Colorado does experience earthquakes, a fact that surprises many residents who associate seismic activity primarily with coastal states. While the overall risk is significantly lower than in places like California, the state is still geologically active. The seismic hazard level is generally considered low to moderate, but the potential for a damaging earthquake is present. Understanding Colorado’s unique geology and the history of its seismic events helps explain this risk.
Historical Record and Current Frequency
Colorado’s history of recorded seismic events dates back to the late 1860s, documenting hundreds of earthquakes above Magnitude 2.5. The state’s largest known event occurred on November 7, 1882, an estimated Magnitude 6.6 earthquake centered near the northern Front Range, likely west of Fort Collins. This powerful event was felt across a vast area, including parts of Wyoming, Utah, and Kansas. A more recent and damaging event took place near Denver on August 9, 1967, a Magnitude 5.3 earthquake centered near Commerce City that caused millions of dollars in damage within the urban corridor. Currently, the state records dozens of small earthquakes annually, though most are Magnitude 2.0 or less and are not felt by residents, indicating that the frequency of major, damaging quakes remains low.
Underlying Geological Mechanisms
Earthquakes in Colorado are classified as intraplate events, occurring far from tectonic plate boundaries. These quakes are caused by stress fields transmitted through the North American Plate, which build up over time and are released along pre-existing weaknesses in the continental crust. The dominant geological feature influencing Colorado’s seismicity is the Rio Grande Rift, a massive zone of crustal extension that cuts north-south through the central mountainous part of the state. This rift slowly pulls the crust apart, reactivating ancient fault systems associated with the Ancestral Rockies. These deep, ancient faults become the rupture zones for natural earthquakes, some of which are capable of generating events in the Magnitude 6.5 to 7.5 range.
Key Seismic Zones in Colorado
Seismic hazard is not distributed uniformly across Colorado, with certain regions showing a greater concentration of activity. The most prominent area is the Rocky Mountain Front Range, especially the portion south of Denver, which is home to numerous potentially active faults. The state’s largest recorded event, the 1882 Magnitude 6.6 quake, was likely situated in the northern Front Range near Fort Collins. The Rio Grande Rift zone, extending into the central and southwestern mountains, is characterized by clusters of faults, including the prominent Sangre de Cristo Fault and the Sawatch Fault. The Sangre de Cristo Fault is considered capable of producing a major earthquake up to Magnitude 7.5.
Induced Seismicity and Human Activity
Beyond natural tectonic forces, certain human activities have been definitively linked to triggering earthquakes in Colorado, a phenomenon known as induced seismicity. The mechanism involves the deep injection of industrial wastewater, often a byproduct of oil and gas extraction, into disposal wells. This high-volume fluid injection increases the pore pressure within underground rock formations, reducing friction on pre-existing faults and causing them to rupture. The state is historically recognized for induced earthquakes, notably the swarm that occurred in the 1960s near Denver at the Rocky Mountain Arsenal, which was directly linked to the disposal of chemical waste fluids and included the damaging Magnitude 5.3 event in 1967. More recently, the Raton Basin experienced a dramatic rise in earthquakes, strongly correlated with the high-volume injection of wastewater from coal-bed methane production.