Critics of fracking raise a myriad of concerns about its environmental repercussions, including methane emissions, water and air pollution. Perhaps no side effect attracts as much opposition to fracking, however, as the earthquakes sometimes set off by the practice or the disposal of related waste. When the U.K. government in November placed a moratorium on new fracking permits, it acted out of concern for associated quakes.

1. How does fracking cause earthquakes?

Hydraulic fracturing, as it’s properly known, blasts water, sand and chemicals at high pressure down a well more than a mile underground to crack rocks known as shale so that the oil or gas trapped inside can escape. Fracturing the rocks creates very tiny tremors, but these are imperceptible. The real danger is that if the shale is situated along a geological fault, the fractures can trigger a chain reaction that moves through the rock, causing two blocks of earth to suddenly slip past one another and evoke an earthquake. This is thought to occur more commonly in areas where the underground rocks are already under stress, creating a predisposition for a slip on a fault.

2. How does disposal of fracking waste cause quakes?

Earthquakes are more commonly caused not by fracking itself but by the disposal of dirty water that comes out of a well along with oil or gas. When this so-called produced water is injected back into the ground via another well, it can cause a pressure change that triggers an earthquake if geological conditions are right. This is a phenomenon in the oil and gas industry generally, but in the U.S., notably, it’s often associated with fracking because the practice has been largely responsible for a huge surge in output of the two fuels over the past decade.

3. How common are fracking-related earthquakes?

There’s no up-to-date repository of data, and it’s not always clear whether a quake is related to fracking. But in the U.S., where fracking is most prevalent, the number of earthquakes has risen sharply in the central longitudes of the country with the ascent of fracking in those areas. In the 36 years before 2009, that part of the country recorded an average 25 earthquakes annually with a magnitude of 3.0 or more. The yearly average since then has soared to 362. Fracking-related quakes have been recorded in other countries, including Canada, the only other nation to embrace the practice, as well as the U.K. and China.

4. How severe are the quakes?

Mostly, they’ve been so small they’re hard to notice, but some have been frightening and a number have caused damage. The disposal of fracking waste is suspected to have contributed to the worst earthquake on record in the oil-rich U.S. state of Oklahoma. The 5.8-magnitude tremor in 2016 shook homes and buildings across the state for almost a minute. A few months later, a 5.0-magnitude quake hit just outside the Oklahoma city of Cushing, home to the country’s biggest oil storage hub. Researchers say fracking operations probably caused a temblor in Sichuan Province, China’s shale hub, in December 2018 that injured 17 people and caused $7.5 million of property damage. In February, three quakes in the province killed two people and damaged nearly 11,000 homes.

5. How is the risk affecting fracking?

Before the U.K. issued its moratorium, frackers there had complained for years that regulations imposed after two industry-related tremors in 2011 were so strict, it was virtually impossible for them to operate. The so-called traffic light system required operators at the two U.K. wells probing shale formations to suspend injection of fluids anytime seismicity exceeded magnitude 0.5. By comparison, the ground movement felt directly beneath the “Big One” roller-coaster in nearby Blackpool is similar to an earthquake measuring 1.6, according to the University of Liverpool. The rule led to a stoppage of all fracking activity months before the government officially proscribed it.

6. What about outside the U.K.?

Earthquake concerns have contributed to the aversion to fracking that has produced bans or limits in countries including Germany, France, the Netherlands and Bulgaria -- as well as in a handful of Canadian provinces and in many U.S. states, counties and municipalities. Worry about quakes accounts for much of the opposition to a plan by the U.S. government to allow fracking on 1.2 million acres of public lands in California, a center of seismic activity. Still, fracking is spreading more widely, in Argentina, Australia, China and Saudi Arabia. The enormous amounts of water it requires are a limitation in many places.

7. Can the quake risks be mitigated?

Oklahoma’s experience suggests it can. Regulators there imposed a traffic-light system that’s more accommodating than the U.K.’s. Operators monitor seismic activity, and if it reaches a magnitude of 2.5 have to halt work at the well. State officials credit the system for reducing the number of seismic events in the state registering at least a 2.7 magnitude, roughly the lowest level humans can feel, from more than five a day in 2015 to fewer than one this year. Operators can also reduce quake risks by thoroughly mapping underground fault lines and avoiding fracking near them.

8. Are human-caused earthquakes something new?

Not at all. In a study published in 2017 covering the previous 150 years, researchers compiled a register of more than 700 sequences, often including multiple earthquakes, thought to have been provoked by human activity. Triggers fell into four categories: extraction of material from the subsurface of the Earth, such as mining (which accounted for 35% of cases); operations on the Earth’s surface, such as damming water (24%); explosions, such as those caused by underground nuclear tests (3%); and introducing material into the subsurface, as is done in fracking (3%).

--With assistance from Jeremy Hodges.

To contact the reporter on this story: David Wethe in Houston at dwethe@bloomberg.net

To contact the editors responsible for this story: Simon Casey at scasey4@bloomberg.net, Lisa Beyer

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