This is how seismic detectives work when they investigate unexplained earthquakes

Fear takes hold of the TV host just when her TV studio starts shaking.

In the middle of a monologue, he stops and remains silent. The movement increases. Other presenters sitting around remain motionless and exchange worried looks.

Then, the tremor becomes stronger. You hear the sound of the lights above them. We look up to see. The presenter is panting. It's time to go.

They hastily remove the microphones and leave the entire South Korean television which retransmits live while the seismic waves of one
5.5 magnitude earthquake shake Pohang.

It was a powerful shake. Images from other places show people who are exhausted because of the collapsing walls.

An entire city of half a million residents was shocked. But this earthquake was not a natural phenomenon, it was triggered by human activities.

This is the conclusion of a report published in March by a team of experts who tried to determine the cause of the event that occurred in Pohang on November 15, 2017, with a balance of 135 people injured.

Thousands of buildings were damaged at a cost of $ 60 million.

To find out whether industrial activity caused the earthquake, South Koreans turned to a new generation of seismologists: seismic detectives.

They are responsible for badyzing seismic records and industry data to determine whether the tremor was natural or not.

The perforations

With more and more drilling and operations
fracking around the world, artificial (or anthropogenic) earthquakes have become
in a growing concern.

Each year about 100,000 oil wells are drilled and the use of geothermal energy, which sometimes involves injecting fluid into a hot rock to generate steam, could be multiplied by six from here 2050.

By removing large amounts of fossil fuels or flooding the fractured rock with liquid, it is possible to alter the balance of the underground stresses and cause tremor.

Although "on dry land" is a well-known saying, on a geological scale, things under our feet are never calm.

It is full of changing material levels with varying densities. There are defects and fractures, often crossed by fringes of liquid. There is sediment, clays and rocks in the background.

Without speaking, on an even larger scale, gigantic tectonic plates that rub or separate. In some places, the floor looks like a brick tower waiting to be knocked over.

Bill Ellsworth of the Center for Induced and Unbalanced Seismicity at Stanford University, California, remembers the first time he saw images of people fleeing buildings when the Pohang earthquake struck the city. city.

"They were very lucky that no one died after seeing some pictures of the security camera," he said. Ellsworth, who was part of the international team that investigated what happened in South Korea.

The earthquakes are measured on the Richter scale, which is "logarithmic", which means that an increase of one point is an increase of 10 times the force.

Residents would experience an earthquake with a magnitude of nearly 3 on the Richter scale, and an earthquake out of 4 would be enough to drop objects from the shelves.

An event of magnitude 5.5 or higher caused by human activity is very rare and, even if it is still considered moderate, it would be enough to damage the buildings.

An error that touched the earth

The day after the Pohang earthquake,
NexGeo, the company that operates the experimental geothermal power plant, denied any responsibility for what happened.

But as Ellsworth and his team began to find the evidence, something different was heard. He and his colleagues evaluated the region's seismic data, as well as information provided by NexGeo, which cooperated with the research, on drilling activities.

Geothermal power plants use the heat of the ground to produce electricity.

There are many ways to do it. Some, for example, use steam released directly from geothermal deposits. In other cases, the rock may be hot but there is not enough liquid to bring the heat to the surface in the form of steam. To break the rock and release this heat, NexGeo has planned to inject fluid into the ground.

Before reaching this stage, they had to drill deep into the ground.

It is during this process that things went wrong.

"For some reason, there was a path that allowed the fluid to escape from the well," says Ellsworth. By injecting more fluid, the drillers sealed their well, but the huge pressure that resulted caused what no one wanted: seismicity.

"It triggered very small events, events so small that they were not noticed at the time," Ellsworth said.

The borehole had actually crossed a fault line, an underground boundary where two planes of earth met. The movement of the earth can occur along these faults. This is what causes earthquakes.

Ideally, fault lines in areas subject to drilling or fluid injection are known and generally avoided.

In this case, in part because there was no indication of a fault line on the surface, the South Korean team had no idea what it had drilled. As Ellsworth points out: "It was very unfortunate."

