On December 11, 2018, the area around Glenwood Springs, CO experienced several earthquakes that ranged from 3.2 to 3.6 magnitude.  According to the USGS and Magnitude – Intensity Comparison, Earthquakes that are in the 3.0 to 3.9 magnitude range area:  “Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.”

A bit about background of earthquakes.  Earthquakes are caused by movements along areas of weakness in the crust called faults.  Earthquakes can be triggered by natural and/or man-made stresses.    Small earthquakes are relatively common in Colorado although Colorado is situated in the interior of the North American Plate.  Earthquakes are most common along the boundaries of Earth’s crustal plates, but can also occur along ancient zones of weakness in the interior of a plate or in areas with active volcanic activity. They can also be induced by human activities such as groundwater/wastewater pumping.  According to the USGS (2010 to 2015 earthquake data) Colorado is ranked 15th of all the states for number of earthquakes with Alaska, Oklahoma, and California ranking, 1,2,3, respectively.  Earthquakes can trigger mass movements and rock falls.

Note in the map, several inferred faults (Grand Hogback monocline) occur in the area of the earthquakes as well as a geologically-active volcano called the Dotsero volcano.  Map of Earthquake Data


useful links