Scientists who watch for earthquakes have discovered a big increase in the number of small quakes in the middle of the country. It's an area that's usually pretty quiet geologically.
The scientists suspect the quakes are caused by wastewater wells. They plan to discuss their findings later this month at a seismology conference, but they've shared the basics with NPR.
Bill Ellsworth, a seismologist at the U.S. Geological Survey, says new technology over the past decade has given scientists a much better feel for when the Earth shakes.
"We've been watching the seismicity across most of the country very carefully for a number of years now," he says, "and one thing we had begun to notice was there was an unusual number of earthquakes occurring in the middle of the country."
Very unusual, in fact. The background rate for the midcontinent 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 they took place, they noticed clusters near wastewater wells, especially in Colorado and Oklahoma. Waste wells are deep holes where various industries pump in wastewater. 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 a lot more wastewater. That requires building more big wells to bury the water.
Ellsworth says in the right place, it doesn't take much to trigger a quake.
"Small perturbations can tip the scales, allowing an earthquake that might not otherwise happen for a very long time," he says.
More evidence linking 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.
What's A Wastewater Well?
Hydraulic fracturing relies on high-pressure water and sand to break apart rock and extract natural gas. The wastewater generated by hydrofracking and other types of gas drilling is injected deep underground for long-term storage. Seismologists believe wastewater injection is causing earthquakes in some parts of the country. Waste wells often go deeper than gas drilling wells, down into basement rock, where faults are more common. Scientists think the water may be lubricating faults in the Earth's crust, causing them to slip.
"The earthquakes that happened then in a swarm followed the startup of two waste-disposal wells that were within 5 kilometers of this fault," he says.
Horton says injecting wastewater into a well raises the pressure of water already trapped in the particles or 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, things like a fault. That's when you get a quake.
Horton says he's convinced there was a cause-and-effect connection between the waste well and the Arkansas quakes.
"The earthquakes started after the injection at the two wells started, and they stopped after the injection stopped," he says.
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.
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.
"The number of earthquakes there has dramatically reduced, which I would think you would take as evidence that the well was triggering the earthquake," he says.
Armbruster says Youngstown was no fluke. He has seen data that reveal more than 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 near waste wells.
"We haven't looked carefully, and we don't know what percentage of these wells would be causing earthquakes," he says.
The state of Ohio is taking action: It's tightening rules for where industry can drill waste wells to avoid quakes.
Also, 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.