Geotimes - June 2005 - Geophenomena

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Geophenomena

Coal-mining-induced earthquakes
Volcanoes in a sensitive climate

Coal-mining-induced earthquakes

About 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 seams.

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 structures.

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 university’s 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 bureau’s 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 dam’s grout curtain and allowing a possibly damaging slow leak.

“All dams seep because you cannot effectively seals all cracks,” says Dan Grundvig, the bureau’s 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 paper’s authors. This continuous monitoring “provided some key insights into relating rates and sizes of mine tremors with aspects of the mining activity,” Arabasz says.

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 in Utah.”

Sara Pratt
Geotimes contributing writer

Volcanoes in a sensitive climate

When 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 planet’s 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 model’s sensitivity.

Mount Pinatubo’s 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 volcano’s contribution to shifting the global climate, and what that says about other sources of change. In 1992, for example, James Hansen of NASA’s Goddard Institute of Space Studies in New York City and co-workers calculated that the Mount Pinatubo’s eruption decreased global temperature by almost a degree Celsius, predicting it would take longer than a year for the atmosphere to equilibrate (see Geotimes, March 2002).

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.

Wigley’s 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 NCAR’s 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 planet’s 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 eruption’s 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.

Naomi Lubick

Link:
"Mount Pinatubo: A Natural Climate Experiment," Geotimes, March 2002

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