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Page 20 ■ Thursday, April 12, 2012


NATION & WORLD


BAKKEN BREAKOUT WEEKLY Study ties oil, gas production to Midwest quakes


By MALCOLM RITTER AP Science Writer


NEW YORK (AP) — Oil and gas


production may explain a sharp in- crease in small earthquakes in the na- tion’s midsection, a new study from the U.S. Geological Survey suggests. The rate has jumped six-fold from


the late 20th century through last year, the team reports, and the changes are “almost certainly man-made.” Outside experts were split in their opinions about the report, which is not yet published but is due to be presented at a meeting later this month. The study said a relatively mild in-


crease starting in 2001 comes from increased quake activity in a methane production area along the state line between Colorado and New Mexico. The increase began about the time that methane production began there, so there’s a “clear possibility” of a link, says lead author William Ellsworth of the USGS. The increase over the nation’s mid- section has gotten steeper since 2009, due to more quakes in a variety of oil


and gas production areas, including some in Arkansas and Oklahoma, the researchers say. It’s not clear how the earthquake


rates might be related to oil and gas production, the study authors said. They note that others have linked earthquakes to injecting huge amounts of leftover wastewater deep into the earth.


There has been concern about po-


tential earthquakes from a smaller- scale injection of fl uids during a pro- cess known as hydraulic fracturing, or fracking, which is used to recover gas. But Ellsworth said April 6 he is confi - dent that fracking is not responsible for the earthquake trends his study found, based on prior studies. The study covers a swath of the


United States that lies roughly west of Ohio and east of Utah. It counted earth- quakes of magnitude 3 and above. Magnitude 3 quakes are mild, and


may be felt by only a few people in the upper fl oors of buildings, or may cause parked cars to rock slightly. The biggest counted in the study was a magnitude-


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5.6 quake that hit Oklahoma last Nov. 5, damaging dozens of homes. Experts said it was too strong to be linked to oil and gas production. The researchers reported that from


1970 to 2000, the region they studied averaged about 21 quakes a year. That rose to about 29 a year for 2001 through 2008, they wrote, and the three follow- ing years produced totals of 50, 87 and 134, respectively. The study results make sense and


are likely due to man-made stress in the ground, said Rowena Lohman, a Cornell University geophysicist. “The key thing to remember is mag- nitude 3s are really small,” Lohman said. “We’ve seen this sort of behavior in the western United States for a long time.” Usually, it’s with geothermal energy, dams or prospecting. With magnitude 4 quakes, a person standing on top of them would at most feel like a sharp jolt, but mostly don’t last long enough to be a problem for buildings, she said. The idea is to understand how the man-made activity triggers quakes, she


said. One possibility is that the injected fl uids change the friction and sticki- ness of minerals on fault lines. Another concept is that they change the below- surface pressure because the fl uid is trapped and builds, and then “sets off something that’s about ready to go any- way,” Lohman said. But another expert was not con-


vinced of a link to oil and gas opera- tions. Austin Holland, the Oklahoma state


seismologist, said the new work pres- ents an “interesting hypothesis” but that the increase in earthquake rates could simply be the result of natural processes. Holland said clusters of quakes can


occur naturally, and that scientists do not yet fully understand the natural cycles of seismic activity in the central United States. Comprehensive earth- quake records for the region go back only a few decades, he said, while natu- ral cycles stretch for tens of thousands of years. So too little is known to rule out natural processes for causing the increase, he said.


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