Earthquakes

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Earthquakes are caused by sudden movements of the earth along a fault. As the rocks on either side of the fault accumulate stress between them, they will eventually overcome friction and slip. The resulting earthquake releases energy in waves that travel through the earth’s crust. These waves can cause noticeable shaking at the 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 considered 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. The active landscape of the state—with the still-rising mountains containing thousands of faults—features over 90 potentially active faults and more than 700 recorded earthquakes of magnitude 2.5 or higher since 1867. Colorado experiences fewer and less frequent earthquakes on average than more seismically active states like California and Alaska. However, the state has experienced large natural (magnitude 6.5 or higher) and human-triggered (induced) earthquakes in recorded history and will continue to periodically experience large earthquakes in the future.

Faults

Because fault movement causes earthquakes, it is important to study the thousands of faults in Colorado to determine whether they have moved in the recent geologic past or capable of moving again in the near future. The CGS conducts scientific studies of fault zones and past earthquakes while monitoring fault movements with a network of seismometers throughout Colorado.

Five faults in Colorado have received sufficient study to be included in the USGS National Seismic (Earthquake) Hazard Map (version 2018 NSHM), and three of those are listed as being capable of generating earthquakes of 7.0 magnitude, or greater. There are many more faults in the state that could probably generate significant earthquakes, but have not received sufficient study, or documentation, to be included in the hazard map. With our current state of knowledge, it is not possible to predict when or where, the next large earthquake might occur in Colorado.

Earthquake Size

Earthquakes are measured in several ways: magnitude, intensity, and ground acceleration.

Magnitude is determined by measurements of recordings on seismographs. Hypothetically, all seismographs around the world should yield the same magnitude for a given earthquake, no matter the distance from the epicenter. Magnitude is always presented in numerals. There are a number of different methods for calculating magnitude: Moment Magnitude, Richter Magnitude, Rayleigh Surface Wave Magnitude, and Gutenberg Body Wave Magnitude. Colorado did not have seismographs prior to 1900, nor did many parts of the U.S. so for older earthquakes we can only estimate their magnitudes from their intensity.

Intensity estimates are based on what people observe or feel during an earthquake. Intensity varies with distance from the fault, and depends on soil conditions and/or height one is in a building when the shaking occurs. Intensity is always presented in Roman numerals. There are methods for plotting up the intensities in different places and deriving an estimated magnitude from the distribution. The magnitude of 6.6 for the earthquake of 1882 that had its epicenter in north-central Colorado was derived in this manner.

Ground acceleration is measured as a percentage of the force of gravity by strong motion instruments. This is also what the National Earthquake Hazard Maps use for their hazard assessment. Like intensities, ground acceleration varyies according to distance from the fault, soil conditions, and type of structure. Ground acceleration values are used in designing earthquake resistant structures. Colorado does not have any buildings instrumented to measure strong motion.

The USGS hosts a detailed discussion of Earthquake Magnitude, Energy Release, and Shaking Intensity for further information.

Hazards

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. Knowledge of both hazards and risk are vitally important in planning a resilient response to earthquake events.

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. In order to reduce risks from earthquake-associated hazards, it is important to understand the local geology and soil conditions.

Colorado’s historic record of earthquakes dates back to 1870.

The table following provides links to more detailed information about Colorado’s largest earthquakes from the CGS publication B-52 Colorado Earthquake Information. Most of these earthquakes only have a Maximum Intensity value that refers to the Modified Mercalli Intensity (MMI) Scale. Some of these earthquakes are accompanied by an isoseismal contour map of MMI values, while others list various types of earthquake magnitudes. The USGS maintains a table summarizing earthquake magnitude types.

Pueblo/Ft. Reynolds | 4 December 1870 | MMI VI

The first earthquake of record to center in Colorado occurred on December 4, 1870. The most complete description of this event is contained in the Pueblo Chieftain on December 8, 1870. Included in this issue are the reactions of various community members to the earthquake and reference is made to a letter from Fort Reynolds that states “Bottles on my washstand standing about one inch apart were violently knocked together. The shock was accompanied by a rumbling noise and appeared to move from east to west. It was also felt on the opposite side of the [Arkansas] river.” Maximum intensity for this event was VI (Hadsell, 1968). According to the Colorado Transcript on December 7, 1870 the earthquake was felt by a least two citizens in Golden. Other references to this earthquake are found in the Rocky Mountain News (12/10/1870) and the Central City Register (12/11/1870). Hadsell (1968) suggested the earthquake was felt over about 160,000 km².

