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<br />r. <br />u <br />magnitude was determined at 3.2 M~ by NEIC and 3.3 by the Colorado School <br />of Mines station, GOL... Felt effects associated with this earthquake appear to <br />follow the Arkansas River Valley. <br />Short-term microearthquake surveys in the Upper Arkansas Valley and <br />the San Luis Valley reported only fifteen events in a total monitoring <br />period of 115 days (Sullivan and others, 1980). Activity in the New <br />Mexico portion of the rifr is considerably greater than in Colorado. <br />Significant earthquakes include the MM intensity VlI and VIII events <br />of 1869, 1893, 1906 and 1918 near Socorro, 410 miles (660 km) south <br />of the project area. <br />Several moderate-sized earthquakes have occurred in central Wyoming <br />north of the project azea. A maximum intensity VII earthquake <br />occurred near Casper on November 14, 1897, causing damage in the <br />city. Two intensity VI earthquakes occurred in 1984, one southeast of <br />Casper on October 18, 1984 (mb=5.4) and the second near Lander on <br />November 3, 1984 (mb=5.0). An M,,=4.3 earthquake occurred 35 <br />miles (56 km) east of Lander on August 16, 1985, and was felt as far <br />north as Sheridan (Rocky Mountain News, August 17, 1985). No <br />damage was reported from this event. <br />The October 18, 1984 event is of interest for two reasons. First, the <br />earthquake was felt over an area of 287,000 km' and was the lazgest <br />earthquake documented in southeastern Wyoming in historical time <br />(Langer and others, 1985). Despite the large felt azea, the earthquake <br />caused little damage. Second, post-event microseismic surveys <br />indicated that aftershock hypocenters ranged in depth from about 2I.0 <br />to 25.5 km (Langer and others, 1985). According to Langer (USGS, <br />personal common., 1985) apparent aftershock focal depths are unusu- <br />ally deep, and only one other intraplate earthquake in the U.S. (south- <br />ern Illinois) occurred at a comparable depth. Focal plane solutions (D. <br />Gordon, USGS, personal common., 1985) suggest high-angle, right- <br />lateral, strike-slip movement striking east-northeast, roughly parallel to <br />the structural grain of Precambrian rocks in southeastem Wyoming. <br />Langer (personal common., 1985) indicates the event may be associ- <br />ated with the transition from brittle to ductile crust beneath the <br />Laramie Mountains. The earthquake was felt as far south as Denver <br />and Colorado Springs, but apparently not in the Buena Vista area. <br />Some workers (Langer, personal common., 1985; Kirkham and <br />Rogers, 1986) have speculated that this event may be similar to the <br />November 7, 1882, earthquake felt over much of Colorado. <br />l9 <br />