Fracked off

DAVID HAMBLING reports on the inconvenient truth that human activity can have seismic effects

In November, the British government brought fracking operations in the

UK to an abrupt halt. Previously, any notion that fracking, or hydraulic fracturing to get oil or gas might produce a rash of earthquakes looked like conspiracy theory. Fracking had been widely practised in the US for years. Then came a report revealing that fracking really was liable to produce earthquakes whose magnitude could not be predicted; the danger that had seemed incredible was real and the government U-turned.

To the ancient Greeks, earthquakes were caused by the shifting of the turtle that carried the Earth, or the elephants supporting the turtle, or perhaps the movement of underground dragons or other giant beasts. We now know that the Earth’s surface is a jigsaw of tectonic plates, each a slab of rock hundreds of miles across. Pressure builds up in the fault lines between these plates, and when a section of rock slips the result is an earthquake. It was always assumed that the forces involved were too gigantic for ant-like humans to have any effect.

When large reservoirs were built though, seismic activity nearby seemed to increase. This was suspected with the Hoover Dam on the Colorado River in the US in the late 1930s. The randomness of the quakes meant that a causal connection was hard to prove, but after a worldwide spate of dam-building in the 1960s, the evidence was piling up. Earthquakes of greater than magnitude 6 – powerful enough to cause severe damage in densely populated areas – occurred near new reservoirs at Kremasta in Greece, Hsinfengkiang in China and Kariba in Zambia. The most destructive happened at Kyona in India in December 1967 when 200 people died and thousands were made homeless. Kyona in particular was a red flag. There were five to six earthquakes a year within 25km (16 miles) of the dam after it was built, but no other seismic activity in a 100km (64-mile) radius.

This attracted plenty of scientific attention, and in 1970 two researchers from the University of Alberta showed that the increased seismic activity was caused by ‘incremental loading’. Damming a river and producing a lake where there had been none before adds millions of tons to the pressure on the bedrock, the final straw making it more likely for tectonic plates already under pressure to slip. The rate of increase of the

The randomness of the quakes meant that a causal connection was hard to prove

water level, and the duration of high water, affected the frequency and magnitude of earthquakes. Later, more nuanced models showed that increasing the ‘pore pressure’, the pressure of the groundwater, could gradually weaken the underlying rock and make slippage more likely.

This was an inconvenient truth for anyone wanting to build a reservoir, which may have contributed to the time taken to officially recognise the danger. Reservoir-induced seismicity or RIS is now an established issue, and areas are surveyed for risk before a new reservoir can be built. Later research found that large oil wells also affected seismic activity. Pumping out millions of tons of oil and replacing it with water affected both the load and the pore pressure, sometimes leading to more earthquakes.

Given this background, it was not surprising that critics were worried by the possible effects of fracking for shale gas. This involves pumping a mixture of water and sand laced with chemicals underground at high pressure to crack the bedrock, allowing deposits of gas, or sometimes oil, to seep out. The scale is tiny compared to a reservoir and Cuadrilla, the company involved in fracking tests in the UK, suggested any earth tremors would be barely noticeable.“Local people should be reassured that any resulting ground motion will be far below anything that could cause harm or damage and is likely to be much less than caused by other industries such as quarrying or construction or even heavy goods vehicles travelling on our roads,” states the company’s websites. They claimed most activity would only be detectable thanks to special seismometers set in Lancashire where the tests were taking place.

In 2011, fracking in England was temporarily halted after a magnitude 1.1 tremor. Things really ratcheted up a level with a 2.9 quake in August 2019. “The walls of my house shook,” a local resident in Lytham St Anne’s told the BBC. “For a moment, I really thought my house was going to fall down.”

Cuadrilla pointed out that the event only lasted a second and the shaking was below the level permitted for construction work. However, a subsequent report in November 2019 by the independent Oil and Gas Authority concluded that it was not possible to predict the size of tremors caused by fracking. This was enough for the government to drop its support of what was originally seen as a “huge opportunity” for the UK, and impose an indefinite fracking ban. This may look like an overreaction. The US has seen widespread fracking for years, and they have not suffered an epidemic of earthquakes. Or have they?

From 1973 to 2008, the US experienced an average of 24 quakes of magnitude 3 and above each year. From 2009 to 2014, the number shot up to 193 a year, an eightfold increase. The US Geological Survey now accepts that fracking is occasionally associated with earthquakes, and in 2019 the Seismological Society of America identified some 600 small quakes that could be tied to fracking.

The US is far more sparsely populated than the UK, with fracking taking place well away from population centres. While there has been no shortage of local protests, there are not enough votes to change matters. The current administration is keen to make the most of American oil and gas. Meanwhile, further research is under way to find out more about the interaction between seismic activity and fracking, and whether the effects can be predicted. The key question is of course whether it can ever be truly safe. Mother Nature is someone you really don’t want to irritate.