References

• Docekal, J., 1970, Earthquakes of the stable interior, with emphasis on the midcontinent: University of Nebraska, Ph.D. Dissertation, 332 p.
• Hadsel, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72.
• Rizzari, F.B., 1951, It CAN’T happen here: A brief history of earthquakes in Colorado: The Denver Westerner’s Monthly Roundup, v. 15, no. 12, p. 5-15.

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Lily Park, Moffat County | October 1871 | MMI VI

Fitzpatrick (1974, p. 26-27) recounted a description of an earthquake in Lily Park, Moffat County in October of 1871 as follows:

The “hunters’ moon” of early October [1871] had come and waned; Suddenly, this deep silence was shattered by the most deafening roar, and rumble, and hiss and bellow that mortal ears ever heard. At the same moment the earth beneath shuttered, heaved, rose, fell as if in mortal agony. The towering cliffs of the canyon walls sent loosened boulders in torrents. The spot where the terrified man crouched, a tiny atom of life in the midst of the awful powers of the upheaval of nature, was in the path of the devastating avalanche of rock, yet the man was unharmed and presently the final rolling stone came to a stop and the silence following the earthquake closed in like some physical thing pressing from all sides.

Not knowing what moment another shock might come, Wallihan endured the night that seemed as long as eternity. A maximum intensity of VI is assigned to this felt report. The date of this October event is poorly constrained, and it is possible that this earthquake and the following event on November 9, 1871 were actually the same event. If the felt reports from Moffat County and the Georgetown-Central City area are from the same earthquake, then the felt area for this earthquake would be fairly large.

References

• Fitzpatrick, V.S., 1974, The Last Frontier: Steamboat Pilot, Steamboat Springs, Colorado.
• Oaks, S.D., and Kirkham, R.M., 1986, Results of a search for felt reports for selected Colorado earthquakes: Colorado Geological Survey Information Series 23, 89 p.
• Stover, C.W., and Coffman, J.L., 1993, Seismicity of the United States, 1568-1989 (revised): U.S. Geological Survey Professional Paper 1527, 418 p.

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Aspen | 17 September 1880 | MMI VI

The only available reference to the September 17, 1880 earthquake that shook Aspen was an article in the Leadville Chronicle which was reprinted in The Denver Tribune on September 21, 1880 (p. 2, c. 1). Unfortunately, no copies of the Aspen newspapers or the Leadville Chronicle for this time period have been located.

The earthquake occurred at midnight on the 16th [local time] and brought many of the residents out of their homes. As they left their homes a second tremor “loosened their teeth and dislocated their locomotion” and “threw the camp into the wildest confusion”.

There were a total of four shocks, separated by intervals of about thirty seconds. During each shock “the earth seemed moved three or four feet out of place, and a low rumbling noise to be compared to nothing save the rolling of distant thunder, was heard”. At about 8 o’clock the following morning a large landslide or rockslide on Aspen Mountain was reported.

Rizzari (1959) discredited this account based on conversations with Don and Jean Griswald, Colorado historians. The Griswalds indicate that the author of the article, Mr. Orth Stein, was prone to exaggeration or even fabrication. Hadsell (1968) rated the event at intensity VI. For complete discussions of this earthquake, please refer to Rizzari (1959) and Oaks and Kirkham (1986).

It is interesting to note that an earthquake was also reported in Salt Lake City at 10:27 p.m. on the 16th (Coffman, von Hake, and Stover, 1982; Oaks and Kirkham, 1986). The report from Aspen might possibly be associated with the Salt Lake City event, but since the times of the reports are somewhat different it is more likely that the report from Aspen represents a local earthquake.

References

• Coffman, J.L., von Hake, C.A., and Stover, C.W., 1982, Earthquake History of the United States: U.S. National Oceanic and Atmospheric Administration and U.S. Geological Survey, Publication 41-1 (through 1980), 258 p.
• Oaks, S.D., Hopper, M.G., Barnhard, L.M., and Algermissen, S.T., 1985, November 7, 1882, Colorado earthquake reinterpreted in light of the October 18, 1984 Wyoming earthquake: Earthquake Notes, v. 55, no. 1, p. 24.
• Hadsell, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72.
• Rizzari, F.B., 1951, It CAN’ T happen here: A brief history of earthquakes in Colorado: The Denver Westerner’s Monthly Roundup, v. 15, no. 12, p. 5-15.

