Evidence is growing that fracking for oil and gas is causing earthquakes that shake the heartland.

States such as Oklahoma, Texas, Kansas and Ohio are being hit by earthquakes that appear linked to oil and gas activity. While the quakes are far more often tied to disposal of drilling waste, scientists also have increasingly have started pointing to hydraulic fracturing itself.

“Certainly I think there may be more of this that has gone on than we previously recognized,” Oklahoma Geological Survey seismologist Austin Holland told colleagues last week.

In addition to what Holland has seen in Oklahoma, a new study in the journal Seismological Research Letters concludes that fracking caused a series of earthquakes in Ohio a year ago. That follows reports of fracking leading to earthquakes in Canada and the United Kingdom.

Hydraulic fracturing is when massive amounts of high-pressure water with chemicals is pumped underground to break shale rock and release the oil and natural gas inside.

The process is responsible for the nation’s energy boom since 2008, as it has allowed access to oil and gas trapped in the shale. But at the same time, earthquakes have spiked in the central and eastern United States.

Before 2008, Oklahoma averaged one earthquake greater than magnitude 3.0 a year. This year there have been 430, Holland said.

Scientists have linked earthquakes in Oklahoma to drilling waste injection. Shale drilling produces large amounts of wastewater, which is often pumped deep underground as a way to dispose of it without contaminating fresh water. Injection raises the underground pressure and can effectively lubricate fault lines according to the U.S Geological Survey.

Geological Survey senior science adviser Bill Leith, speaking at an earthquake forum last week held by the U.S. Energy Association, said communities need to be worried about earthquakes from drilling waste injection. But quakes from fracking itself are rare, Leith said.

After a presentation at the earthquake forum, however, Oklahoma seismologist Holland suggested that fracking could explain about 10 percent of the earthquakes.

A study in the November edition of Seismological Research Letters concluded that fracking triggered a series of earthquakes up to magnitude 2.2 last year in Harrison County, Ohio.