Magnitude Recurrence Relations for Colorado Earthquakes

Colorado has a significant potential for damaging earthquakes. The Colorado Geological Survey has identified 92 potentially active faults. Two faults have documented slip-rates approaching 1 mm per year. Four hundred and seventy-seven Colorado earthquakes have been felt and/or equaled or exceeded magnitude of 2.0 between 1870 and 1996. Eighty-two earthquakes have equaled or exceeded an MMI Scale of V. Colorado's largest historical earthquake, which occurred on 7 November 1882 (8 November UCT), had an estimated magnitude of 6.5 and maximum MMI of VII to VIII. Colorado's maximum credible earthquake has been estimated at 7.5 ML. In this paper we analyze independent earthquakes (foreshocks, aftershocks, and fluid-injection induced earthquakes removed) to develop magnitude-recurrence relations. Analysis of instrumentally measured earthquakes predicts that a 6.5 ML or larger earthquake occurring somewhere in Colorado has a mean recurrence interval of about 420 years. A magnitude 6.6 ML earthquake has a 10 percent Poisson's probability of exceedance in 50 years. A 7.5 ML earthquake has a 2 percent Poisson's probability of exceedance in 50 years. Colorado's magnitude-recurrence (Gutenberg-Richter) relation is log N=2.58−0.80 ML.