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The Big One | 7 November 1882 | Mw 6.6 MMI VII

Figure 1. Isoseismal map for the north-central Colorado earthquake – 8 November 1882 – MMI VII.

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 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.

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 isoseismal 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 below. 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 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 and others, 1985; Spence and others, 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 by Kirkham and Rogers (1986), but this location is probably accurate to only 1/2° latitude or longitude.

The 1882 earthquake has also been investigated by Spence 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 by Spence and others (1996).

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.

* The sum of Building-related, Transportation and Utility Lifeline, and Utility System losses.

References

• Oaks, S.D., and Kirkham, R.M., 1986, Results of a search for felt reports for selected Colorado earthquakes
• Heck, N. H., 1928, Earthquake History of the United States
• Hadsell, F.A., 1968, History of earthquakes in Colorado
• Dames & Moore, 1981, Geologic and seismologic investigations for Rocky Flats Plant
• McGuire, R.K., Krusi, Alan, and Oaks, S.D., 1982, The Colorado earthquake of November 7, 1882
• Eckert, A.D., 1982, The geology and seismology of the Dudley Gulch graben
• Clift, A.E., and Billington, S., 1986, Results of a limited microseismicity survey in the Piceance Creek Basin
• Kirkham, R.M., and Rogers, W.P., 1986, An interpretation of the November 7, 1882 earthquake
• Oaks, S.D., Hopper, M.G., Barnhard, L.M., and Algermissen, S.T., 1985, November 7, 1882, Colorado earthquake reinterpreted in light of the October 18, 1984 Wyoming earthquake
• Spence W., Langer, C.J., and Choy, G.L., 1996, Rare, large earthquakes at the Laramide deformation front-Colorado (1882) and Wyoming (1984)
• Kirkham, R.M., and Rogers, W.P., 1985, Colorado earthquake data and interpretations–1867 to 1985 (CGS Bulletin 46)

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Axel Basin (Maybell) | 1 December 1891 | MMI VI

Hadsell (1968) listed an intensity VI earthquake occurring during December of 1891 in Axial Basin. Rizzari (1959) cited an article in the August 18, 1942 issue of The Steamboat Pilot that describes this earthquake. Oaks and Kirkham (1986) searched numerous sources for additional reports, but discovered only two, an eyewitness account in Fitzpatrick (1974) and a brief mention of the earthquake in The Daily Sentinel (Grand Junction) on November 14, 1901. Oaks and Kirkham (1986) also contains an article from the August 13, 1942 issue of The Steamboat Pilot which is nearly identical to the article cited by Rizzari (1959). The Steamboat Pilot said “The earth was convulsed by ‘waves’ that rolled at intervals of a few seconds, lasting for a minute or more” and summarized the effects recalled by Hortense Fitzpatrick (Fitzpatrick, 1974). The earthquake was reported in Lily Park and by settlers scattered across the area. Fitzpatrick (1974) described the earthquake as a sound that “was as if a hundred-mile wind had struck with a roar and a seething hiss, while some stupendous power had hurled tons of loose earth against the side of the house”. Their house moved, calendars and pictures on the wall oscillated, dishes in the cupboard clattered, the lids on the cookstove made noise, and the earth rolled and vibrated. Their cow fell against the house. Thousands of tons of boulders rolled down Fitzpatrick’s cliff. The Grand Junction Daily Sentinel on November 14, 1901, mentioned that an earthquake was felt in town in 1891 and that it was “not nearly so distinct” in their town as the November 13, 1901 earthquake.

References

• Hadsel, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72
• Oaks, S.D., and Kirkham, R.M., 1986, Results of a search for felt reports for selected Colorado earthquakes: Colorado Geological Survey Information Series 23, 89 p.
• Rizzari, F.B., 1951, It CAN’ T happen here: A brief history of earthquakes in Colorado: The Denver Westerner’s Monthly Roundup, v. 15, no. 12, p. 5-15.
• Stover, C.W., and Coffman, J.L., 1993, Seismicity of the United States, 1568-1989 (revised): U.S. Geological Survey Professional Paper 1527, 418 p.

