Fracking in Ohio confirmed as cause of rare earthquake strong enough to be felt

A new study links the March 2014 earthquakes in Poland Township, Ohio to hydraulic fracturing that activated a previously unknown fault. The induced seismic sequence included a rare felt earthquake of magnitude 3.0, according to research published online by the Bulletin of the Seismological Society of America (BSSA).

In March 2014, a series of five recorded earthquakes, ranging from magnitude 2.1 to 3.0, occurred within one kilometer (0.6 miles) of 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 10, 2014.

Hydraulic fracturing, or fracking, is a method for extracting 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. The process of fracturing the rocks normally results in micro-earthquakes much smaller than humans can feel.

It remains rare for hydraulic fracturing to cause larger earthquakes that are felt by humans. However, due to seismic monitoring advances and the increasing popularity of hydraulic fracturing to recover hydrocarbons, 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 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."

Using a technique called template matching, the researchers sifted 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.

Skoumal and his colleagues 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."

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2011 Japan earthquake: Fault had been relieving stress at accelerating rate for years

Stanford scientists have found evidence that sections of the fault responsible for the 9.0 magnitude Tohoku earthquake that devastated northern Japan in 2011 were relieving seismic stress at a gradually accelerating rate for years before the quake.

This "decoupling" process, in which the edges of two tectonic plates that are frictionally locked together slowly became unstuck, transferred stress to adjacent sections that were still locked. As a result, the quake, which was the most powerful ever recorded to hit Japan, may have occurred earlier than it might have otherwise, said Andreas Mavrommatis, a graduate student in Stanford's School of Earth Sciences.

Mavrommatis and his advisor, Paul Segall, a professor of geophysics at Stanford, reached their conclusions after analyzing 15 years' worth of GPS measurements from the Japanese island of Honshu. Their results were published earlier this year in the journal Geophysical Research Letters.

"We looked at northeastern Japan, which has one of the densest and longest running high-precision GPS networks in the world," Mavrommatis said.

Segall said, "The measurements indicated the plate boundary was gradually becoming less locked over time. That was surprising."

The scientists will present their work, "Decadal-Scale Decoupling of the Japan Trench Prior to the 2011 Tohoku-Oki Earthquake from Geodetic and Repeating-Earthquake Observations," Dec. 17 at the American Geophysical Union's Fall Meeting in San Francisco. The talk will take place at 5 p.m. PT at the Moscone Convention Center in Moscone South, Room 306.

The pair's hypothesis is further supported by a recent analysis they conducted of so-called repeating earthquakes offshore of northern Honshu. The small quakes, which were typically magnitude 3 or 4, occurred along the entire length of the fault line, but each one occurred at the same spot every few years. Furthermore, many of them were repeating not at a constant but an accelerating rate, the scientists found. This acceleration would be expected if the fault were becoming less locked over time, Mavrommatis said, because the decoupling process would have relieved pent-up stress along some sections of the fault but increased stress on adjacent sections.

"According to our model, the decoupling process would have had the effect of adding stress to the section of the fault that nucleated the Tohoku quake," Segall said. "We suspect this could have accelerated the occurrence of the earthquake."

The scientists caution that their results cannot be used to predict the occurrence of the next major earthquake in Japan, but it could shed light on the physical processes that operate on faults that generate the world's largest quakes.

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