COLORADO PALEOSEISMIC CONUNDRUMS – IS THE HAZARD REALLY AS LOW AS IT SEEMS?

Seismic hazard in Colorado is certainly low relative to other parts of the western United States, whether viewed by historical earthquake observations, or by comparisons of late Quaternary fault data. However, the support for this perception is strongly anchored in the absence of adequate paleoseismic event data for faults in Colorado. Of five Colorado faults included in the 2014 National Seismic Hazard Maps, three have paleoseismic event data from trenches, and these data document no more than two to three Holocene events on the Northern Sangre de Cristo fault and a single Holocene event on the Cheraw fault. Trench data from the Southern Sawatch fault suggests the MRE there was likely pre-Holocene, with recurrences intervals much greater than 10 kyr. Existing characterizations for other faults are primarily based on geomorphic offsets of late Pleistocene landforms (~ 12 to 35 (?) ka in age), and few constraints on the age of MRE. In contrast, if the limited historical seismicity record is viewed as indicative of and consistent with the cumulative rates of larger events on faults in the same region, one might expect at least five to ten events with M>~6.5, with associated surface ruptures on major faults such as those included in the NSHM.

The characterization of each Colorado fault in the NSHM has limitations, but the shorter scarp lengths and lower slip rates on the Gore Range Frontal and Southern Sawatch faults suggest existing data may have fewer limitations. Paradoxically, in the 2014 NSHM, the highest annual event rate of any fault in Colorado is attributed to the Williams Fork fault due to the magnitude cap imposed by the very short fault length, despite the absence of event data. New data from the Cheraw fault indicates a longer fault length than used in 2014 NSHM, and increases the potential for additional Holocene events on the northeastern sections of that fault. On the 185-km-long Northern Sangre de Cristo fault, the few existing trenches leave most potential rupture sections for surface-rupture earthquakes >M6.5 untested and present NSHM modeling allows significant weight to long ruptures.