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Buena Vista | 15 November 1901 | MMI VI

Numerous newspapers published accounts of an earthquake felt in and near Buena Vista on November 15, 1901 (Oaks and Kirkham, 1986). It is somewhat difficult to discriminate the felt reports of the November 13 and 14 events from those for the November 15 earthquake. It is our interpretation that the felt reports from Buena Vista and Cottonwood Lake are for a distinct earthquake on November 15 that was felt only in that immediate area. It is possible, but we believe unlikely, that the date of the Buena Vista report is incorrect and that these felt reports resulted from the Utah earthquakes on November 13 and 14. An interesting aspect of this earthquake is that moderate intensities (VI ) are reported for this event in the immediate vicinity of Buena Vista, but that surrounding towns did not report the event. Many residents of Buena Vista ran excitedly into the streets wearing only night clothes, and a large plate glass window was broken. The water in Cottonwood Lake was agitated, and numerous boulders rolled down Mount Princeton and Mount Harvard. Newspapers in Salida described the effects in Buena Vista, but did not report any local effects (Salida Mail, November 19, 1901). Likewise, newspapers in both Leadville and Aspen failed to mention any local manifestations to the earthquake. Such evidence suggests the Buena Vista earthquake may have occurred at a fairly shallow depth, and may have been only magnitude 4.0 to 5.0 or perhaps even smaller.

References

• Hadsel, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72.
• Oaks, S.D., and Kirkham, R.M., 1986, Results of a search for felt reports for selected Colorado earthquakes: Colorado Geological Survey Information Series 23, 89 p.

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Ridgway Area | 11 November 1913 | MMI VI

Figure 4. Isoseismal map for the Ridgway-area earthquake – 11 November 1913 – MMI VI.

Coffman, von Hake, and Stover (1982) described the three earthquakes on November 11, 1913 based on unpublished data from H.F. Reid. They indicate the quake was strong at Montrose, Ouray, and Telluride, and that objects were thrown from shelves and rocks rolled down cliffs at Ouray. According to Coffman, von Hake, and Stover (1982) the earthquake was felt over nearly 20,000 km², and the maximum intensity was V. Oaks and Kirkham (1986) discovered a recollection of these events in a newspaper article describing the September 9, 1944 earthquake in the Montrose Daily Press on September 9, 1944. This article suggests that Ridgway was hardest hit in the area and that the quake centered near Portland, about 10 km south of Ridgway. Pictures fell from walls, dishes were broken, and the Ridgway school ceiling was damaged during the earthquake. Similar types of damage were reported over a wide area. Original newspaper accounts of this earthquake have not been located. We assign the damage reported in Ridgway an intensity VI and the reports from Montrose, Ouray, and Telluride an intensity V.

References

• Coffman, J.L., von Hake, C.A., and Stover, C.W., 1982, Earthquake History of the United States: U.S. National Oceanic and Atmospheric Administration and U.S. Geological Survey, Publication 41-1 (through 1980), 258 p.
• Oaks, S.D., and Kirkham, R.M., 1986, Results of a search for felt reports for selected Colorado earthquakes: Colorado Geological Survey Information Series 23, 89 p.
• Reid, H.F., unpublished earthquake catalog, John Hopkins University, Baltimore, Md.
• Stover, C.W., Reagor, B.G., and Algermissen, S.T., 1984, Seismicity map of the State of Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-1694.

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Montrose/Basalt | 9 September 1944 | MMI VI

Figure 8. Isoseismal map for the Montrose-Basalt-area earthquake – 9 September 1944 – MMI VI.

A moderate earthquake was felt over part of western Colorado during the evening of September 8, 1944 (local time). Bodle (1946) described felt reports ranging up to intensity VI at Basalt, Montrose, and Riland. The earthquake cracked walls and chimneys, rattled windows, and overturned small objects in Basalt. Buildings creaked, loose objects rattled, and some plaster was cracked in Montrose. Reports from Riland indicate a strongly built log house was moved slightly out of line. Aspen, Eagle, Edwards, Gilman, and Grand Junction experienced intensity V shaking. Additional felt reports were contained in Bodle (1946), while “not felt” reports were in USCGS “Abstracts of Earthquake Reports”.

