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.