The 14 August 1983 Cimarron, Colorado Earthquake and the Cimarron Fault

On 14 August 1983, a small earthquake of Richter magnitude 3.4 occurred near the town of Cimarron in southwestern Colorado. The analysis of data collected from seismographic stations located within the intermountain U.S. suggests the earthquake occurred on the Cimarron fault, a major west-northwest trending Precambrian fault which has experienced displacement possibly since Oligocene time. A fault plane solution determined for the earthquake exhibits normal faulting on a moderately dipping, west-northwest- or east-west-trending fault plane in good agreement with the geologic observations of the Cimarron fault. The fault plane solution also displays a north-northeast trending minimum compressive stress suggesting that this earthquake occurred in response to a reactivation of the Cimarron fault in an extensional tectonic stress field similar to that observed in the Rio Grande rift to the southeast or the Southern Great Plains to the east. The location and the faulting characteristics of the Cimarron earthquake represent the best evidence to date that associates an earthquake with a known major fault in Colorado.