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Colorado Earthquake Hazard earthquakes is being compiled under the Colorado Earthquake Hazard Reduction Program (CEHRP). Recorded Earthquake History Like many other states in the Rocky Mountain West, Colorado has a relatively short history of recorded earthquakes. Much of what is known about earth- quakes in Colorado comes from citizens who settled Colorado and from scientists who have studied earthquakes. Observations of the severity of earth- quake effects on people, buildings, and the physical environment are categorized by the Modified Mer- calli (MM) intensity scale. The table on the previous page lists some of the significant earthquakes felt in Colorado since 1870. The largest known earth- quake, the 6.5 magnitude event on November 7, 1882, caused damage on the eastern and western slopes of Colorado and was felt throughout the state. That earthquake was also felt as far west as Salt Lake City, Utah, as far east as Salina, Kansas, and as far north as Ft. Washakie in north central Wyo- ming. Scientists and engineers are re-investigating the November 7, 1882 earthquake in order to under- stand more about its size, the location of the epicen- ter, and the damaging effects from the largest earthquake in Colorado's recorded history. Induced Seismicity Colorado is one of the few places in the world where earthquakes have been generated or triggered by human activities. The most obvious cases of human- induced earthquakes involve experimental nuclear explosions to stimulate natural gas recovery. Three such explosions have been detonated in or very near Colorado: Projects Gasbuggy, Rulison, and Rio Blanco. At the Rangely oil field, scientists were actu- ally able to turn on and off earthquakes by control- led fluid injection. Another case of possible induced seismicity occurred in the 1960s northeast of Denver. In 1961, a deep in- jection well was drilled at the Rocky Mountain Arse- nal. Fluid injection began in 1962 and a series of earthquakes occurred shortly thereafter. The largest of the earthquakes, the August 9, 1967, 5.3 magni- tude and the November 27, 1967, 5.2 magnitude earthquakes, occurred about 18 months after injec- tions stopped. The relationship between the fluid in- jections and earthquakes was never conclusively solved, but the 5.3 and 5.2 magnitude events in 1967 caused over one million dollars in damage in Denver and its northern suburbs. For the past several dec- ades, the seismicity at the Rocky Mountain Arsenal has been monitored, and scientists and engineers from government and the private sector are working to understand earthquakes there. 1967. Earthquake Damage Northglenn, CO Colorado Population At Risk Nearly three-fourths of Colorado's 3.4 million resi- dents live in the Front Range communities. In addi- tion to permanent residents, Colorado is visited each year by nearly five million tourists and business peo- ple. Besides people, most of Colorado's infrastruc- 30