Abstract

As part of a seismic hazards evaluation for a site in Moab, Utah, we have characterized potential seismic sources in much of the southeastern Utah portion of the Paradox Basin in terms of their probability of being active, geometry, maximum earthquake, and slip rates. These potential seismic sources included 11 faults and two seismic source zones. Ten of the faults including the Moab, Lisbon Valley, Salt Valley, and Paradox Valley faults are associated with salt structures and are probably not seismogenic. To accommodate the remote possibility, however, that these faults could generate earthquakes, they were considered in the seismic hazard analysis at low probabilities. Seismogenic structures included the frontal faults associated with the Uncompahgre uplift.

Although the Colorado Plateau interior in which the Paradox Basin is located is seismically active, very little seismicity has been definitively associated with known faults. The contribution to seismic hazard from such “background” earthquakes was incorporated into the hazard analysis as the Colorado Plateau interior seismic source zone. We adopted a maximum earthquake of moment magnitude (M_w) 6 ± for this zone. The second seismic source zone (maximum magnitude M_w 5 ± ) included in this analysis was a northeast-trending zone of microseismicity aligned approximately along the stretch of the Colorado River southwest of Moab.

Due to the large uncertainties in characterizing seismic sources, a probabilistic seismic hazard analysis using a logic tree approach was performed to estimate the ground motions at the Moab site. Based on the estimated seismic source parameters and empirical relations for seismic wave attenuation, ground shaking, in terms of peak horizontal acceleration, at Moab was estimated as a function of return period. For return periods of 500 to 10,000 years, the peak horizontal accelerations ranged from about 0.05 to 0.18 g. The largest contributor to the seismic hazard at Moab were the background earthquakes of the Colorado Plateau interior.