Historic Earthquakes
Colorado's Largest Historical Earthquake
November 8, 1882
Probably in the Front Range, West of Fort Collins
Magnitude: 6.6 ± 0.6
By William Spence
U.S. Geological Survey, Central Region Geologic Hazards Team
P.O. Box 25046, MS 966, Denver, CO 80225
The 1882 earthquake occurred during the peak of Colorado's mining era, before recording seismometers were invented.
Intensity map for the 1882 earthquake.
This map shows that the 1882 earthquake was felt throughout most of Colorado and Wyoming and well into Utah, Idaho, and Nebraska. The dashed lines, termed isoseismals, separate zones of different levels of shaking. The innermost contours enclose areas of strongest shaking and are nearest to the source of the earthquake. (An isoseismal line is a line connecting points on the Earth's surface at which earthquake intensity is the same.) The heavy dashed line extrapolates where this earthquake would have been felt, if a reporting population had existed in the sector to the north and east. The revised shape of this larger intensity map makes it more consistent with the intensity maps of the modern earthquakes shown below and to the right. The estimated magnitude of Mw = 6.6 ± 0.6 is based on the total area (about 850,000 km2 ) over which USGS scientists infer that this earthquake could have been felt.
The inner, stippled zone is the felt area of the largest after-shock, which occurred about 10 hours after the 1882 main shock. Generally, after-shocks occur near to a main shock's rupture; thus, this smaller felt zone helps scientists pinpoint the location of the main shock. The solid black star approximates this location, which is the Front Range, west of Fort Collins. This location is good to approximately 50 km.
A modern recurrence of an earthquake like that of 1882 could cause significant damage. Such an earthquake may be possible anywhere in the Laramie Mountains and in the Front Range from Casper, Wyoming, to Pueblo, Colorado (see back page). However, our limited knowledge of earthquake rates in this region indicates that the repeat period for such an earthquake is very great (on the order of every 1000 years). Smaller earthquakes, in the magnitude range 5.0-5.5, would be far less threatening but occur more frequently (perhaps one within every several tens of years).
Intensity map for the 1984 Laramie Mountains, Wyoming, Earthquake
This earthquake's felt area of 287,000 km2
is anomalously great for a magnitude 5.3 earthquake in this region.
This is due to the higher stress drop than usual (sharper jolt) of this midcrustal earthquake.
Isoseismals for the two largest Denver earthquakes August 9, 1967; Mb=5.0 (ISC) and November 9, 1967; Mb=5.1 (ISC)
Intensity maps for the two largest Denver earthquakes show far less radial symmetry than that for the 1984 earthquake. This asymmetry is possibly due to focusing and defocusing of seismic energy by the north-trending Front Range as well as high absorption of energy within the Denver Basin. The multi-year Denver earthquakes were induced by human activity: the high-pressure injection of toxic fluids into a 3.7-km-deep (2.3-mi) disposal well at the Rocky Mountain Arsenal.
Known earthquakes of Colorado and southern Wyoming, 1867-1993
These earthquakes occurred diffusely throughout the Southern Rocky Mountains and the Colorado Plateau, typically at depths of less than 15 km. Notable earthquakes occurred in 1882 (center of map in the Front Range), in 1984 (north-central, in the Laramie Mountains), and in the mid-1960's (the famous Denver earthquakes). Depths of 25-27 km for the 1984 earthquakes are anomalously great.
REFERENCE:
Spence, William, Langer, C.J., and Choy, George L., 1996, Rare, Large Earthquakes at the Laramide Deformation Front Colorado (1882) and Wyoming (1984): Bulletin of the Seismological Society of America, v. 86, no. 6, p. 1804-1819.
