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SEC' UTHREE Seismotectonic Setting and Historical Seismicity (1983-1993) (Bolt et al., 2003); and the Advanced National Seismic System(ANSS)catalog for more recent events. Colorado is located in a transition zone between the seismically active Basin and Range Province to the west and the significantly less active, cratonic Midcontinent to the east (Figure 1). The majority of the historical seismicity in Colorado is located in the western two-thirds of the state, with relatively few events in the Great Plains east of the Front Range, with the exception of the induced seismicity at the Rocky Mountain Arsenal (RMA) near Denver beginning in the mid- 1960's (Figure 1). Earthquakes up to about Richter magnitude(ML)4, less frequently ML 5, and swarm activity are commonly observed (Bolt and Wong, 1995). This seismicity appears to be the result of extensional reactivation of Laramide thrust/reverse faults (Wong et al., 1994). The apparent concentrated seismicity along the Front Range south of Boulder(Figure 1) is a result of the Front Range network that operated from 1983 to 1993 for Denver Water. Earthquakes as small as about ML 1.0 were recorded, in contrast with other parts of the region where the magnitude threshold is probably ML 3.0(Bolt et al.,2003). A recent and prominent area of seismicity has been the Raton Basin near Trinidad, Colorado. The largest event was a M 5.3 earthquake on 22 August 2011 at a shallow depth of 4 km. The earthquake caused damage to 46 buildings in the sparsely populated epicentral region. The area has been historically seismically active but the recent seismicity has been located near fluid- injection wells suggesting a possible induced cause. 3.2.1 1882 Earthquake Based on the brief historical record, the largest known earthquake that has occurred within Colorado is the enigmatic 1882 M 6.6 earthquake (Figure 1). The event occurred in the early evening of 7 November 1882. It shook most of western and central Colorado, southern Wyoming and parts of eastern and northern Utah(Figure 5). Although this earthquake has been studied in great detail because of its size and implications for seismic hazard in Colorado (Hadsell, 1968; McGuire et al., 1982; Kirkham and Rogers, 1986; Spence et al., 1996), its location and size remain uncertain and controversial. No associated surface faulting or deformation was reported or has yet been found. A high intensity from the earthquake was reported in the Douglas Pass area, north of Grand Junction, where travelers saw landsliding and rockfalls and could hardly stand up. McGuire et al. (1982)give this report some credence and assign a Modified Mercalli(MM)intensity of VIII, whereas Kirkham and Rogers (1986) discount this report as an exaggerated tale and only assign it as felt at this locality. Landsliding also was reported in the mountains (MM VII-VII), but the earthquake was only felt with MM III in the town of Pueblo (Figure 5). MM VI intensities have been assigned to Boulder and Denver in central Colorado, in Rangely, where trees were visibly shaken, and as far north as Laramie, Wyoming where plaster cracked and windows were broken (Figure 5). The site may have been shaken at a MM VI level(Figure 5). The most current and definitive analysis to date on the 1882 earthquake was performed by Spence et al. (1996). Based on an analysis of the 1984 Laramie Mountains earthquake and comparison with the 1882 event, they support Kirkham and Rogers (1986) interpretation of a location in the northern Front Range (Figure 5). They estimate that the magnitude of the 1882 earthquake was M 6.6±0.3. w-MNLi8,7