IS COLORADO A LOW-HAZARD AREA FOR EARTHQUAKES?

Colorado has suffered from a relative lack of statewide research on earthquake hazards. Much of the knowledge that exists derives from targeted studies of dams and nuclear facilities. Research lagged because many people considered Colorado to have little chance of suffering a strong earthquake. The earthquake record is incomplete because of a lack of ability to detect and accurately locate earthquakes.

Known Quaternary faults in the state have increased from 8 in 1970 to more than 90 in 1998. A compilation of Maximum Credible Earthquakes (MCE) from geotechnical reports reveals a total of thirteen faults with MCE of 6.25 to 7.5. Some areas of the state have extensive Neogene faulting but have not undergone adequate evaluation for Quaternary faulting.

A variety of factors appear to have worked together to cause research into the earthquake hazard in Colorado to have lagged:

Colorado's faults were long considered to be Laramide or older in age, with only broad, regional upwarping occurring after the Laramide orogeny.

Quaternary faults were not recognized in the state prior to 1970.

Colorado's geology makes it difficult to unravel Quaternary faulting in many areas of the state.

The abundance of induced earthquakes at the Rocky Mountain Arsenal and Rangely drew attention away from Colorado's natural earthquakes.

The strongest earthquake in Colorado was not definitively located until 1986.

Microseismic events were claimed not to cluster or to be associated with specific faults and an analysis of the 1:24,000 geologic mapping in the state shows identification of faults is erratic.

Paleoseismic discoveries in areas such as California, Washington, New Madrid, and South Carolina drew attention and resources away from the findings in Colorado.

The accumulated evidence from a variety of disciplines indicates that Colorado needs much more research into its earthquake hazard.