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Yesterday, earthquakes struck off the coast of Indonesia in western Mexico, and off the coast of Oregon, places where quakes are fairly common. But seismologists are also noticing a big increase in earthquakes in the Midwest, a region that is usually stable. One group of scientists suspects the quakes are a side effect of natural gas drilling. They plan to discuss their findings at a seismology conference this month, but first shared the basics with NPR's Christopher Joyce.
CHRISTOPHER JOYCE, BYLINE: Bill Ellsworth is a seismologist at the U.S. Geological Survey. He says new technology over the last decade or so has given scientists a much better feel for when the Earth shakes.
BILL ELLSWORTH: We've been watching the seismicity across most of the country very carefully for a number of years now, and one thing we had begun to notice was there were an unusual number of earthquakes occurring in the middle of the country.
JOYCE: Very unusual, in fact. The background rate for the mid-continent had been about 20 quakes a year. That rose to 29 by 2008. Then it really jumped - 50 quakes in 2009, 87 the next year, and a whopping 134 last year. When USGS scientists zeroed in on where the quakes took place, they noticed clusters near waste water wells, especially in Colorado and Oklahoma.
Waste wells are deep holes where various industries pump in waste water at very high pressure. This has been a common practice for decades and once in a while the pumping has created quakes. But the boom in natural gas drilling across the country has created lots more waste water. That requires building more big wells to bury that water. Ellsworth says in the right place it doesn't take much to trigger a quake.
ELLSWORTH: Small perturbations can tip the scales, allowing an earthquake to happen that might not otherwise happen for a very long time.
JOYCE: More evidence linking small quakes and waste wells keeps coming in from around the country. Seismologist Steve Horton at the University of Memphis tracked a swarm of quakes along a fault in Arkansas in 2010 and 2011.
STEVE HORTON: The earthquakes that happened then in a swarm followed the startup of two waste disposal wells that were within five kilometers of this fault.
JOYCE: Horton says injecting waste water into a well raises the pressure of water already trapped in the particles of rock around it. It's kind of like sticking a straw into a soupy souffle and blowing water into it. It moves things around, underground(ph), things like a fault. And that's when you get a quake. Horton says he's convinced that there was a cause-and-effect connection between the waste well and the Arkansas quakes.
HORTON: The earthquakes started after injection at the two wells started, and they stopped after the injection stopped.
JOYCE: Horton points out that a lot of the evidence connecting waste wells and quakes is preliminary. Since there are small quakes in most parts of the country, it's hard to be sure that a waste well is at fault. But that doesn't look to be the case in Youngstown, Ohio. There were significant quakes last year associated possibly with a waste well there - the biggest hit the day after the well stopped injecting water. John Armbruster, a seismologist with Columbia University, has monitored the well since then.
JOHN ARMBRUSTER: And the number of earthquakes there has dramatically reduced, which I think you would take as evidence that the well was triggering the earthquakes.
JOYCE: Armbruster says Youngstown was no fluke. He's seen data that reveal over a dozen small quakes in another part of Ohio, near the town of Marietta, over the past year-and-a-half. It's not a seismically active part of the state, but it has several waste wells.
Seismologists, as well as industry experts, point out that there are tens of thousands of waste wells in the country and only a few create problems. But Armbruster says it's early days yet. Scientists need to put more instruments out there near waste wells.
ARMBRUSTER: We haven't looked carefully and we don't know what percentage of these wells would be causing earthquakes.
JOYCE: The state of Ohio is taking action. It's tightening rules for where industry can drill waste wells to avoid quakes. And the federal Environmental Protection Agency is working with the U.S. Geological Survey and other scientists to draft guidelines for waste wells in the rest of the country.
Christopher Joyce, NPR News.