What this study means is that with the shifts that are reflected in the earthquakes there are new faults and fractures created.  These can be new pathways along which materials can move – pathways that did not exist before the wells were put in and hydraulically fractured.

Earthquakes have also been linked to the use of injection wells for the disposal of wastewater from oil and gas drilling.

The controversial oil and gas drilling technique of hydraulic fracturing, commonly known as fracking, triggered 400 small earthquakes in Ohio over a three-month period in 2013, according to new peer-reviewed research published Tuesday.

Conducted by seismologists Paul Friberg, Ilya Dricker, and Glenda Besana-Ostman, the research reveals a previously undiscovered fault line approximately two miles below three horizontal gas wells near the town of Uhrichsville, Ohio. In one instance, the researchers say they detected 190 tiny earthquakes below one of those wells during a 39-hour period starting just after that well was fracked.

The earthquakes were too small to be felt in all cases, but Friberg said they were stronger than he initially expected.

“Hydraulic fracturing has the potential to trigger earthquakes, and in this case, small ones that could not be felt,” he said in a statement, “however the earthquakes were three orders of magnitude larger than normally expected.”

The technique of fracking works, essentially, by “fracturing” underground shale rock. Companies drill a well underground, then inject a high-pressure mixture of water, sand, and chemicals into that well to crack the rock and let gas flow out more easily. According to Friberg, the process of cracking the rocks causes “micro-earthquakes,” the magnitudes of which actually come up as negative numbers on the Richter scale — usually in the range of -3 to -1.

Only ten of the earthquakes measured by Friberg and his team were above negative magnitudes, all occurring between October 2 and October 19, 2013, and ranging from a magnitude of 1.7 to 2.2. Like the micro-quakes, those earthquakes weren’t felt by the public either, because of how deep underground they occurred.

Still, the findings have big implications for Ohio, which has some of the most strict rules governing fracking-induced earthquakes in the country. Concerned with growing reports of seismic activity in fracking-rich states, Ohio’s government in April announced that it would begin requiring oil and gas companies to install earthquake monitors before drilling within three miles of a known fault line, or in any area that has ever experienced an earthquake greater than a 2.0 magnitude. If those monitors detect a quake of 1.0 or more, the company has to halt its operations, and regulators must investigate whether drilling was the cause.

The earthquakes detected by Friberg and his team happened before the regulations were in place, but at least ten of them were above the 1.0 magnitude threshold.

This certainly isn’t the first time seismologists have reported increased frequency and intensity of earthquakes in areas where fracking is prolific. But what makes this research different from most is that it credits earthquakes to the process of fracking itself — the actual process of cracking underground shale rock with high-pressure fracking fluid.

Most other studies on artificial earthquakes don’t credit fracking itself. Instead, they credit the process of wastewater injection: a.k.a, taking the leftover water used to frack the well and disposing of it by injecting it back underground. Scientists increasingly believe that the underground fluid migrates along dormant fault lines, re-activating them and causing earthquakes.

Like the quakes in Ohio, they have for now been mostly too small to be felt by humans. But some scientists say that larger quakes triggered by wastewater injection from fracking could become the norm as more and more water is stored underground.

“I think ultimately, as fluids propagate and cover a larger space, the likelihood that it could find a larger fault and generate larger seismic events goes up,” Western University earth sciences professor Gail Atkinson reportedly said at a meeting of the Seismological Society of America in May.

The research linking fracking and the fracking wastewater disposal process to earthquakes is still preliminary, but evidence is getting stronger. Seismologists say more research needs to be done on the subject, particularly because it’s so difficult to tell which seismic events are natural and which others are caused by human activity.

Meanwhile, U.S. Geological Survey scientists in September found evidence “directly linking” the uptick in Colorado and New Mexico earthquakes since 2001 to wastewater injection. A group of more than 50 earthquakes in 2009 and 2010, in Cleburn, Texas was also linked to the injection of fracking wastewater into the ground. Before 2008, the area had never recorded an earthquake.