Earthquakes occur in Colorado, but the state experiences a lower frequency and magnitude of seismic events compared to regions located directly on tectonic plate boundaries, such as California or Alaska. Colorado sits on the interior of the North American Plate, classifying its seismic activity as intraplate seismicity. While the overall hazard is considered low to moderate, the state is not immune to ground shaking and has a history that includes both naturally occurring and human-triggered events.
Historical Frequency and Magnitude
Colorado’s historical earthquake record, dating back to 1867, includes over 700 recorded events measuring magnitude 2.5 or higher. The vast majority of these tremors are minor, with many going unnoticed by the general public.
The largest known natural earthquake in Colorado’s history is estimated to have been a magnitude 6.6 event on November 7, 1882. This powerful tremor was likely centered in the northern Front Range, west of Fort Collins, and was felt across several states, causing damage to structures in Denver and Boulder. Modern analysis suggests that the faults in the state are capable of generating an event in the magnitude 6.5 to 7.5 range.
While small quakes are common, large, damaging earthquakes of magnitude 6.0 or greater are rare events. The long time intervals between these significant events complicate efforts to predict when or where the next major earthquake will occur. Scientists rely heavily on geologic studies of ancient faults to assess the potential for future seismic hazards.
Natural Geological Triggers
Seismic activity away from plate edges, known as intraplate seismicity, is driven by the slow build-up of stress within the continental crust. Colorado’s earthquakes are a result of tectonic compression and extension acting on ancient weaknesses deep within the North American Plate. This stress field can re-activate faults that have been dormant for millions of years, causing them to slip and release energy.
The major geological feature influencing natural seismicity is the Rio Grande Rift, a zone where the crust is slowly being pulled apart, creating a region of crustal stretching. This extensional force is responsible for the formation of the mountainous landscape and the existence of large, potentially active fault systems, such as the Sangre de Cristo Fault.
Another force contributing to fault movement is crustal unloading, sometimes referred to as tectonic rebound. Research suggests that the massive weight of glaciers during the last ice age suppressed movement on certain faults. As the ice melted and the load was removed, the underlying crust rebounded, allowing these faults to become more active.
The Role of Induced Seismicity
Colorado holds a unique place in seismic history as the location of one of the first scientifically recognized cases of human-caused, or induced, seismicity. This episode occurred in the 1960s, linked to a deep injection well at the U.S. Army’s Rocky Mountain Arsenal near Denver. From 1962 to 1966, the Army injected millions of gallons of contaminated liquid waste more than two miles deep into fractured Precambrian rock.
Shortly after injection began, the previously quiet Denver area experienced an earthquake swarm that lasted for years. The largest event in this series was a magnitude 5.3 earthquake in August 1967, which caused property damage in the Denver metropolitan area. Scientists confirmed that the fluid injection triggered the seismicity by increasing the pore pressure within the rock formation.
The heightened pressure acted as a lubricant, reducing the friction holding dormant faults in place. Subsequent experiments in the Rangely oil field further confirmed this link, demonstrating that manipulating fluid pressure in a deep well could control earthquakes.
This understanding led to modern regulatory responses concerning wastewater disposal. Today, areas like the Raton Basin in southern Colorado and locations near Greeley have experienced recent swarms of induced seismicity attributed to high-volume wastewater injection from oil and gas operations.
Colorado’s Primary Seismic Zones
Seismic activity in Colorado is not uniformly distributed across the state, but is concentrated in specific geographic regions tied to underlying geologic structures.
Southern Colorado and the Rio Grande Rift
One of the most active areas is the southern portion of the state, particularly the region that encompasses the Rio Grande Rift. This zone includes the San Luis Valley and the mountain ranges bordering it, featuring major faults like the Sangre de Cristo Fault.
Central Rocky Mountains
The central Rocky Mountains also show clustered activity, with events recorded in areas near Buena Vista and Crested Butte. This region contains numerous ancient fault systems that are susceptible to reactivation by the regional stress field. Historically, parts of the Western Slope, such as the Paradox Valley, have also shown localized seismicity.
The Front Range
The Front Range, which is the most densely populated corridor, is another significant zone, largely due to the historic and ongoing potential for induced earthquakes. The Denver area remains a focus of seismic interest because of the potential for re-activation of the underlying Derby Fault. Furthermore, the far southern Front Range, including the Raton Basin, has been a site of recent earthquake swarms linked to fluid injection activities.