Most people are surprised to learn that earthquakes occur in Colorado. The map pictured to the right, shows just how many historic earthquakes we've had. They are even more surprised to learn that we experienced a Magnitude 6.6 earthquake in the late 19th Century. Colorado is most famous in the earthquake literature for the swarm of earthquakes during the 1960s that were triggered by pumping waste fluids down a 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. Read more in our 
Earthquake RockTalk.
The CGS maintains the Colorado Earthquake Mapserver which contains the location of all cataloged earthquakes in Colorado. In addition to earthquakes, the mapserver also has the location of fault lines that were determined to have ruptured within the last 23 million years. CGS also has an Earthquake Reference Collection (ERC) which contains more than 500 references to earthquakes and faulting within the state, some rather hard to find in most libraries. To access the ERC and those publications that are offered online as PDFs, click here.
Colorado is comprised of areas with low to moderate potential for damaging earthquakes, based on research by geologists and geophysicists who specialize in seismology. There are about 90 potentially active faults that have been identified in Colorado, with documented movement within the last 2 million years. However, there are several thousand other faults that have been mapped in Colorado that have not been sufficiently studied to know whether they are capable of generating earthquakes or not. It is not possible to accurately estimate the timing or location of future dangerous earthquakes in Colorado. The lack of an adequate network of seismometers in Colorado makes it difficult to detect and locate small earthquakes. Moreover, the historical record is quite short (~150 years). Nevertheless, the available seismic hazard information can provide a basis for a reasoned and prudent approach to seismic safety.
Faulting
Sudden movement on faults is responsible for large earthquakes. By studying the geologic characteristics of faults, geoscientists can often determine when the fault last moved and estimate the magnitude of the earthquake that produced the last movement. In some cases, it is possible to evaluate how frequently large earthquakes occurred on a specific fault during the recent geological past.
Geological studies in Colorado indicate that there are about 90 faults that moved during the Quaternary Period (approximately the last 2 million years) and should be considered potentially active. The Sangre de Cristo fault, which lies at the base of the Sangre de Cristo Mountains along the eastern edge of the San Luis Valley, and the Sawatch fault, which runs along the eastern margin of the Sawatch Range, are two of the most prominent potentially active faults in Colorado. Not all of Colorado’s potentially active faults are in the mountains and some can not be seen at the earth’s surface. For example, the Cheraw fault, which is in the Great Plains Physiographic Province in southeast Colorado, appears to have had movement during the recent geologic past. The Derby fault near Commerce City 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 6½ to 7¼. 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 (2009) contains much usable information.
Past and Possible Future Earthquakes
More than 500 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 6½. The location of this earthquake was in the northern Front Range west of Fort Collins.
Colorado's Largest Historical Earthquakes (Records date back to 1867)
Although many of Colorado’s earthquakes occurred in mountainous regions of the state, some have been located in the western valley and plateau region or east of the mountains. The most economically damaging earthquake in Colorado’s history occurred on August 9, 1967 in the northeast Denver metropolitan area. This magnitude 5.3 earthquake, which was 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 Rocky Mountain Arsenal. It was followed by an earthquake of magnitude 5.2 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 propose that a magnitude 6.0 earthquake is possible on the fault that passes under the Arsenal. Such an earthquake would cause more than $10 billion dollars damage. This would be Colorado's Katrina-- the event that we know is possible, but aren't prepared for.
Relative to other western states, Colorado’s earthquake hazard is higher than Kansas or Oklahoma, 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. It is prudent to expect future earthquakes as large as magnitude 6.5, the largest event of record. Calculations based on the historical earthquake record and geological evidence of recent fault activity suggest that an earthquake of magnitude 6 or greater may be expected somewhere in Colorado every several centuries.
Summary and Conclusions
Based on the historical earthquake record and geologic studies in Colorado, an event of magnitude 6½ to 7¼ 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.