Fracking — the hydraulic fracturing of rock by the injection of water at high pressure, to release shale gas — is a hot topic these days. One of the areas which creates most concern is whether, and to what extent, it causes, or increases the likelihood of, earthquakes.
Man-made earthquakes are nothing new. We’ve long known that activities such as mining and quarrying produce seismic events, some of which are even separately identified on the United States Geological Survey’s real time earthquake maps.
In the central and southern United States, particularly Oklahoma, there’s also evidence that human activity is increasing seismicity, with the USGS noting that: “The increase in seismicity has been found to coincide with the injection of wastewater in deep disposal wells in several locations, including Colorado, Texas, Arkansas, Oklahoma and Ohio. Much of this wastewater is a byproduct of oil and gas production and is routinely disposed of by injection into wells specifically designed and approved for this purpose.”
But that’s wastewater injection and it’s important to get the terminology correct. They go on to point out that: “Hydraulic fracturing, commonly known as ‘fracking,’ does not appear to be linked to the increased rate of magnitude 3 and larger earthquakes.”
Studying the Earthquake Cluster in Poland Township
A new study by a team of seismologists includes an assessment of a recent cluster of small earthquakes (up to magnitude ~3) in Poland Township, Ohio. Like much of the central and eastern US, the area is generally tectonically stable and most seismic activity is associated with movement along existing faults.
Such seismic activity can reach significant magnitudes and Ohio’s largest recorded earthquake, which predated modern fracking methods, was a tremor of M5.4 in 1937.
In March 2014, a series of small earth tremors (M2.1-M3.0) occurred in Poland Township, Ohio, close to an area in which fracking operations were taking place. As a result, the Ohio Department of Natural Resources (ODNR) commissioned an academic study to determine whether there were links between the two.
The study used a technique known as template-matching, a statistical methodology which matches the expected pattern of events with an observed pattern.
As the introduction to the study notes:
“If it can be demonstrated that the Poland Township earthquakes were induced by hydraulic fracturing, the ML 3.0 event in the sequence would be one of the largest earthquakes directly linked to the process.”
Study Findings: Linking Fracking and Low-Level Seismicity
The results of the study indicated that the observed seismicity was not the result of fracking rupturing the rock directly but rather of the reactivation of existing faults in the underlying (basement) rock as a result of the fracking process.
“The earthquakes occurred in the upper Precambrian basement, a very old rock that we know is highly fractured,” lead author Rob Skoumal told Decoded Science.
“The hydraulic fracturing did not create a fault, but rather it reactivated a pre-existing one in the Precambrian basement that we did not know about.”
“The Poland Township sequence included a magnitude 3 event. While this is a relatively small earthquake, it is one of the largest documented earthquakes that has been induced by hydraulic fracturing in the United States.”
Looking Forward: Applying the Study Results
The significance of this research lies in how it might be applied in future to reducing the likelihood of seismicity associated with both fracking and wastewater injection by identifying the mechanisms which induce earthquakes — although as Rob Skoumal points out, there’s no evidence of any wrongdoing by the operators.
Skoumal says, “In the Poland Township case, it appears that only a small portion of the operation was responsible for the earthquakes — this has also been observed in other cases of earthquakes induced by hydraulic fracturing.”
So what’s the risk? Well, in terms of fracking (as opposed to wastewater injection, which has been blamed for larger seismic events than those recorded in Poland Township) it’s very low. While many environmental groups are concerned about the possible increase in seismicity, Skoumal is at pains to lay them to rest.
“It is unlikely that a larger earthquake will be induced by hydraulic fracturing in Ohio in the near future,” he says, pointing out that: “The ODNR has been very proactive in their attempts to reduce the likelihood of induced earthquakes. Since the magnitude 4 Youngstown earthquake in 2011 , the induced earthquakes in Ohio have all been small. As the government, industry, and academia continue to work together on this issue, the likelihood of induced earthquakes in Ohio could decrease.”