Oaks and Kirkham (1986) described numerous newspaper articles concerning this earthquake. Additional felt reports were obtained from these articles and, along with those of Bodle (1946), were utilized to construct the isoseismal map below. This map suggests the felt area for the earthquake was approximately 19,000 km². Other estimates of the felt area size were 7,800 km² (Bodle, 1946) and 41,000 km² (Hadsell, 1968).

References

• Bodle, R.R., 1946, United States earthquakes 1944: U.S. Coast and Geodetic Survey, Serial No. 682, 43 pg.
• Hadsel, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72.
• Murphy, L.M., and Cloud, W.K., 1957, United States earthquakes 1955: U.S. Coast and Geodetic Survey, 83 p.

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Lake City | 3 August 1955 | MMI VI

Figure 15. Isoseismal map for the Lake City-area earthquake – 3 August 1955 – MMI VI.

Southwestern Colorado experienced a moderate earthquake just before midnight on August 2, 1955 (local time). Murphy and Cloud (1957) and the U.S. Coast and Geodetic Survey quarterly series “Abstracts of Earthquake Reports” described the effects of the shock and assigned intensity values. Intensity VI was reported at Lake City, where one chimney was cracked and another fell (Figure 15). The U.S. Fisheries Station 19 km southwest of Creede, a location 32 km southwest of Creede, and Silverton reported and intensity of V. Two foreshocks were also recorded. Murphy and Cloud (1957) suggested the earthquake was felt over 5,200 km², whereas the isoseismal map they published, indicates the felt area to be approximately 4,600 km². Hadsell (1968) reported the size at 15,600 km².

References

• Hadsel, F.A., 1968, History of earthquake activity in Colorado, in Hollister, J.C., and Weimer, R.J., eds., Geophysical and geological studies of the relationship between the Denver earthquakes and the Rocky Mountain Arsenal well: Colorado School of Mines Quarterly, v. 63, no. 1, p. 57-72.
• Murphy, L.M., and Cloud, W.K., 1957, United States earthquakes 1955: U.S. Coast and Geodetic Survey, 83 p.
• U.S. Geological Survey, 2008, Computer-file printouts of Colorado earthquakes, 1981-present: prepared by U.S. Geological Survey, National Earthquake Information Center, July 17, 1985, January 12, 1996, February 21, 1997, January 6, 2003, and April 15, 2004. Web-based searches from USGS NEIC website, 2004-present.

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Montrose/Ridgway | 11 October 1960 | mb 5.5 MMI V

Figure 18. Isoseismal map for the Montrose-Ridgway-area earthquake – 11 October 1960 – MMI V.

In the early morning hours on October 11, 1960 (local time) a large part of southwestern Colorado was shaken by one of the largest earthquakes to occur historically in the state. Tally and Cloud (1962) described the effects of the tremor. Intensity VI damage was reported in Cimarron, Lake City, Montrose, Ophir, Ouray, Placerville, Powderhorn, Ridgway, and Telluride. Plaster, chimneys, and windows were cracked in many of these locations. Perhaps the hardest hit was Montrose, where a foundation was cracked in three places. Numerous other locations reported the earthquake effects at lesser intensity. Grand Junction experienced somewhat high intensities for its distance to the epicenter. Talley and Cloud (1962) reported a magnitude of mb 5.5 for this earthquake. Their isoseismal map for the event indicates the quake was felt over an area of about 26,000 km². The isoseismal map shown below was prepared from information in Talley and Cloud (1959), the U.S. Coast and Geodetic Survey quarterly series “Abstracts of Earthquake Reports”, and the NOAA intensity file computer printout. The felt area is somewhat larger on isoseismal map, being approximately 39,000 km². Unruh and others (1993) relocated this event and report a slightly different epicentral location.

References

• Coffman, J.L., von Hake, C.A., and Stover, C.W., 1982, Earthquake History of the United States: U.S. National Oceanic and Atmospheric Administration and U.S. Geological Survey, Publication 41-1 (through 1980), 258 p.
• Stover, C.W., Reagor, B.G., and Algermissen, S.T., 1984, Seismicity map of the State of Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-1694.
  Talley, H.C., and Cloud, W.K., 1962, United States earthquakes 1960: U.S. Coast and Geodetic Survey, 90 p.
• Unruh, J.R., Wong, I.G., Bott, J.D.J., Silva, W.J., and Lettis, W.R., 1993, Seismotectonic evaluation, Rifle Gap Dam, Silt Project, Ruedi Dam, Fryingpan-Arkansas Project, northwestern Colorado: unpublished report prepared by William Lettis & Associates and Woodward-Clyde Consultants for U.S. Bureau of Reclamation, Denver, Colorado, 154 p.
• Wong, I.G., 1991, The 1986 Crested Butte, Colorado earthquake swarm: The Mountain Geologist, v. 28, no. 1, p. 3-8.

