapped active and potentially active faults in the region <br />of the project (e.g., within about 100 km [62 mi] of the site), this region is generally <br />chazacterized by a relatively low level of historical seismicity. This is reflected in the Eact <br />that only 12 historical earthquakes have been recorded in the region (100 km [62 mi] <br />radius of the site) for the period from 1568 to 1998 that was seazched for this <br />investigation (LJ.S. Geological Survey, 1998). These 12 earthquakes ranged from <br />magnitude M 2.5 to 4.0, and had focal depths ranging from 1 km to 10 km (0.6-6 mi), <br />with an average depth of about S km (3 mi). <br />The earliest recorded of the 12 earthquakes was the January 6, 1979 magnitude MI 2.9 <br />Divide earthquake, that occurred on the Oil Creek fault, about 28 km (17 mi) north of the <br />site (U.S. Geological Survey, 1998; Butler and Nicholl, 1986). The largest earthquake was <br />magnitude M 4.0, and it occurred on December 25, 1994 neaz Castle Rock, Colorado. The <br />closest earthquakes to the site were two events that occurred in December of 1995, and <br />were located about 15 km to the east, between the southern extent of the Oil Creek fault <br />and the southern extent of the Ute Pass fault zone. The December 23, 1995 earthquake <br />was magnitude M 3.6, while the December 31,1995 event was magnitude M 2.8. Both <br />had focal depths of 5 km (3 mi). <br />Because of the few number of earthquakes, there aze no clear patterns or spatial <br />assodations with mapped active or potentially active faults. However, 6 of 12 historical <br />events are neaz (within 1 to S km [0.6-5 mi]) either the Oil Creek fault or the Ute Pass <br />fault zone. <br />The previous investigations of the earthquake ground motions at the site by Woodward- <br />Clyde Consultants (1990) and Vector Engineering (1993) have resulted in PGAs for the <br />site which were 0.10 g and 0.14 g, respectively. Examination of regional probabilistic <br />mapping of PGA by the U.S. Geological Survey (1996) and Algermissen et al (1990) has <br />resulted in the following tabulation of approximate PGAs for the site azea, for various <br />return periods: <br />Golder Associates <br />