Most people are surprised to learn that natural earthquakes occur in Colorado! They are even more surprised to learn that we experienced a magnitude 6.6 earthquake in the late 19th Century.
The map shows the historic earthquakes recorded since 1867. The CGS maintains the ON-19-01 Colorado Earthquake and Fault Map, which contains the location and information on all cataloged earthquakes in Colorado. In addition to earthquakes, the map also has the location of, and information on, fault lines that were determined to have ruptured within the last 23 million years.
Colorado is most famous in the earthquake literature for the swarm of earthquakes during the 1960’s that were triggered by pumping waste fluids down a deep well at the Rocky Mountain Arsenal. All of this contributes to a false sense of security concerning the possibility of a damaging earthquake(s) hitting Colorado.
CGS also has an Earthquake Reference Collection (ERC) containing more than 500 references to earthquakes and faulting within the state, some rather hard to find in most libraries.
Because earthquakes are caused by the movement of faults, it is important to understand the history of Colorado’s thousands of faults. This study is an ongoing project. So far, geological studies in Colorado indicate that there are about 90 faults reported in the literature that have moved during the Quaternary Period (approximately the last 2 million years) and should be considered potentially active. Extensive information on Colorado’s potentially active faults is provided in the Interactive Earthquake and Fault Mapserver.
The National Seismic Hazard Map that is prepared by the USGS includes three faults in Colorado. The Sangre de Cristo fault (at the base of the Sangre de Cristo Mountains along the eastern edge of the San Luis Valley), and the Sawatch fault, (along the eastern margin of the Sawatch Range) are two of the faults included in the national map. Not all of Colorado’s potentially active faults are in the mountains. For example, the Cheraw fault, which is north of La Junta, is also on the national hazard map. The Derby fault near Commerce City has caused earthquakes and lies thousands of feet below the earth’s surface, but has not been recognized at ground level.
Several potentially active faults in Colorado are thought to be capable of causing earthquakes as large as magnitude 6.5 to 7.5. In comparison, California has hundreds of hazardous faults, some of which can cause earthquakes of magnitude 8 or larger. The time interval between large earthquakes on faults in Colorado is generally much longer than on faults in California. The Colorado Earthquake Hazard Mitigation Council’s Earthquake Hazards Map (2008) contains much usable information.
Past and Possible Future Earthquakes
More than 700 earthquake tremors of magnitude 2½ or higher have been recorded in Colorado since 1867. More earthquakes of magnitude 2½ to 3 probably occurred during that time, but were not recorded because of the sparse distribution of population and limited instrumental coverage in much of the state. For comparison, more than 20,500 similar-sized events have been recorded in California during the same time period. The largest known earthquake in Colorado occurred on November 7, 1882 and had an estimated magnitude of 6.6. The location of this earthquake was in the northern Front Range west of Fort Collins.
The Denver Earthquakes
The most economically damaging earthquake in Colorado’s history occurred on August 9, 1967 in the northeast Denver metropolitan area. This magnitude 4.8 earthquake centered near Commerce City caused more than a million dollars damage in Denver and the northern suburbs. This earthquake is believed to have been triggered by the deep injection of liquid waste into a borehole at the Rocky Mountain Arsenal. It was followed by an earthquake of magnitude 4.5 three months later in November 1967.
Although these events cannot be classified as major earthquakes, they should not be discounted as insignificant. They occurred within Colorado’s Front Range Urban Corridor, an area where nearly 75% of Colorado residents and many critical facilities are located. Since March 1971, well after the initial injection of fluids ceased, 15 earthquakes of approximate magnitude 2½ or larger have occurred in the vicinity of the northern Denver suburbs. At least two published articles or abstracts (Bott, JDJ and Wong, JG, 1996; Evans, D.M., 1966b) available in the ERC (link above) propose that a magnitude 6.0 earthquake is possible on the fault that passes under the Rocky Mountain Arsenal. Such an earthquake would cause more than $10 billion dollars damage. This would be Colorado’s version of Hurricane Katrina – the event that we know is possible, but are not necessarily prepared for.
Summary and Conclusions
Relative to other western states, Colorado’s earthquake hazard is higher than Kansas or Nebraska, but lower than Utah and certainly much lower than Nevada and California. Even though the seismic hazard in Colorado is low to moderate, it is likely that future damaging earthquakes will occur.
Based on the historical earthquake record and geologic studies in Colorado, an event of magnitude 6.5 to 7.5 could occur somewhere in the state. Scientists are unable to accurately predict when the next major earthquake will occur in Colorado, only that one will occur. The major factor preventing the precise identification of the time or location of the next damaging earthquake is the limited knowledge of potentially active faults.
Given Colorado’s continuing active economic growth and the accompanying expansion of population and infrastructure, it is prudent to continue the study and analysis of earthquake hazards. Existing knowledge should be used to incorporate appropriate levels of seismic safety in building codes and practices. The continued and expanded use of seismic safety provisions in critical and vulnerable structures and in emergency planning statewide is also recommended. Concurrently, we should expand earthquake monitoring, geological and geophysical research, and mitigation planning.