The damage was modest: A number of ostrich eggs fell off a table in the quake and were destroyed, and beyond that, hundreds of cracks in building walls, but not all of them likely attributable to the quake. Still, the people of the Swiss border city were alarmed. After all, this earthquake was caused by human activity.

“The Basel area has favorable conditions for generating geothermal energy,” says Professor Serge Shapiro of the Institute of Geological Sciences at Freie Universität. Shapiro is in charge of a project entitled PHASE (Physics and Application of Seismic Emissions), which began in 2004. In the project, he and his Applied and Earthquake Seismology Research Group are studying the processes behind minor earthquakes like the one in Basel that can be observed when human activity causes changes in deeper layers of the Earth’s crust. “The Earth’s crust is very hot in the area around Basel,” Shapiro explains. “The temperature is 200 degrees Celsius there at a depth of 5,000 meters.”

Through a method known as “deep heat mining,” a plan was launched to use this heat to supply about 5,000 households in Basel with electricity and hot water in the future. The goal was to drill two boreholes down into the hot layers of rock, then feed cold water into one of the holes and pump it back up to the surface through the second one, after the water had absorbed the heat from the Earth.

“But Basel is on hard granite. To be able to circulate the water, it was necessary to split these masses of stone,” Shapiro explains. Starting in late October 2006, the operator of the project fed huge volumes of water down into the ground through the first borehole that was completed. Every minute, 3,500 liters of water – some 20 bathtubs’ worth – flowed into the ground at high pressure. These kinds of masses of water exert tremendous pressure on the rock, creating cracks.

“Quakes are possible anywhere people change the natural geology”

On December 8, 2006, just a month and a half later, the pressure had built to the point that it was too high. By then, 12,000 cubic meters of water had been pumped into the ground. For comparison, the Teufelssee lake in Berlin’s Grunewald district has a volume of about 72,000 cubic meters of water. The accumulated pressure was released as a quake.

“Quakes like this are possible anywhere that people change the natural geology,” the geophysicist explains. Earthquakes similar in magnitude to the one felt in Basel have been observed around the Groningen gas fields, in the Netherlands, since the 1990s. In Koyna, India, a dam caused an earthquake that killed about 200 people in 1967. And in November 2017, a magnitude 5.4 quake in the South Korean city of Pohang left almost 100 people injured by falling rubble, and many homes have been uninhabitable since then. A geothermal energy project is suspected of being the cause there as well.

Lisa Johann, a research associate in Shapiro’s research group, recently analyzed data from Oklahoma to better understand the processes taking place inside the Earth. Oil has long been produced in quantity there, on the U.S. Great Plains. “In modern fracking methods, water is pumped into the ground to expand the spaces in the rock, thereby releasing gases and oil. But the water that was naturally bound in the rock is also carried along, so huge amounts of process water are used at production sites.”

In Oklahoma, geologists have identified layers of rock located about a kilometer below the surface that are highly porous, so they absorb the excess water like a sponge; it seeps down based on its own weight rather than having to be pumped into the ground, which uses energy. The water is pumped up through several hundred wells and then fed back into the ground over an area of more than 1,000 square kilometers. This human intervention had no effect at first.

“Since 2009, however, there have been as many as 900 perceptible earthquakes per year measured in the region,” Johann says. She analyzed the official figures published by the United States Geological Survey (USGS) and Stanford University, with which there has been lively scientific exchange as part of this project. The data from oil and gas companies on water fed into the ground are also being used.

“The first thing that struck me was that the number of earthquakes has fallen since Oklahoma put a cap on the volume of water than can be put into the ground,” says Johann. “At the same time, we have also found that the area of seismic unrest has expanded, so in some places, there are shocks located at increasingly long distances from injection points.”

She has developed a model that could explain what is going on. In simplified terms, the water injected into the ground exerts pressure on the layers of rock deeper down. The more water is pumped into the ground, the higher the pressure, until the point that the strain that has accumulated in the ground due to tectonic processes is released in the form of a quake. But the water injected doesn’t only have this vertical effect on the layers of rock. The water pressure also spreads horizontally, over increasing distances. This would explain why shocks are also felt in places located far away from the wells. The study showed astonish parallels with the data on earthquakes that have occurred in connection with dam projects.

“The physics is the same, whether the water is collected on the Earth’s surface or an underground reservoir is created,” says Shapiro, who is supervising Johann’s dissertation. He hopes to expand the model now, thereby explaining the quakes’ spread to larger areas. “We also hope our research will help take the concerns of people in areas where geothermal facilities are planned into account. Examples include the towns of Landau and Insheim, in the German state of Rhineland-Palatinate, where the Earth shook near the geothermal energy plants in 2009,” he explains. “We already have a better understanding of the processes that led to these incidents. Now the question is how we can also get the risk of quakes better under control.”

---

This text originally appeared in German on December 15, 2019, in the Tagesspiegel newspaper supplement published by Freie Universität.