Earthquakes are caused by a sudden slip on a fault. Rocks on either side of a fault must overcome friction on the fault to slip, but once built-up stress on the fault overcomes the friction keeping rocks in place, the resulting earthquake releases energy in waves that travel through the Earth’s crust. These waves can cause noticeable shaking at the Earth’s surface and, in the case of large earthquakes, damage to roads, buildings, and other infrastructure that may pose a threat to public safety.
Colorado is an active tectonic province that is essentially being pulled apart where the Rio Grande Rift cuts north/south across the mountainous, central part of the state. Colorado’s high mountains are a result of uplift on faults (with associated earthquakes) that are part of the rift system. Colorado’s active landscape with still-rising mountains contains thousands of faults, with over 90 potentially active faults and more than 700 recorded earthquakes of magnitude 2.5 or higher since 1867. Although Colorado experiences fewer and less frequent earthquakes on average than more seismically active states like California, Colorado has experienced large natural (magnitude 6.5 or higher) and human-triggered earthquakes in recorded history and will continue to experience large earthquakes into the future.
Earthquake hazard is the likelihood of a certain level of shaking, also known as “Peak Ground Acceleration,” that may occur from an earthquake in a particular area. Colorado has low-to-moderate earthquake hazard as rated by the U.S. Geological Survey Earthquake Hazards Program. Earthquake risk is the likelihood of economic or personal loss occurring from an earthquake in a particular area, and is determined by looking at the level of earthquake hazard, the number of people and properties in the hazard area, and the vulnerability of people and infrastructure to earthquakes.1 Knowledge of both hazards and risk are vitally important for planning for earthquakes.
Earthquakes can cause damage to buildings and infrastructure directly through shaking, but they are sometimes also responsible for causing related hazards, such as ground fissures, soil liquefaction, landslides and rockfalls, and more. In order to reduce risks from earthquake-associated hazards, it is important to study local geology and soil conditions.