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Mining-Triggered Quakes May Threaten Earth-Filled Dams

Mining-Triggered Quakes May Threaten Earth-Filled Dams

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Ed Yeates reportingWhile Utah's rich coal reserves keep power plants running to satisfy our appetite for electricity, there may be a risky tradeoff.

Three new published studies show coal mining triggered earthquakes might pose a risk for nearby earth-filled dams.

Dr. Walter Arabasz/Director, Univ. of Utah Seismograph Stations: "this happened in March, in the Willow Creek mine, where a magnitude 4.2 earthquake was triggered by a slip on a shallow geological fault."

As they ream out coal, mining engineers do their best to control underground stress - but sometimes Nature is fickle - and the mining operation can trigger movement on a nearby fault. Studies by the University of Utah, the USGS and the Bureau of Reclamation show, unlike the Wyoming mine incident, Utah's coal mining technique- where machinery chomps away small sections at a time allowing the roof to collapse behind it -- produces very little stress.

But still, the mining triggers not deep seated, but shallow low yield earthquakes.

Dr. Walter Arabasz: "When you bring it up shallow in the crust, the frequency content, the character of the shaking would be different."

In fact, the studies show this proposed Cottonwood coal tract, yet to be mined, would come within about a half mile of the Joes Valley Dam. Underground mining near the dam might cause a 3.9 tremor.

Three-point-nine is not a large earthquake, perhaps enough to knock some things off a shelf. Not enough to damage a building. But earth filled dams are different.

Bureau of Reclamation area director Bruce Barrett says even a low yield quake could loosen a piece in the dam's grout curtain, allowing water to gradually start seeping out. Over time, that could prove disastrous.

Barrett is proposing a setback that would keep any future underground mining a mile to a mile and a half away. How much coal would that lock up.

Dr. Michael Kim McCarter/Univ. of Utah Mining Engineering: "The coal seam would probably be in the order of eight to ten feet thick and a square mile or more."

That's more than 5-million tons of coal. But based on these latest studies, Barrett says the Bureau must now lean heavily on the side of caution - even though the risk is minimal.

The Cottonwood coal tract near Joes Valley is only a proposal at this point. The Bureau of Reclamation wanted the studies completed before any company decides to mine it.

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