If you hit the ground with a hammer, it creates a micro-earthquake, but it is obviously too small to be detected. The ancient Chinese used to use a drum in the ground to listen for enemy sappers mining underneath their fortifications.

The process of hydraulic fracturing, or fracking, extracts gas and oil from shale rock by injecting a high-pressure water mixture directed at the rock to release the oil and gas trapped inside. Like any geological event, that results in micro-earthquakes much smaller than humans can feel. 

But in March 2014, a series of five recorded earthquakes in Poland Township, Ohio, ranging from magnitude 2.1 to 3.0, occurred about a half a mile from a group of oil and gas wells operated by Hilcorp Energy, which was conducting active hydraulic fracturing operations at the time. Due to the proximity of a magnitude 3.0 event near a well, the Ohio Department of Natural Resources (ODNR) halted operations at the Hilcorp well on March 10th. 

A new study links the March 2014 earthquakes in Poland Township to hydraulic fracturing, saying it activated a previously unknown fault. The induced seismic sequence included a rare felt earthquake of magnitude 3.0, according to the paper. 

Earthquakes happen thousands and thousands of times per year, they are just too faint to be detected. Improved seismic monitoring has made it possible to detect them better than ever and the authors say the number of earthquakes - felt and unfelt - associated with hydraulic fracturing has increased in the past decade. 

"These earthquakes near Poland Township occurred in the Precambrian basement, a very old layer of rock where there are likely to be many pre-existing faults," said Robert Skoumal, graduate student at  at Miami University in Ohio, who co-authored the study with Michael Brudzinski and Brian Currie at Miami University in Ohio. "This activity did not create a new fault, rather it activated one that we didn't know about prior to the seismic activity."

They used template matching to sift through seismic data recorded by the Earthscope Transportable Array, a network of seismic stations, looking for repeating signals similar to the known Poland Township earthquakes, which were treated like seismic "fingerprints." They identified 77 earthquakes with magnitudes from 1.0 and 3.0 that occurred between March 4 and 12 in the Poland Township area. The local community reported feeling only one earthquake, the magnitude 3.0, on March 10.

They then compared the identified earthquakes to well stimulation reports released in August 2014 by the ODNR and found the earthquakes coincided temporally and spatially with hydraulic fracturing at specific stages of the stimulation. The seismic activity outlined a roughly vertical, east-west oriented fault within one kilometer of the well.

Industry activities at other nearby wells produced no seismicity, suggesting to the authors that the fault is limited in extent. 

"Because earthquakes were identified at only the northeastern extent of the operation, it appears that a relatively small portion of the operation is responsible for the events," said Skoumal, who suggests the template matching technique offers a cost-effective and reliable means to monitor seismicity induced by hydraulic fracturing operations.

"We just don't know where all the faults are located," said Skoumal. "It makes sense to have close cooperation among government, industry and the scientific community as hydraulic fracturing operations expand in areas where there's the potential for unknown pre-existing faults."

Article: "Earthquakes Induced by Hydraulic fracturing in Poland Township, Ohio," Jan. 6, 2015 Bulletin of the Seismological Society of America.