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NE of Denver | 5 January 1966 | mb 5.0 MMI V

Figure 30. Isoseismal map for the earthquake north-east of Denver – 5 January 1966 – MMI V.

The January 5, 1966 earthquake was felt over a small area in the Denver-Brighton vicinity. An isoseismal map for this event is shown in Figure 30. Von Hake and Cloud (1968) and the U.S. Coast and Geodetic Survey quarterly series provided the felt reports and intensity ratings for the earthquake. Intensity V was reported in Denver, Commerce City, and Northglenn. Major and Simon (1968) indicated a magnitude of ML 3.4 for the tremor, while von Hake and Cloud (1968) described it as mb 5.0. The isoseismal map below suggests the earthquake was felt over an estimated 1,100 km², while Hadsell (1968) reported a felt area of 2,100 km².

References

• U.S. Geological Survey, Preliminary determination of epicenters and associated earthquake data report, January 1961-present, (formerly by U.S. Coast and Geodetic Survey, U.S. Environmental Science Services Administration, and U.S. Oceanic and Atmospheric Administration).
• von Hake, C.A., and Cloud, W.K., 1968, United States earthquakes 1966: U.S. Coast and Geodetic Survey, 110 p.

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CO-NM border near Dulce, NM | 23 January 1966 | mb 5.5 MMI VII

Figure 31. Isoseismal map for the Dulce, NM-area earthquake – 23 January 1966 – MMI VII.

The Colorado-New Mexico border region was rocked by a moderate earthquake on January 23, 1966 that centered near Dulce, New Mexico. Felt effects and intensities for the earthquake were described by von Hake and Cloud (1968) and intensities for the U.S. Coast and Geodetic Survey quarterly series “Abstracts of Earthquake Reports”. Figure 31 is an isoseismal map of the event prepared from the available felt reports. It indicates the quake was felt over a rectangular-shaped region covering about 27,000 km². Von Hake and Cloud (1968) reported a felt area of 39,000 km² for this event. Between January 22 and January 28 (local time) the USCGS seismological center in Albuquerque recorded 119 events in the Dulce area. Three temporary seismographs were installed in the Dulce area on January 28th. During the first week of operation 218 earthquakes were recorded, all located in a small area near Dulce. 532 aftershocks were recorded during 30 days of monitoring, with hypocentral depths of about 5 to 10 km (Hoffman and Northrop, 1977). Earthquake activity continued at a high rate for about a year following the main shock. Minor activity continues to be instrumentally detected in the Dulce area. There are discrepancies in the epicentral locations for many of the aftershocks following the Dulce earthquake in the various catalogs.

Nearly every house in Dulce was damaged to some degree by the earthquake, but the principal damage was to structures at the Bureau of Indian Affairs (BIA) complex and local schools. Damage was estimated at $200,000, but no deaths or injuries were reported. The BIA dormitories suffered extensive damage to interior and exterior walls. The steam-heating plant for the BIA school was severely damaged. Older homes in the Dulce area experienced considerable structural damage, but new homes suffered mostly only cosmetic damage. A number of chimneys were damaged, especially those 0.3 to 0.7 meters high. Considerable rockfall occurred on buttes in the area. Photographs of some of the damage are contained in von Hake and Cloud (1968). A maximum intensity of VII was originally reported for this quake in Dulce, but Hoffman and Northrup (1977) suggest a VII to VIII or VII+ rating may be more appropriate. Intensity VI damage was reported in several locations along the Colorado-New Mexico border. Earthquake magnitude was originally determined at mb 5.5. Later studies by Herrmann and others (1980) reported a magnitude of mbLg 5.1 magnitude for the main shock.

