ISSUES IN ASSESSING SEISMIC HAZARDS IN THE ROCKY MOUNTAIN-CENTRAL U.S. TRANSITION REGION IN COLORADO

Central Colorado lies mostly within the Southern Rocky Mountains and is home to the vast majority of the State’s 5.4 million people. The region exhibits a low to moderate level of seismicity of generally small magnitude (moment magnitude [M] < 5). Colorado’s largest historical earthquake, the enigmatic 1882 M6.6 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 in the State are concentrated within the northern Rio Grande rift that bisects the Southern Rocky Mountains. 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. Although this suggests average recurrence intervals of a few thousand to more than 10,000 yrs, timing of the most recent earthquake on these faults is highly uncertain so it is unknown how far along these faults are in their seismic cycles. The lengths of these faults suggest potential surface-faulting earthquakes of M > 6.5.

Site-specific probabilistic seismic hazard analyses of critical and important facilities indicate a low to moderate hazard depending on the proximity to active faults. However, there are two key issues that need to be resolved to obtain a more accurate quantification of the seismic hazard in central Colorado. The first issue is what ground motion prediction models should be used in seismic hazard analyses: models for tectonically active regions like the western U.S. or models appropriate for the central and eastern U.S. The difference in the models is significant with the latter predicting significantly higher ground motions for the same magnitude and distance. The second issue is how to accurately address the hazard from background earthquakes such as 1882 and Quaternary faults that have not been adequately evaluated. Studies to date indicate that previously recognized faults are more active than previously thought. The generally slow progress in understanding the seismic hazards in Colorado has been largely due to the perception that there is very little to any seismic hazard in the State. Paleoseismic and seismicity data, however, indicate a level of hazard that warrants further investigations.