Denver is located in the stable continental interior, far from the nearest tectonic plate boundary. This position means the city does not face the high seismic risk common to coastal areas like California or Alaska. Earthquakes in the Denver area are infrequent and typically minor, though not entirely absent. The U.S. Geological Survey (USGS) considers Colorado a zone of low to moderate seismic activity. Understanding Denver’s seismic reality requires looking closely at both natural geological stresses and human-caused tremors.
Historical Earthquakes Near Denver
Natural earthquakes have been felt in the Denver area and along the Front Range, though rare. The largest known natural earthquake in Colorado history occurred on November 7, 1882, estimated at magnitude 6.6. Centered in the northern Front Range, this tremor caused shaking strong enough to damage buildings in Denver and Boulder. Few documented tremors were felt prior to the mid-20th century.
Most naturally occurring seismic events in the Front Range are small, generally magnitude 2.0 to 4.0. While instruments detect these smaller tremors, they are often not felt by residents or cause significant damage. This pattern of infrequent, moderate-to-large events and continuous, minor background seismicity is characteristic of intraplate regions.
Geological Setting of Colorado Earthquakes
Earthquakes occur in Colorado despite the state’s location far from the North American tectonic plate edges. This is known as intraplate seismicity, where stresses accumulate within the continental crust until released along pre-existing weaknesses. The stress field across the western U.S. is transmitted inland, pushing the crust beneath Colorado.
Ancient, deeply buried faults, associated with the uplift of the Rocky Mountains and the Rio Grande Rift, play a significant role. These faults, inactive for millions of years, can be reactivated by regional stress. The Rio Grande Rift, a major continental rift zone, is the dominant seismotectonic feature, contributing structural weaknesses that allow for the occasional release of strain energy. Movement along these fractured faults beneath the Denver basin is the mechanism for natural tremors.
The Unique History of Induced Seismicity
Denver holds a unique place in seismology due to a well-documented case of human-induced earthquakes in the 1960s. This episode centered around the Rocky Mountain Arsenal (RMA) facility northeast of the city, where the U.S. Army drilled a deep injection well to dispose of contaminated chemical wastewater.
Beginning in 1962, injecting millions of gallons of fluid increased the pore pressure in the fractured Precambrian bedrock. This elevated pressure lubricated the ancient Derby Fault, reducing friction and triggering a sequence of over 1,500 earthquakes. The largest induced event reached magnitude 5.3 in August 1967, causing structural damage in the northern Denver suburbs. The link between the volume of injected waste and the frequency of tremors established the RMA as an early, recognized example of induced seismicity.
Current Seismic Risk and Building Requirements
Denver’s seismic risk remains low to moderate compared to coastal high-hazard zones. Risk factors include the potential for natural earthquakes and the lingering, though diminished, potential for induced events. The USGS seismic hazard maps reflect this moderate threat, which informs regional safety standards.
Building requirements in Denver address seismic loads through the adoption of the International Building Code (IBC). Denver mandates a minimum of Seismic Design Category (SDC) B for new construction, meaning seismic resistance must be factored into the design. This standard requires structural engineers to ensure buildings can withstand a moderate level of ground shaking without collapse. Residents are advised to practice basic preparedness, such as securing heavy objects and knowing to Drop, Cover, and Hold On.