"There's been no doubt in my mind what's causing them," Spotts said. "Sadly, it's really taken a long time for people to come around. Our lives are being placed at risk. Our homes are being broken."
Yet another study, this one published Tuesday in the journal Nature Communications, connected a swarm of small quakes west of Fort Worth, Texas, to nearby natural gas wells and wastewater disposal.
The American Petroleum Institute said the industry is working with scientists and regulators "to better understand the issue and work toward collaborative solutions."
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The Environmental Protection Agency said there no plans for new regulations as a result of the USGS study.
"We knew there would be challenges there, but they can be overcome," EPA Administrator Gina McCarthy said Thursday at an energy conference in Houston.
For decades, earthquakes were an afterthought in the central and eastern U.S., which worried more about tornadoes, floods and hurricanes. Since 2009, quakes have sharply increased, and in some surprising places.
The ground has been trembling in regions that were once seismically stable, including parts of Alabama, Arkansas, Colorado, Kansas, New Mexico, Ohio, Oklahoma and Texas.
The largest jolt linked to wastewater injection — a magnitude-5.6 that hit Prague, Oklahoma, in 2011 — damaged 200 buildings and shook a college football stadium.
The uptick in Oklahoma quakes has prompted state regulators to require a seismic review of all proposed disposal wells. The Oklahoma Corporation Commission, which regulates the oil and gas industry, has ordered dozens of disposal wells to stop operating or change the way they are run because of concerns they might be triggering earthquakes, said spokesman Matt Skinner.
"There are far more steps that will be taken," Skinner said.
Last year, regulators in Colorado ordered an operator to temporarily stop injecting wastewater after the job was believed to be linked to several small quakes.