References

• Herrmann, R.B., Dewey, J.W., and Park, S.K., 1980, The Dulce, New Mexico earthquake of 23 January 1966: Seismological Society of America Bulletin, v. 70, no. 6, p. 2171-2183.
• Hoffman, J.P., and Northrop, S.A., 1978, The Dulce, New Mexico, earthquake January 23, 1966: Earthquake Notes, v. 48, p. 3-20.
• National Oceanic and Atmospheric Administration, 1985, Earthquake intensity file for Colorado-computer printout: National Oceanic and Atmospheric Administration, Environmental Data Services, Boulder, Colorado.
• U.S. Geological Survey, 2008, Computer-file printouts of Colorado earthquakes, 1981-present: prepared by U.S. Geological Survey, National Earthquake Information Center, July 17, 1985, January 12, 1996, February 21, 1997, January 6, 2003, and April 15, 2004. Web-based searches from USGS NEIC website, 2004-present.
• von Hake, C.A., and Cloud, W.K., 1968, United States earthquakes 1966: U.S. Coast and Geodetic Survey, 110 p.

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NE of Denver | 9 August 1967 | mb 5.3 MMI VII

Figure 37. Isoseismal map for the earthquake north-east of Denver – 9 August 1967 – MMI VII.

One of the strongest shocks to affect the Denver area in the 1960’s occurred on August 9, 1967. An isoseismal map for this event is shown bellow. Felt reports and intensity ratings were described by von Hake and Cloud (1969). Intensity VII damage was reported in Northglenn, where plate glass windows broke, many walls, ceilings, foundations, and concrete floors cracked, and several businesses sustained damage due to fallen merchandise. A liquor store estimated damage at $10,000 to $20,000. Intensity VI damage was reported in 28 locations, many of which suffered considerable cracked plaster and mortar, broken windows, damaged foundations and chimneys, and damage to household goods. The earthquake was felt as far as Sterling, Pueblo, and Laramie. Based on the isoseismal map, we estimate the felt area to be about 50,000 km². Von Hake and Cloud (1969) proposed a size of 39,000 km², while Hadsell indicated it was felt over 117,000 km². Docekal (1970) reported a felt area of 52,000 km². A magnitude of mb 5.3 was reported for this earthquake by von Hake and Cloud (1969). Nuttli and others (1979) calculated an mb of 4.9 and Ms of 4.4. Herrmann and others (1981) suggested a focal depth of 3 km for this event. The overall felt area is prominently elongated in directions parallel and perpendicular to the mountain front. The intensity V and VI contours also are oriented in an elongate pattern perpendicular to the mountain front.

References

• Herrmann, R.B., Park, S.K., and Wang, C.Y., 1981, The Denver earthquakes of 1967-1968: Seismological Society of America Bulletin, v. 71, no. 3, p. 731-745.
• Nuttli, O.W., Bollinger, G.A., and Griffiths, D.W., 1979, On the relation between Modified Mercalli intensity and body-wave magnitude: Seismological Society of America Bulletin, v. 69, no. 3, p. 893-909.
• Stover, C.W., and Coffman, J.L., 1993, Seismicity of the United States, 1568-1989 (revised): U.S. Geological Survey Professional Paper 1527, 418 p.
• U.S. Geological Survey, Preliminary determination of epicenters and associated earthquake data report, January 1961-present, (formerly by U.S. Coast and Geodetic Survey, U.S. Environmental Science Services Administration, and U.S. Oceanic and Atmospheric Administration).
• von Hake, C.A., and Cloud, W.K., 1969, United States earthquakes 1967: U.S. Coast and Geodetic Survey, 90 p.

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NE of Denver | 27 November 1967 | mb 5.2 MMI VI

Figure 34. Isoseismal map for the earthquake north-east of Denver – 27 November 1967 – MMI VI.

The Denver area was hit by another damaging earthquake on November 27, 1967. Figure 38 illustrates an isoseismal map for this event. Descriptions of felt effects and intensity ratings were recorded in von Hake and Cloud (1967) and the USCGS quarterly series. Intensity VI damage occurred at 20 locations. Damage was described as “chiefly cracked plaster, enlargement of existing cracks, and loss due to fallen merchandise in stores”. The earthquake was felt over an estimated 56,000 km², based on Figure 38. Von Hake and Cloud (1969) reported a felt area of 44,000 km², whereas Docekal (1970) suggested it was felt over 62,000 km². The outline of the felt area is strongly skewed in directions parallel and perpendicular to the mountain front. The distribution of higher intensities is elongated perpendicular to the mountain front. Von Hake and Cloud (1969) place the earthquake magnitude at mb 5.2, while Nuttli and others (1979) report magnitudes of mb 4.6 and Ms 3.8. Stover, Reagor, and Algermissen (1984) report an mbLg of 4.6. Hermann and others (1981) obtained a focal depth of 5 km and a seismic moment of 7.2 x 10²² dynes-cm for the earthquake.

