The ground under North Texas didn’t always shake, but today the tremors never really stop. Researchers have pinned the recent rise in small earthquakes around the region on fracking, the process of injecting water into the ground at high pressure to break apart the rock and release oil and natural gas. All of the earthquakes in the last seven years have occurred above the Barnett Shale, a geological formation that has become a major fracking site for petroleum companies. It’s “most likely” that many of the quakes were manmade, according to a recent report by researchers from Southern Methodist University in Dallas.

The study, published in April, looked at the uptick in earthquakes near the towns of Reno and Azle, both located atop the Barnett Shale. Between late 2013 and early 2014, there were more than two dozen small earthquakes in the area that researchers said were most likely due to fracking. The report was the first time a study found fracking a likely cause of small earthquakes and not just a “possible” one, and adds to the growing body of evidence that fracking could be causing more earthquakes.

"While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events," Heather DeShon, a professor of geophysics at SMU and the report’s head researcher, said in a statement. Researchers from the university have been monitoring the increase in North Texas earthquakes since 2008.

The rise in seismic activity has also been felt near Dallas, which has seen 40 small earthquakes since January – a dramatic increase from years past, CNN reported. Before 2008, the area had just one recorded earthquake in 58 years, according to the U.S. Geological Survey, or USGS. The federal agency has identified 17 areas across the country that have seen increased earthquake activity related to fracking, including regions in Alabama, Arkansas, Colorado, Kansas, New Mexico, Ohio, Oklahoma and Texas. In April, officials in Oklahoma declared oil and gas operations the leading cause of the state’s earthquakes. 

Some experts have their doubts that fracking could be causing more earthquakes. "There just isn't enough evidence to support the claim yet in this case," USGS researcher George Choy said, according to CNN. “The connection has not been established, but we cannot rule them out.”