Induced Seismicity and Energy Technologies - Induced Seismicity Potential in Energy Technologies - page 6

BOX 1.2
The Rocky Mountain Arsenal Earthquakes
During the spring of 1962 seismological stations in Colorado began recording a number of small earth-
quakes near Denver. Although Denver had previously been considered to be in an area of low seismicity, between
April 1962 and August 1967 over 1,500 earthquakes were recorded at the seismograph station at Bergen
Park, Colorado. Some of the earthquakes were noticeable to local residents and exceeded M 3 and M 4. The
earthquakes were eventually attributed to the underground injection of fluid using a deep well drilled on land
known as the Rocky Mountain Arsenal approximately 6 miles northeast of downtown Denver.
The Rocky Mountain Arsenal was used by the U.S. Army from 1942 through 1985 for both the manu-
facture and the disposal of chemical weapons. In 1961 the army drilled a well on the arsenal grounds for the
disposal of chemical fluid wastes by underground injection. The well was drilled to a depth of 12,045 feet into
Precambrian crystalline rocks (rocks greater than about 700 million years old) beneath the sedimentary rocks of
the Denver basin. Fluid injection began in March 1962, and from that time through September 1963, fluid was
injected at an average rate of 181,000 gallons per day (gal/day). Injection was stopped in October 1963, but
commenced again from August 1964 through April 1965. During this second injection cycle the fluid was not
injected under pressure but was fed to the well under gravity flow at a rate of 65,800 gal/day. In April 1965
pressure injection resumed at a rate of 148,000 gal/day. The maximum injection pressure at any time was
72 bars (1,044 pounds per square inch [psi]). a
In April and May 1962, two seismological observatories in the Denver area began recording a series of
small earthquakes.
In June of 1962 several earthquakes occurred which were large enough to be felt by residents and
caused considerable concern. By November of 1965 over 700 shocks had been recorded and,
although 75 of these had been felt, no damage was reported….” (McClain, 1970)
Research conducted in the mid-1960s on the deep injection well located on the Arsenal grounds detailed
the correlation between the amount of fluid injected into the Arsenal well and the number of Denver earthquakes
(Evans, 1966). This research indicated a strong relationship between injection volumes and earthquake frequency
(see Figure). More detailed investigation by several local universities and the U.S. Geological Survey (USGS)
gave further support to this conclusion. The research showed the majority of the earthquakes had epicenters
within 5 miles of the Arsenal's injection well. The depths of the earthquakes varied from 12,140 to 23,000 feet
(3,700 to 7,000 meters) below the surface, which is the depth of Precambrian rocks in the area. Research also
showed that the epicenters for the earthquakes aligned in a generally northwest-to-southeast direction, similar
to the orientation of a system of natural vertical fractures found in the Precambrian rocks in the area.
Although injection into the Arsenal well ceased in February 1966, earthquake activity continued for several
more years. The strongest earthquakes actually occurred after injection into the well was discontinued. A detailed
analysis of seismograms (Herrmann et al., 1981) indicated seismic moments of the largest earthquakes that can
be converted to M 4.5 (April 1967), M 4.8 (August 1967), and M 4.5 (November 1967). These magnitudes
are more accurately determined and somewhat smaller than the magnitudes reported in earlier papers on the