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Western Colorado/Rio Grande Rift -Source 3 The Western Colorado Source contains the Rio Grande rift, the western Rocky Mountains and the Colorado Plateau. The majority of seismic activity in the state of Colorado is located within this source zone. Much of the seismicity is associated with the Rio Grande rift system in southern and south-central Colorado. A diffuse pattern of seismic activity is present in the Colorado Plateau with no appazent regional causative structures. The Rio Grande Rift system contains late Quaternary normal faults and it is possible that the extensional stresses that chazacterize this source could lead to reactivation of preexisting faults within other parts of the source. We treat major seismogenic faults in the Rio Grande rift as line sources in SEISRISK III as discussed in the preced- ing sections. To account for other possible unrecognized or reacti- vated faults, we assign a maximum magnitude of 7.0 to this source zone. Recurrence relations aze illustrated on Figure 5. The;Great Plains -Sources 4 & 5 The Great Plains physiographic province lies east of the Rocky Mountains and south and east of the Denver Basin (Figure 1). Risk Engineering (1994) divided the province into northern and southern source zones at approximately 38.5° north latitude. According to Risk Engineering (1994), the boundary between the northern and southern Great Plains sources is somewhat azbitrary. In general, Source 4 to the south represents an area of higher relief and exposure of bedrock at the surface. Only diffuse and minor earthquake activity is present in the Source 4 and is entirely absent in Source 5 to the north. The maximum earthquake magnitudes assigned to Sources 4 and 5 aze 6.0. Recur- rence relations are illustrated on Figures 6 and 7, respectively. 7.2 ATTENUATION RELATION SEISRISK III requires an attenuation table as input to the program. For this study, we used the attenuation relationship of Joyner and Boore (1981) formulated as: 32