The Mystery of ‘Unexplained’ Earthquakes

July 10, 2019

Fear seizes the TV presenter at the very moment her TV studio starts to shake. She stops, mid-monologue, and falls silent. The shaking worsens. Other presenters seated around the awkwardly large plastic table sit stock still, save for a few worried glances left and right. Then the shaking gets stronger. The rattling of equipment above them can be heard. One broadcaster turns his gaze upward to see. The main presenter gasps. It’s time to go.

As the South Korean live TV team hastily discarded body microphones and abandoned their set, the seismic ripples of a 5.5 magnitude earthquake continued to shudder across Pohang. It was a powerful jolting. Other footage shows people running from buildings as walls collapse behind them. An entire city of half a million residents was left in shock. But this quake wasn’t a freak natural event. It was started by people.

That’s the conclusion of a report published in March by a team of experts who tried to find out what caused the event in Pohang on 15 November 2017. It left 135 people injured and 1,700 had to be temporarily relocated to emergency housing. Thousands of buildings were damaged, costing $75m ($60m). Because a geothermal drilling project had been operational nearby at the time, a big question needed to be answered: Whodunit? Humans or nature? To find out if industrial activity had set off the quake, the South Koreans called on a new breed of seismologist: the earthquake detectives.

They are the ones tasked with combing through seismic records and industry data to see if the shaking was natural or not. It is not an easy thing to prove either way. But these scientists are now coming up with surer methods of identifying the culprit. They are forensics for the Earth.

With more drilling and fracking occurring around the world, human-induced or anthropogenic earthquakes have become an increasingly common concern. About 100,000 oil wells are now drilled every year and the use of geothermal energy, which sometimes involves injecting fluid into hot rock in order to create steam, could increase six-fold by 2050. By removing large quantities of fossil fuel or by flooding fractured rock with liquid, it’s possible to upset the balance of stresses below and set an earthquake in motion.

While we like to use metaphors like “on solid ground” in English, on a geological scale the stuff below our feet is anything but. It’s full of shifting planes of material with varying densities. There are faults and fractures, often with ribbons of fluid running through them. There are sediments, clays and bedrock. Not to mention, on an even bigger scale, gigantic tectonic plates rubbing against or pulling apart from one another. In some places, the ground is like a tower of toy bricks just waiting to topple.

Bill Ellsworth remembers the first time he saw images of people fleeing buildings as the Pohang earthquake rattled the city.

“They were very fortunate that no-one was killed, having seen some of the security cam footage,” he says. Ellsworth, from Stanford University’s Center for Induced and Triggered Seismicity, was part of the international team that investigated what happened.

The stakes for these investigators were high. They knew at the outset that to label Pohang a human-induced earthquake would be a big deal. Earthquakes are measured on the Richter scale – which is “logarithmic” – meaning an increase in one point signifies a 10-fold increase in strength. A quake of about 3 on the Richter scale would be felt by inhabitants, and 4 would be enough to knock objects off shelves. A 5.5 or higher magnitude event caused by human activity is very rare, and although it is still considered moderate, it would be enough to damage buildings.

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