QUATERNARY FAULTING AND SEISMIC HAZARDS IN THE COLORADO ROCKY MOUNTAINS

The Rocky Mountains constitute most of Colorado and are home to the vast majority of the State's 4.3 million people. Flanked on the west and east by the tectonically quiescent Colorado Plateau and Great Plains, respectively, the Colorado Rocky Mountains still exhibit a low to moderate level of seismicity albeit of generally small magnitude (M < 5). A possible exception to this pattern is the Front Range, which has exhibited a surprisingly high level of small magnitude seismicity. This pattern may be an artifact of the relatively dense seismographic coverage that has existed in the region since 1983. Colorado's largest known earthquake, the enigmatic 1882 M 6.6 event whose source still remains unknown to this day is testament to the State's earthquake potential.

The few late Quaternary faults that have been identified to date are concentrated within the northern Rio Grande rift that bisects the Rockies in central Colorado. Paleoseismic investigations of the Williams Fork Mountain, Frontal, Mosquito, and Sawatch faults indicate mid- to late-Quaternary activity with slip rates ranging from less than 0.05 mm/yr to possibly 0.5 mm/yr. This suggests average recurrence intervals of a few to more than 10,000 years. The lengths of these faults suggest that surface-faulting earthquakes of M > 6.5 are possible. This paleoseismic record stands in contrast to the historical earthquake record where no surface-faulting earthquake has been observed.. Site-specific probabilistic seismic hazard analyses performed throughout Colorado's Rocky Mountains for critical and important facilities indicate a low to moderate hazard depending on the proximity to active faults. To date, very little consideration has been given to seismic hazards in Colorado by the vast majority of its population and yet the paleoseismic and seismicity data indicate a level of hazard that warrants further investigations and should be addressed by mitigation actions.