Fracking is an issue – a big issue.
The increasing pressure on finding alternative – and preferably clean – sources of energy has led to an expansion of the process of hydraulic fracturing, in which gas trapped in sedimentary rocks is released by means of water injected into the rocks, opening up fractures and allowing trapped gas to escape.
Opponents have raised concerns about many aspects of this particular approach to energy exploitation, with key concerns including pollution of both surface and groundwater, the release of toxic chemicals into the environment – and the possibility of earthquakes.
On the latter point, at least, there seems to be general agreement; the relationship between quakes and the extraction of oil and gas is an accepted one, as studies of tremors in Oklahoma and in Blackpool, in the United Kingdom, have demonstrated.
But it’s worth taking a closer look at why and how these tremors occur – as well as the scale of the associated risks.
How Does Fracking Create Earthquakes?
It goes without saying that an earthquake is a natural phenomenon, caused when movement occurs along a fault in the earth’s crust. This movement is usually the result of major earth forces of compression and extension. But there’s also a phenomenon known as induced seismicity, whereby human activity alters the stresses on a fault and causes it to reactivate.
In fracking, the use of water is the key to inducing earthquakes – increased water injection increases pore pressure in the rock and effectively weakens it. Such an impact is more significant at deeper levels where the rock is stronger and can accumulate greater amounts of strain before it breaks.
According to the British Geological Survey’s Brian Baptie, with induced earthquakes – just as in natural ones – the risk of a tremor is higher in areas where the rock is already weakened. “In fracture and fault zones the water lubricates the faults and brings them closer to failure, which is what happened in Blackpool,” he explained. “The incidence of earthquakes depends on structure rather than rock type.”
“The fracking process involves two different elements,” explained Dr Baptie. “There’s fracking itself, where the water is pumped into the rock at relatively shallow levels and then out again. And there’s the disposal of contaminated wastewater, when it’s pumped into the rock at a deeper level (usually around 2.5km). Most of the fracking-related earthquakes come from this second stage.”
How Big Can an Induced Earthquake Be?
The magnitude of an earthquake depends on the strength of the rock. The biggest earthquake in recorded history is the magnitude 9.5 (M9.5) earthquake which struck off Chile in 1960. Like most other major earthquakes, this occurred at a subduction zone and was entirely natural.
Induced earthquakes, though they may be significant in their local tectonic context, do not compare with these ‘megathrust’ earthquakes, where massive strain builds up over enormous distances along faults fault.
In Oklahoma, the largest tremor in the state was the M5.6 which struck Prague in 2011. Although this seismic event may not have been directly fracking-induced, the USGS notes that research “suggests that a magnitude 5.0 foreshock to the 2011 Prague, Okla., earthquake was human-induced by fluid injection; that earthquake may have then triggered the mainshock and its aftershocks.”
Most earthquakes we know to be fracking-induced are smaller than this, however.
To place this in context, it’s important to remember than the earthquake magnitude scale is logarithmic – an earthquake of M5 is ten times bigger than an M4 and one hundred times the size of an M3 and so on. This means that the magnitude difference of 3.5 between Oklahoma in 2011 and Chile in 1960 actually represents a difference in size of almost 8000 times – and in terms of energy released, the Chile ‘quake was a mind-boggling 707,000 time larger than the Oklahoma tremor.
Induced Seismicity: Other Culprits?
Fracking isn’t the only process which causes induced seismicity. Many methods of energy exploitation involve altering the strains on rocks with consequent earthquake activity.
Mining has long been a culprit – Dr Baptie pointed out that there have been several mining earthquakes in the UK of up to ~M3, and larger ones have been recorded elsewhere (up to ~M5.6 in South Africa).
Even geothermal energy, which is regarded as ‘green’, involves injection of water in to rock and can cause rock to fracture or faults to slip; while processes such as research (drilling boreholes) or even building dams can also increase the strain upon bedrock.
A recent paper from Durham University (April 2013) summarises the current position as follows: “The greatest magnitudes of induced seismicity recorded are as follows: reservoir impoundment (max. M=7.9), oil and gas field depletion (7.3), mining (5.6), underground waste disposal (5.3), water injection into oil fields (5.1), geothermal energy generation (4.6), shale gas hydraulic fracturing (3.8), and seismicity research boreholes (3.1).”
Put in these terms, then, the seismic hazard issue relating to fracking is relatively limited.
Hazard Mitigation: What Can We Do?
But hazard can be mitigated, and Dr Baptie suggested three possible approaches by which we can tackle the hazards of fracking.
“Given the clear relationship between injected fluid volume and seismicity – less water injected leads to smaller earthquakes,” he said, “so using less water is one mitigating factor. Alternatively, it might be possible to keep the water flowing for a longer period and this reduces the pore pressure and therefore the possibility/magnitude of an earthquake.” And of course there’s monitoring: “If seismic activity, and if it exceeds an acceptable level, change what you’re doing or stop altogether.”
Not all Fracking Induces Earthquakes
What we should remember, however, is that not all fracking induces earthquakes – and, overall, the seismic hazard is low even though the chance of small induced earthquakes is high.
And fracking isn’t the only source of earthquakes. It appears, then, that if we want to tap into certain energy sources we must accept the seismic hazard that goes with them, which in most cases is relatively low.
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