Fracking, in which liquids are pumped into the ground under high pressure to crack rocks that then release oil or gas, is highly controversial. Opponents of hydraulic fracturing point to ground water contamination and other environmental damages. But a less obvious side-effect is now being discussed by scientists from the Seismological Society of America (SSA): fracking can also cause earthquakes.

The earth shook in Ohio

The connection between seismological events and the contentious extraction method has been known for some time. But 2014 saw one of the strongest fracking-related earthquakes ever in the US. In March, there was a quake of magnitude three on the Richter scale in Poland Township, Ohio.

Seismologists from the SSA researched the event and found that it was directly related to fracking going on in the area - and that it wasn't an isolated incident. In total, they found 77 earthquakes over the course of their study, with magnitudes ranging from one to three. They said these showed a clear chronological and spatial relation to fracking activities in and around Poland Township.

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"The earthquakes in Poland Township occurred in a very old, Precambrian stratum, which probably already had numerous seismological faults," researcher Robert Skoumal said in an SSA statement.

By pumping liquids into the ground at high pressure, the energy companies opened up old wounds, so to speak - they re-activated faults in the rock, which led to an earthquake that was felt on the surface. After that, the drilling was stopped and the seismic activity quickly ended.

Eyes to the ground

The state of Ohio reacted promptly. It now has a regulatory system in place that seeks to identify and investigate earthquakes in drilling areas, seismologist Michael Brudzinski of Miami University in Ohio, which participated in the SSA study, told DW. When a quake of magnitude one is recorded, the incident is investigated. Once it hits magnitude two, the fracking operation is halted.

Some people have been wondering why risky faults aren't identified before the drilling even starts, so these situations can be avoided altogether. Easier said than done, according to Brudzinski: "It's very difficult to identify these faults that get activated before the drilling takes place. They're several miles deep in the ground and often not very large in extent."

The companies that do the fracking do some scans of the area they plan to work in, but so far there's no preemptive system to make the faults visible and prevent quakes from the get-go.

Finding faults

"Whenever you drill or dig into the ground, seismic activity is a possibility," German seismologist Klaus-Günter Hinzen said. In Germany, earthquakes occur with a higher frequency in coal-mining areas. As the country slowly switches to renewable energy sources, this is becoming less of an issue, but the same cannot be said for fracking, in the US or elsewhere.

Michael Brudzinski says it's often hard to identify quake-prone faults

Still, Brudzinski says the risks appear to be manageable. "We can't rule out that large earthquakes associated with fracking could happen," he said. "But there's been a large number of fracking operations over the last five years or so and we still only have a handful of cases of any felt earthquakes - less than ten. So the numbers suggest to me that it's really quite rare to have any earthquake large enough to be felt, let alone cause damage."

With natural earthquakes, a magnitude of three like the one measured in Ohio doesn't even usually register aboveground. The difference between natural quakes and those triggered by drilling is how deep below ground they occur.

"Earthquakes caused by mining happen at a very shallow depth, sometimes less than 1,000 meters below the surface," Hinzen explained. "Regular earthquakes usually occur around 15 kilometers underground."

So deep down, a magnitude three tremor probably wouldn't even tip over a bag of rice. But in former German mining areas like the Ruhr, they definitely once influenced the lives of local residents.

"Your life wasn't endangered," Hinzen said."But still, being woken up every night because the earth shakes must have been quite irritating."