References

• Herrmann, R.B., Park, S.K., and Wang, C.Y., 1981, The Denver earthquakes of 1967-1968: Seismological Society of America Bulletin, v. 71, no. 3, p. 731-745.
• Nuttli, O.W., Bollinger, G.A., and Griffiths, D.W., 1979, On the relation between Modified Mercalli intensity and body-wave magnitude: Seismological Society of America Bulletin, v. 69, no. 3, p. 893-909.
  Stover, C.W., and Coffman, J.L., 1993, Seismicity of the United States, 1568-1989 (revised): U.S. Geological Survey Professional Paper 1527, 418 p.
• von Hake, C.A., and Cloud, W.K., 1969, United States earthquakes 1967: U.S. Coast and Geodetic Survey, 90 p.

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Trinidad | 22 August 2011 | Mw 5.3 MMI VII

A magnitude 5.3 earthquake was recorded by U.S. Geological Survey seismometers, locating the epicenter approximately 9 miles (+/- 7.8 miles) WSW of Trinidad, Colorado. Early reports by local news media indicated that significant damage to buildings had occurred in the towns of Segundo and Valdez, 15 miles west of Trinidad on Colorado State Highway 12. Prior to the magnitude 5.3 event, three smaller events were also recorded in the same vicinity, the largest being a magnitude 4.6 event at 5:30 PM MDT. The CGS investigated both of these events and provided scientific information for affected communities. In both occurrences, the CGS collaborated with, and cooperated with, the Colorado Division of Emergency Management, the Colorado Oil and Gas Conservation Commission, and the National Earthquake Information Center (USGS).

References

• Investigation of an Earthquake Swarm – Trinidad, Colorado, August-October 2001 (Mark E. Meremonte and others, 2002)
• Case Study: The Trinidad, Colorado Earthquakes

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ON-001 — Colorado Earthquake and Fault Map — shows a variety of information including Cenozoic faults. (supersedes publications B-46; B-52; B-52B, and IS-60).

ON-006-03 — AM.RD349 Raspberry Shake live link — A direct link to our RaspberryShake seismometer unit located at our laboratory. This site shows many other units located around the world along with their live traces.

ON-006-05 — Seismic Stations in Colorado — Includes information on seismic stations operated by US Bureau of Reclamation, Colorado Mesa University, the CGS, NEIC, University of New Mexico, and USGS NEHRP.

Colorado Division of Homeland Security and Emergency Management – Earthquakes — Information on earthquake preparedness, Colorado earthquake hazards, and additional preparedness resources.

Colorado Earthquakes — YouTube video about Colorado earthquakes.

Earthquake Reference Collection (ERC) — A comprehensive library of Colorado-specific reports, maps, documents, and papers examining the geoscience behind earthquakes and faulting around the state.

Induced Earthquake Bibliography — This comprehensive bibliography contains several hundred references concerning human-induced earthquakes and other seismicity.

Denver Basin Earthquake Studies — Induced seismicity project covering Weld County, Colorado by researcher Anne Sheehan at the Cooperative Institute for Research in Environmental Sciences (CIRES), CU-Boulder.

Federal Emergency Management Agency Earthquakes — Federal earthquake hazard maps, federal funding for risk management and building codes.

The Great ShakeOut — Information on the annual nationwide earthquake drill, drill procedures, and tips for earthquake preparedness.

USGS Hazards Program – Colorado — Colorado seismicity, hazard maps, and Colorado-specific earthquake topics.

USGS Earthquake Hazards Program — Information on faults, hazard maps, and earthquake ground motion.

USGS Earthquake Notification Service — A customizable service for receiving timely notifications about any earthquakes — national and international.

Western States Seismic Policy Council (WSSPC) — A non-profit earthquake consortium for western US states

ON-001 — Colorado Earthquake and Fault Map — shows a variety of information including Cenozoic faults. (supersedes publications B-46, B-52, B-52B, and IS-60).

County-based HAZUS reports — A primary resource for planners and emergency responders state-wide.