Despite yesterday's earthquake that hit the Trinidad region, "Colorado is considered a region of minor earthquake activity," according to the U.S. Geological Survey. But forty years ago, a series of quakes rocked the Denver area -- quakes caused not by Mother Nature, but by Uncle Sam.

How? The Army was dumping dangerous chemicals into a deep injection well out at the Rocky Mountain Arsenal.

The Rocky Mountain Arsenal was created out of farmland on the eastern edge of metro Denver during World War II to arm the U.S. Army. After the war, it became a bustling center of industrial activity -- a top-secret center that created a lot of dangerous waste.

Here's the history of the Arsenal quakes from the USGS site:

In 1961, a 12,000-foot well was drilled at the Rocky Mountain Arsenal, northeast of Denver, for disposing of waste fluids from Arsenal operations. Injection was commenced March 1962, and an unusual series of earthquakes erupted in the area shortly after. It was 32 minutes after 4 a.m. on April 24 when the first shock of the Denver series was recorded at the Cecil H. Green Geophysical Observatory at Bergen Park, Colorado. Rated magnitude 1.5, it was not strong enough to be felt by area residents. By the end of December 1962, 190 earthquakes had occurred. Several were felt, but none caused damage until the window breaker that surprised Dupont and Irondale on the night of December 4. The shock shuffled furniture around in homes, and left electrical wall outlets hanging by their wires at Irondale. Over 1,300 earthquakes were recorded at Bergen Park between January 1963 and August 9, 1967. Three shocks in 1965 -- February 16, September 29, and November 20 -- caused intensity VI damage in Commerce City and environs. The Denver series was forgotten, however temporarily, in October 1966, when a southeast Colorado tremor rocked a 15,000 square-mile area of that State and bordering New Mexico. Minor damage, in the form of broken windows and dishes and cracked walls and plaster, occurred at Aguilar, Segundo, Trinchera, and Trinidad. Another strong shock rumbled through the Denver area on November 14, 1966, causing some damage at Commerce City and Eastlake. Slighter rumblings (below magnitude 3.0) occurred throughout the remainder of 1966, and through the first week of April 1967. Then, on April 10, the largest since the series began in 1962 occurred; 118 windowpanes were broken in buildings at the Rocky Mountain Arsenal, a crack in an asphalt parking lot was noted in the Derby area, and schools were dismissed in Boulder, where walls sustained cracks. Legislators quickly moved from beneath chandeliers in the Denver Capitol Building, fearing they might fall. The Colorado School of Mines rated this shock magnitude 5.0. Boulder sustained minor damage to walls and acoustical tile ceilings on April 27, 1967, as result of a magnitude 4.4 earthquake. Then a year and half after the Rocky Mountain Arsenal waste dumping practice stopped, the strongest and most widely felt shock in Denver's history struck that area on August 9, 1967, at 6:25 in the morning. The magnitude 5.3 tremor caused the most serious damage at Northglenn, where concrete pillar supports to a church roof were weakened, and 20 windows were broken. An acoustical ceiling and light fixtures fell at one school. Many homeowners reported wall, ceiling, floor, patio, sidewalk, and foundation cracks. Several reported basement floors separated from walls. Extremely loud, explosivelike earth noises were heard. Damage on a lesser scale occurred throughout the area. During November 1967, the Denver region was shaken by five moderate earthquakes. Two early morning shocks occurred November 14. They awakened many residents, but were not widely felt. A similar shock, magnitude 4.1, centered in the Denver area November 15. Residents were generally shaken, but no damage was sustained. A local shock awakened a few persons in Commerce City November 25. Houses creaked and objects rattled during this magnitude 2.1 earthquake. The second largest earthquake in the Denver series occurred on November 26, 1967. The magnitude 5.2 event caused widespread minor damage in the suburban areas of northeast Denver. Many residents reported it was the strongest earthquake they had ever experienced. It was felt at Laramie, Wyoming, to the northwest, east to Goodland, Kansas, and south to Pueblo, Colorado. At Commerce City merchandise fell in several supermarkets and walls cracked in larger buildings. Several persons scurried into the streets when buildings started shaking back and forth. During 1968, ten slight shocks were felt in Colorado. Only one, on July 15, caused minor damage at Commerce City. In September of that year, the Army began removing fluid from the Arsenal well at a very slow rate, in hope that earthquake activity would lessen. The program consisted of four tests between September 3 and October 26. Many slight shocks occurred near the well during this period.

In its own account of the cleanup at the Rocky Mountain Arsenal, the Army web site offers this explanation:

Deep well injection for liquid waste has been safely used for many years at sites throughout the United States without documented damage to human health or the environment. After an extensive study of deep injection wells across the country by the U.S. Environmental Protection Agency (EPA), it was concluded that this procedure is effective and protective of the environment. The Rocky Mountain Arsenal deep injection well was constructed in 1961, and was drilled to a depth of 12,045 feet. The well was cased and sealed to a depth of 11,975 feet, with the remaining 70 feet left as an open hole for the injection of Basin F liquids. For testing purposes, the well was injected with approximately 568,000 gallons of city water prior to injecting any waste. However, when the Basin F liquids were actually introduced, the process required more time than anticipated to complete because of the impermeability of the rock. The end result was approximately 165 million gallons of Basin F liquid waste being injected into the well during the period from 1962 through 1966. The waste fluid chemistry is not known precisely. However, the Army estimates that the waste was a more dilute version of the Basin F liquid which is now being incinerated. Current Basin F liquid consists of very salty water that includes some metals, chlorides, wastewater and toxic organics. From 1962 -- 1963, the fluids were pumped from Basin F into the well. From 1964 -- 1966, waste was removed from an isolated section of Basin F and was combined with waste from a pre-treatment plant, located near Basin F, and then pumped into the well. The waste from the pre-treatment plant was generally a solution containing 13,000 parts per million sodium chloride (salt), with a pH ranging from 3.5 to 11.5. The organic content of the solution was high but is largely unknown. The injected fluids had very little potential for reaching the surface or useable groundwater supply since the injection point had 11,900 feet of rock above it and was sealed at the opening. The Army discontinued use of the well in Feb. 1966 because of the possibility that the fluid injection was triggering earthquakes in the area. The well remained unused for nearly 20 years. In 1985 the Army permanently sealed the disposal well in stages. First, the well casing was tested to evaluate its integrity. Any detected voids behind the casing were cemented to prevent possible contamination of other formations. Next, the injection zone at the bottom 70 feet of the well was closed by plugging with cement. Additional cement barriers were placed inside the casing across zones that could access water-bearing formations (aquifers). The final step was adding Bentonite, a heavy clay mud that later solidified, to close the rest of the hole up to the ground surface.

Today, the Rocky Mountain Arsenal, once some of the most contaminated land in the country, is a cleaned-up nature center. Just watch where you step.

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