Validated by the government

The data collected by Ellsworth and his colleagues convinced them that the event was due to human activity.

Although the results have been debated, their conclusions have already been accepted by the South Korean government, which announces the dismantling of the geothermal power plant.

Could the platform have noticed the initial seismicity and stop drilling just in time?

It's possible, says Ellsworth, but they relied on a relatively simple traffic light system to help them judge the safety of drilling.

This involves monitoring seismicity and stopping drilling if a certain magnitude of the earthquake is reached.

Listening to the ground and interpreting the various problems that occur is not an easy task.

How come?

At 5,500 km away, in southern England, a scientist was involved in another case of seismic detectives, this time with an oil drilling operation located in the Surrey field.

Stephen Hicks shows a big black box near a huge solar panel installed in a field. "We have five in the region," he says.

Hicks is a seismologist at the Imperial College of London. He was charged with a local investigation to find the cause of a series of small earthquakes in the area. This is not something that Surrey is used to doing.

But on February 27, the city was shaken by an earthquake of magnitude 3.1 in the early hours of the morning.

It was the strongest so far and, although it was not very serious, it was an unusual event. The UK only has two or three of these tremors a year.

Because a company called
UK Oil and Gas (UKOG) mines oil nearby, many fear that this activity will disrupt ancient faults and cause earthquakes.

There was a series of demonstrations at the drilling site. Many have turned to scientists to see if they can prove what is really happening, which is Hicks' work.

"That's what we call the digitizer," he says, eagerly pointing to a small box inside the black drawer.

"It just converts the badog signal into digital and we can convert it into speed, meters per second or acceleration."

After touching the ground, he shows me the huge peaks that appear a few minutes later on the near-real-time tremor map that can be viewed online.

Having several instruments on the ground means that accidental noise, for example pbading vehicles (or hitting scientists), can be ruled out.

It is only when the tremors appear uniformly on several seismic monitors that it indicates that there is a tremor.

Since the summer of 2018, Hicks and his colleagues monitor the signals. But unlike Pohang, nothing seems unusual here.

Almost every 90 or more tremors that Hicks has detected in the past eight months are small, less than 1 magnitude.

And they occur at a relatively shallow depth, about 2.5 km, but not as shallow as a borehole at about 700 or 800 meters.

In addition, earthquakes have occurred around the area, known as the Weald Basin, not clustered near the drill site.

"We think it's a coincidence," he says.

And although he does not believe they are man-made, they are still interesting, because earthquakes like this, at such a shallow depth, usually do not register such a high resolution in the realm -United.

"Whatever the cause, the sequence is interesting," he says.

Jackie Wilson, near the epicenter of the largest earthquake, explains that "someone has come to ask those of us who are here to sign against all of these ongoing drilling," adds -t it.

Hicks has talked with people about what's going on, but for now, he's coming to his conclusion that earthquakes are natural.

Extended around the world

Public concern that tremors can be caused by humans is becoming more common around the world. Especially when a geothermal or drilling activity occurs in the same place as tremors.

People are clearly getting used to the concept of anthropogenic seismicity, which can be induced by humans or activated.

The latter is a slightly different condition in which earthquakes are caused mainly by tectonic activity, while human activity plays a role in the way they occur.

It is natural that there are concerns, says Francesco Grigoli in
ETH Zurich.

The expert has studied the tools available for seismic detectives when they are trying to discover the cause of seismic disturbances.

"There is no standard recipe for identifying an event, nor for excluding an event," he says.

He and his colleagues studied the Pohang earthquake. He points out that more open data can make all the difference in deciding whether an earthquake is anthropogenic or not.

If they are motivated to do so, drillers can now use extremely sensitive listening equipment, able to identify "every last accident," says James Verdon of Bristol University.

"This gives us thousands, if not hundreds, of thousands of data points with which to conduct a much more detailed badessment of the seismic threat," he says.

Seismic detectives can, in theory, help badess the situation during drilling, not just after the fact, and activate the alarm if drilling becomes dangerous.

By listening carefully to what the ground says, businesses and governments may be better equipped to respond before it is too late.

IN ADDITION