Volcanoes in a sensitive climate
75 miles south of Provo, Utah, inside Manti-La Sal National Forest, lies Joes
Valley Dam, a 190-foot-high earthen dam built in the mid-1960s for irrigation.
Perched behind the dam, at nearly 7,000 feet above sea level, is Joes Valley Reservoir,
a 1,160-acre-lake surrounded by sheer-walled canyons carved into the surrounding
Blackhawk sandstone, an upper Cretaceous-Tertiary formation interlaced with coal
A recent study in the Joes Valley Dam region
investigated the effects of nearby coal mining on earthquake risk in the area.
Courtesy of U.S. Bureau of Reclamation.
The same juxtaposition that makes Joes Valley popular with both fly fishermen
and rock climbers a dammed reservoir and canyons in a coal-bearing sandstone
also makes it an ideal natural laboratory for seismologists studying earthquakes
triggered by mining and whether vibrations from those quakes will reach surface
The nearby Trail Mountain Coal Mine plans to extend its shallow, underground mining
operation into an area about half a mile from the Joes Valley Dam. To investigate
the seismic hazard to the dam, seismologists from the University of Utah, the
U.S. Geological Survey (USGS), and the U.S. Bureau of Reclamation studied earthquakes
within the Trail Mountain Mine, adjacent to the tract where future coal mining
is planned. The multi-part study, published as a trio of papers in the February
Bulletin of the Seismological Society of America, concluded that the planned
mining could cause an earthquake as large as magnitude 3.9 at the dam.
In general, one would expect an earthfill dam to be fairly resilient to
ground shaking from a magnitude-3.9 shock, says team leader Walter J. Arabasz,
a seismologist at the University of Utah, Salt Lake City, and director of the
universitys network of seismograph stations. However, Arabasz notes that
the question of what, if any, potential damage could arise from such a quake ultimately
has to be answered by an engineer with some understanding of the fragility
of the dam a task that would most likely fall to the U.S. Bureau
of Reclamation, which manages the dam.
In response to the seismology study, Bruce Barrett, area manager of the bureaus
Upper Colorado Region based in Provo, Utah, proposed in March the establishment
of a setback to keep any future mining at least 1 mile away from Joes
Valley Dam. He cited the potential risk of a quake loosening the dams grout
curtain and allowing a possibly damaging slow leak.
All dams seep because you cannot effectively seals all cracks, says
Dan Grundvig, the bureaus chief geologist in the region. The key is
to keep seepage to a manageable level and monitor for increases to detect if conditions
are changing in the dam.
Coal-mining-induced earthquakes are common in Utah, but large ones are rare. The
largest coal-mining-related event historically observed in Utah took place in
2000, when a magnitude-4.2 earthquake occurred at the Willow Creek Mine, about
30 miles north of the Joes Valley Dam, triggering rock falls that disrupted traffic
on a highway and a rail line.
The method of coal mining used at Trail Mountain called longwall mining
induces tremors, not by blasting, but by carving away coal along the length
of a seam and allowing the overburden to collapse in areas that have already been
mined. The collapse redistributes stress in the overlying rock and coal, causing
it to fracture or burst, producing minor quakes.
Between late-2000 and mid-2001, the researchers recorded 1,913 earthquakes, all
less than magnitude 2.2 and about 1,600 feet deep, which were highly correlated
with mining activity both in space and time, according to the papers
authors. This continuous monitoring provided some key insights into relating
rates and sizes of mine tremors with aspects of the mining activity, Arabasz
The recorded ground motions also helped Arthur F. McGarr and Jon B. Fletcher,
geophysicists with the Earthquake Hazards Team at USGS in Menlo Park, Calif.,
to develop the first equations to predict how the ground will move in response
to shallow quakes of various sizes at short distances from Trail Mountain (between
1,600 feet and 6 miles). Until now, ground-motion equations were based on either
natural earthquakes, which are much larger and deeper, or on quakes caused by
deep gold mining in South Africa, which occur in a much different geologic setting
than shallow coal mining.
There are many other situations around the world where the seismic hazard
is due to very shallow earthquakes, usually caused by mining or quarry operations,
McGarr says. So, I think the ground-motion prediction relations developed
in this paper will have a broader impact than just on coal mines near reservoirs
Geotimes contributing writer
Back to top
in a sensitive climate
Mount Pinatubo violently erupted in June 1991, the subsequent ejection of ash
and particulate matter into the atmosphere allowed climate scientists to use
the volcano as a laboratory for climate effects. Now, the volcano is helping
researchers to tune their climate models and determine how sensitive the planets
atmosphere is to change.
Mount Pinatubo spewed ash hours before its cataclysmic eruption on June 15,
1991. The ash cloud, shown here, mixed with a typhoon, and the later eruption
sent particulate matter around the planet, affecting global climate. Researchers
are using the event to calibrate the sensitivity of their climate models. Courtesy
of NOAA and NASA Goddard.
One key to climate modeling is calculating how long it takes for the whole system
to respond to changes, from clouds to ocean temperature changes to global average
temperature shifts. Many events can factor into changes in the system, including
volcanic eruptions, and each event can help calibrate a models sensitivity.
Mount Pinatubos eruption is the largest climatic forcing from a volcano
since Krakatoa in Indonesia erupted in 1883. Studies of particulate matter have
allowed scientists to determine the volcanos contribution to shifting
the global climate, and what that says about other sources of change. In 1992,
for example, James Hansen of NASAs Goddard Institute of Space Studies
in New York City and co-workers calculated that the Mount Pinatubos eruption
decreased global temperature by almost a degree Celsius, predicting it would
take longer than a year for the atmosphere to equilibrate (see Geotimes,
In a study in the May 6 Journal of Geophysical Research Atmospheres,
Tom Wigley of the National Center for Atmospheric Research (NCAR) in Boulder,
Colo., and his co-workers compared the climatic effects of Pinatubo in the Philippines,
as well as El Chichón in Mexico and Mount Agung in Indonesia. Using models
linking both ocean and atmosphere (that are still simple compared to the all-inclusive
models used by the Intergovernmental Panel on Climate Change), they looked at
which models can get at climate sensitivity most accurately.
Wigleys team compared 16 models that contain some kind of volcanic forcing,
along with solar impacts and greenhouse gases. Comparing the models to data
observations over the past century or so, they found that the climate system
would take 27 to 43 months to come to equilibrium after a large eruption. That
amount of time corresponds to what would happen if current atmospheric carbon-dioxide
levels doubled, causing a 1 to 4 degrees Celsius warming.
Wigley says that their findings show that their relatively simplified models
do a good job of replicating the most complex ones out there, such as NCARs
Parallel Climate Model. Such determinations can have further impacts on conclusions
by the IPCC, for example, and future climate change policies. You need
a model that is sensible and credible, Wigley says, particularly when
trying to tease out human impacts on a sensitive climate.
However, another recent publication also using Mount Pinatubo questions the
sensitivity of the planets climate system. David Douglass and Robert Knox
of the University of Rochester in New York, publishing in the March 11 Geophysical
Research Letters, found that the 1991 eruptions impact on climate
lasted less than a year, with temperature peaking at 7 months, and leaving
no volcano effect in the pipeline for future climate change. The researchers
thus concluded that the climate system is fairly insensitive, so to speak, and
comes back to equilibrium quickly. Their results, however, are sparking controversy
in the climate community, with rebuttals in the works.
Pinatubo: A Natural Climate Experiment," Geotimes, March 2002
Back to top