Coal Mining Causing Earthquakes, Study Says
Richard A. Lovett
for National Geographic News
|January 3, 2007|
The most damaging earthquake in Australia's history was caused by humans, new research says.
The magnitude-5.6 quake that struck Newcastle in New South Wales on December 28, 1989, killed 13 people, injured 160, and caused 3.5 billion U.S. dollars worth of damage (Australia map).
That quake was triggered by changes in tectonic forces caused by 200 years of underground coal mining, according to a study by Christian D. Klose of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.
The quake wasn't enormous, but Australia isn't generally considered to be seismically active and the city's buildings weren't designed to withstand a temblor of that magnitude, Klose said.
All told, he added, the monetary damage done by the earthquake exceeded the total value of the coal extracted in the area.
Klose presented his findings at a meeting of the American Geophysical Union in San Francisco, California, last month.
The removal of millions of tons of coal from the area caused much of the stress that triggered the Newcastle quake, Klose said. (Related: "Mountaintop Mining Raises Debate in Coal Country" [January 13, 2006].)
But even more significant was groundwater pumping needed to keep the mines from flooding.
"For each ton of coal produced, 4.3 times more water was extracted," Klose said.
Other mining operations, he added, sometimes require as much as 150 tons of water to be removed for each ton of coal produced.
"So this is on the low end," he said.
Coal mining isn't the only human activity that can trigger earthquakes.
Klose has identified more than 200 human-caused temblors, mostly in the past 60 years. "They were rare before World War II," he said.
Most were caused by mining, he said, but nearly a third came from reservoir construction.
Oil and gas production can also trigger earthquakes, he added.
Three of the biggest human-caused earthquakes of all time, he pointed out, occurred in Uzbekistan's Gazli natural gas field between 1976 and 1984 (map of Uzbekistan).
Each of the three had a magnitude greater than 6.8, and the largest had a magnitude of 7.3.
Human-triggered earthquakes are particularly dangerous, Klose said, if they occur in seismically inactive areas.
That's partly because people aren't prepared for them. But also, he said, "regions that are naturally inactive are very trigger-sensitive, because stress has built up over long periods of time."
Klose's presentation drew considerable attention from the assembled geophysicists, who wondered if there were ways to reduce the risk by altering mining practices.
"One way would be to find a way that doesn't reduce the water in the mine," Klose said.
But as far as he knows, mining engineers aren't examining this, because they are currently unaware of the earthquake risk.
The danger is also relevant to proposals to sequester carbon dioxide by injecting it into geologic formations deep underground where the gas cannot escape and contribute to global warming.
"That alters stress in the crust [too]," Klose said, adding that the risk of earthquakes should be taken into account in planning the locations of such facilities.
Basically, he said, "don't put the injection fields close to large cities."
The research could also have an impact on earthquake-insurance premiums, André Unger of the University of Waterloo, in Ontario, told National Geographic News by email.
The precise method by which premiums are calculated is a deeply guarded trade secret, but they appear to be based on a region's historical earthquake risk—"a purely statistical methodology," he said.
The new finding indicates that other factors are now at work, he said.
Furthermore, Unger noted that underground carbon sequestration might be a mixed blessing from insurance companies' points of view.
A carbon-sequestration plan could reduce the risk of some types of damage (such as from hurricanes, which some scientists say are being strengthened by global warming), while increasing the risk of others, like earthquakes.
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