Is there time underground?
The subsurface does not offer favorable conditions everywhere
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But even if the computer hits the brakes in good time, the danger is not over. After the earth shook in Basel, the pumps were immediately switched off and the borehole opened to let the water pressure escape. Nevertheless, there were repeated smaller earthquakes in the following two months, three of which had a magnitude of 3.0 or more.
Even two years later, the underground has not yet completely settled, the sensors show. Relatively strong tremors were also registered elsewhere after the pumps were switched off, reports Deichmann. "That doesn't fit the picture of seismology," he admits. If there is no water pressure, the shaking should stop. "This may be due to chemical processes in which the water changes the rock over the long term, reducing its strength." As soon as the rock is weaker than the natural tension that still exists, it breaks - the seismographs twitch.
Deep geothermal energy can also run quieter when the natural conditions are better. For example at the test facility of the Potsdam Geo Research Center (GFZ), which is located in Groß Schönebeck near Berlin. "We pumped twice as much water per unit of time as our colleagues in Basel," reports project manager Ernst Huenges. "But we only had tremors with a magnitude of -1, which is less than a ten-thousandth of the Basel quake."
There are two reasons for this, explains the scientist. On the one hand, the natural tension in the rocks of the north German subsurface is much lower and the rock fractures that are generated are generally smaller. Second, the Potsdamers drilled into a porous sandstone and not a dense granite like in Basel. "The sandstone is more flexible," says Huenges. "If there is a quake, it can better absorb the released energy." Because the sandstone is full of holes like a sponge, it can let more water through anyway, the artificial breaking up of the rock is only necessary to a small extent.
But the subsoil does not offer such favorable conditions everywhere. "In certain places it is better not to start geothermal projects that involve artificially breaking up the rock," says the GFZ scientist. This includes those areas that are under great natural tension, such as around Istanbul. In those regions that are somewhere between "very safe" and "fairly endangered", you have to feel your way gradually, he says.
In principle, however, the geoscientist considers the process to be manageable. "You can calculate exactly what maximum earthquake strength can be triggered with a limited amount of water in seismically calm zones." But that's only one side.
Especially in areas that are already under strong tension, small artificial earthquakes in the immediate vicinity can trigger far stronger tremors that would naturally have occurred at some point in the next few decades. The researchers call this chain reaction "triggering". "We can't predict that," says Huenges.
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