Before 2011, Youngstown, Ohio, had never experienced an earthquake. Or at least, the area hadn't had one since 1776, when record-keeping began. So it was strange, during a nine-month period of 2011, that nine of them shook the town.
At first these were small; none topped 2.7 on the Richter scale—tiny things, not perceived much differently than the passing of a heavy truck. But then, the quakes escalated. On Dec. 31, 2011, at 3:05 p.m., Youngstown was stirred by a 3.9 (a local newspaper crowdsourced its readers to respond to the question "Did you feel the boom?" You'll want to read some of the answers.) To be sure, this still wasn't a huge quake. For some perspective, the one that hit the D.C. area in July 2011 was a stronger 5.8. But still, while earthquakes in the greater Ohio area aren't unheard of (there have been at least 200 since 1776), having such a concentrated number was odd for the area, especially in a city that had previously experienced none.
Located suspiciously near all of this seismic activity was the Northstar 1 well, which pumped chemical-laden hydraulic-fracturing, or fracking, fluid deep within the ground, at a rate around 1,300 gallons per hour. Right after the 3.9 quake, the Ohio Department of Natural Resources director took to a news conference to say, "The seismic events are not a direct result of fracking." In any case, the well was shut down for investigation. (It had actually been shut down just a few days prior in response to a 2.7 quake. The New Year's Eve quake turned it off for the long term.)
Now, the data is in, and it appears the Natural Resources director's assessment was flat-out wrong. "We conclude that the recent, 2011-2012, earthquakes in Youngstown, Ohio were induced by the fluid injection at Northstar 1 deep injection well due to increased pore pressure along the preexisting faults located close to the wellbore," an article in the most recent Journal of Geophysical Research concludes. Pore pressure refers to the force exerted on underground rock by fluids. Furthermore, the research finds a correlation between the quakes and the daily volume of material being injected into the well. After a peak of injection pressure into the well, five days later, seismic activity would spike as well.
What seemed to happen was that the high-pressure fluid moved along a preexisting fault in a west-southwest direction (away from the well), increasing pressure on the rock formations as it progressed. This decreased the resistance the rock had to faulting, which increased the risk of an earthquake. And the location of the seismic activity moved as the the fluid did, along the fault line. When the pumping at the well stopped, the study concludes, the earthquakes waned.
However, it's not the case that fracking fluid will always cause earthquakes. In the case of Youngstown, the placement of the well over an unknown fault led to the town shaking. After all, the Northstar 1 "is the only well out of 177 ... waste disposal wells operating in the state of Ohio during 2011 that has been linked to potentially induced earthquakes."