PROBABILISTIC SEISMIC HAZARD ANALYSIS FOR CHERRY CREEK AND CHATFIELD DAMS, DENVER, COLORADO

A probabilistic seismic hazard analysis (PSHA) was performed for Cherry Creek and Chatfield Dams, located in south Denver and operated by the U.S. Army Corps of Engineers. The seismic source model included 13 faults in the state of Colorado and 3 areal zones. Faults along the Front Range were modeled as a continuous fault of varying dip, and the individual Golden, Rampart Range, and Ute Pass faults as alternate scenarios. Areal zones of “background” seismicity included a low activity zone in eastern Colorado, a more active zones along the Front Range and one comprising western Colorado and most of Wyoming. Next Generation Attenuation functions were used west of the Front Range – Great Plains boundary, and three central and eastern U.S. functions for sources to the east. Seismicity was derived from catalogs used to develop the 2008 U.S.G.S. National Hazard Maps, and seismicity from a local Front Range network operated by the Denver Water Board was relocated with a progressive velocity-hypocenter inversion technique. Observations from the results include a seismogenic thickness of 25 km along the Front Range and the concentration of deep hypocenters near the Pikes Peak Batholith. Microtremor refraction surveys were performed at each dam to obtain near-surface shear wave velocities. These ranged between 450 and 800 m/s. The PSHA results show the hazard at Chatfield Dam to be higher than for Cherry Creek, due it its closer distance to the more active Front Range sources (about 5 and 25 km, respectively). For Chatfield Dam the 10,000 year mean ground motions are 0.19 g for peak horizontal acceleration, and 0.15 g for 1.0 second spectral acceleration at 5% damping. For Cherry Creek Dam the results are 0.13 g and 0.11 g, respectively. The deaggregation analysis shows that for Chatfield Dam areal zones east of the Front Range-Plains boundary dominate the hazard for all return periods and spectral response periods up to 1.0 second. For Cherry Creek Dam the Front Range areal source zone generally dominates the hazard, but with the Plains areal zone and Front Range fault system becoming important at long return periods and long response periods.