The Big One

Colorado’s Largest Historic Earthquake: November 7, 1882 – Magnitude 6.6

On November 7, 1882 at about 6:30 p.m. local Denver time, a moderately strong earthquake shook much of Colorado and parts of southern Wyoming and northeastern Utah. The following quote from the Rocky Mountain News gives an indication of the shaking in Denver, 60 miles from the epicenter.

A general stampede was caused among the employees of The News office, especially in the editorial rooms.The editors and reporters were seated engaged at work when the floors of the editorial rooms began to tremble violently. …for a short time it appeared as if the building was about to tumble in.

The shaking was so great in Denver that it broke the electrical generators loose from their mounts and knocked out power. The earthquake was apparently felt as far east as Salina, Kansas and perhaps even in Plattsmouth, Nebraska (Rockwood, 1883; Oaks and Kirkham, 1986); and as far west as Salt Lake City.

The Colorado Front Range was rockin’ and rollin’ on November 7, 1882, when shock waves from a magnitude 6.6 earthquake (centered just west of Fort Collins) rumbled through thestate. Image and caption from US Geological Survey postcard.

An aftershock followed on the morning of November 8 (local time) and was felt in Denver, Boulder, Greeley, Laramie, and near Meeker. The main event was the largest earthquake to occur in the Colorado region during the historical period (1867-present) and has received considerable study by numerous researchers. Heck (1928) reported the felt area as 28,000 square kilometers. Hadsell (1968), as part of the investigation of the earthquakes at the Rocky Mountain Arsenal, conducted the first extensive evaluation of this event. Hadsell concluded the earthquake may have been centered north of Denver and east of Boulder, had maximum intensity of VII, and was ML 5.0 ± 0.6 based on the maximum observed intensity or ML 6.7 ± 0.6 based on its circular felt area of just under 1,200,000 square kilometers.

Dames and Moore (1981) studied this event as part of their seismic hazard evaluation for the Rocky Flats Plant. A summary of their investigation into the 1882 earthquake was described by McGuire et al (1982). They collected additional felt reports and reassessed others, particularly the account of landsliding and severe ground shaking in the Douglas Pass area that was discounted byHadsell (1968). Dames and Moore (1981) discredited the felt report from Salina, Kansas because the event was not reported in Salina’s newspaper. They placed the earthquake epicenter in northwestern Colorado and suggested that the Dudley Gulch graben in the Piceance Creek Basin was a possible causative structure. A radiocarbon date on charcoal collected by the authors and W.R. Junge from unfaulted deposits that cover the fault and detailed studies of the Dudley Gulch graben by Eckert (1982) and Clift and Billington (1986) have since demonstrated that no surface rupture has occurred on the Dudley Gulch graben for over one thousand years.Dames and Moore (1981)estimated the felt area at 500,000 km2, assigned a maximum intensity of VIII to the event, and suggested the earthquake had a magnitude of around ML 6.5. A re-evaluation of existing felt reports, combined with newly discovered felt reports and confirmation of the felt report in Salina, Kansas byOaks and Kirkham (1986), led Kirkham and Rogers (1986) to somewhat different conclusions. Their isoseismal map is reproduced as Figure 1 below.

Kirkham and Rogers (1986) reported a felt area of 470,000 square kilometers and suggested the earthquake had a magnitude of ML 6.2 ± 0.3 based on regression equations that compare magnitude and felt area for Rocky Mountain earthquakes. The relatively large felt area, combined with only moderate reported intensities, suggested the earthquake occurred at a moderate hypocentral depth, perhaps about 20 km or more, similar to the 1984 Laramie Mountains, Wyoming earthquake.

An intensity map for the aftershock during the morning of November 8 (local time) that was felt in Denver, Boulder, Greeley, Laramie, and near Meeker is presented in Figure 2. No intensity ratings have been assigned to the known aftershock felt reports. In Laramie and Denver, however, the aftershock was reported to be nearly as strong as the main quake. If all known felt reports for the aftershock are included in a single felt area, it would encompass an area of about 61,000 square kilometers. This suggests the aftershock may have been around magnitude 4.5 to 5.0. An equally plausible interpretation could include the Denver, Boulder, Greeley, and Laramie felt reports in a smaller felt area and show the report from near Meeker as an isolated felt location.

The felt reports for the main shock do not define a consistent epicentral location. The felt area (Figure 1) for the aftershock on November 8th at 4:45 a.m. local time and 1) the large number of higher intensity reports in the Denver-Fort Collins-Laramie area, 2) the Salina, Kansas felt report, 3) the unusually shaped isoseismal maps of recent Denver area earthquakes (as demonstrated by the isoseismal maps contained in this report), and 4) the intensity patterns associated with the October 15, 1984 Laramie Mountains earthquake (Oaks et al 1985); Spence et al 1996) suggest an epicentral area somewhere in north-central Colorado, perhaps in the northern Front Range or southern Laramie Mountains. An epicentral location of 40 1/2°N and 105 1/2° W has been assigned to both the main quake and the aftershock byKirkham and Rogers (1986), but this location is probably accurate to only 1/2° latitude or longitude.

FIGURE 1

As part of a recent seismotectonic evaluation of the Rocky Flats facility conducted subsequent to the original release of Bulletin 46 (Kirkham and Rogers, 1985), G.A. Bollinger re-evaluated the 1882 earthquake (Risk Engineering, Inc., 1994, Appendix C). In general, he agreed with the epicentral location of Kirkham and Rogers (1986), but rather than selecting a specific location for the event, he assigned confidence levels of the earthquake having occurred in various areas defined by isoseismal or felt areas. Bollinger also estimated the magnitude of the earthquake using several methods. He suggested it had a moment magnitude of 6.44 and assigned a confidence of 80 percent that this value was correct.

Risk Engineering, Inc. (1994); p. 16, concluded that “Given currently-available data, the epicenter may have been anywhere in north-central or northwestern Colorado, southern Wyoming, or northeastern Utah, and the moment magnitude is estimated to have been in the range 5.0 ± 0.4 to 6.9 ± 0.2, with a best estimate of 6.4 ± 0.3”.

The 1882 earthquake has also been investigated bySpence and others (1996), who compared it with the 1984 Laramie Mountains, Wyoming earthquake. They concluded the epicenter for the 1882 event was probably in the northern Front Range in the general vicinity of the location proposed by Kirkham and Rogers (1986), estimated the earthquake would have had a felt area of 870,000 square kilometers (Figure 2) based on the current population distribution, and suggested it had a magnitude of Mw 6.6 ± 0.6. They stated that earthquakes of this size may occur at shallow to mid-crustal depths throughout the Laramie Mountains and Front Range. We have elected to use the magnitude determined bySpence and others (1996).

FIGURE 2

A repeat of this 1882 earthquake today would cause a total of $2.8 billion* in economic loss from all sources, according to an analysis using FEMA’s HAZUS program for estimating loss. To read the full report, click here.
* The sum of Building-related, Transportation and Utility Lifeline, and Utility System losses.

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