Last updated March 1, 2018 at 4:35 pm
It’s all a question of how far you stay away from faults – and the answer is remarkably specific.
From the start of this century, something odd started happening in the United States. In the middle of the North American continent, large areas of land long considered to be geologically stable began to experience earthquakes.
Most were small enough to be missed by residents and were recorded only by seismologists. Others, however, came it at greater than five on the Richter scale and caused significant damage. In 2014, Oklahoma recorded more earthquakes than the notoriously shaky state of California.
Although initially puzzling, the cause of the sudden upswing in seismic events was eventually pinpointed by researchers.
“To a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production,” concluded a team in 2015, led by researchers at the US Geological Survey and writing in the journal Science.
The link between fracking and earthquakes
The method to which the scientists referred is, of course, hydraulic fracturing, or fracking, a method of extracting hydrocarbon fuels from rock deposits through the high-pressure injection of water, sand and a cocktail of chemicals.
Since that study, the link between fracking and earthquakes has become commonly acknowledged, with papers recording its occurrence not only in the US, but also in the UK, China and Poland.
A 2016 study, again published in Science, looked at a cluster of earthquakes recorded in the Fox Creek area of Alberta in Canada, the site of fracking activities. The researchers found that the quakes occurred up to four months after a fracking event, and did not occur on a diminishing scale. (The largest in the cluster studied occurred several weeks after the main injection.)
The scientists, led by geologist Xuewei Bao of Canada’s University of Calgary, found that the pressure induced by injection of the fracturing material could diffuse through the target rock mass and impact on nearby fault-lines, thus triggering seismic movement.
This and other, similar, studies produced a single, important question: at what distance does the diffusion decrease to a negligible level and thereby not affect faults?
The answer, as it turns out, is a remarkably specific 895 metres.
That’s the finding reached by a team led by Miles Wilson of the UK’s University of Durham.
Wilson and colleagues were invited to address the question by an organisation called ReFINE, a collaboration between the universities of Durham and Newcastle launched in 2013 after a series of tremors in the coastal town of Blackpool led to a temporary moratorium on all fracking operations.
Faultlines and fracking
To make its finding, Wilson’s team used data from 109 fracking operations and measured the distance between the injection point and the closest micro-seismic event – a tiny tremor too small to be felt except by detectors.
The scientists found that the distances ranged between 59 and 720 metres. A 2017 study by ReFINE, using a different model, suggested that the minimum safe distance to avoid triggering seismic activity was 433 metres. However, the latest analysis found 12 real-world tremors beyond that boundary.
In a paper published in the journal Geomechanics and Geophysics for Geo-Energy and Geo-Resources, Wilson and colleagues calculate that chance of a fracking-induced quake at 433 metres at 32%. At 895 metres, the chance drops to just 1%.
Head of the ReFINE project, Richard Davies from Newcastle University, praises the latest work and urges regulators around the world to adopt the central finding.
“We strongly recommend that for the time being, fracking is not carried out where faults are within 895 metres of the fracked borehole to avoid the risk of fracking causing earthquakes and that this guideline is adopted world-wide,” he says.