Geologist André Niemeijer of Utrecht University had not expected it to be as severe as the earthquake that woke up a large part of Groningen on Thursday night. Certainly not more than a year after gas extraction has stopped.
Last summer he was in NRC still be tempted to make a cautious prediction. “I cannot imagine that we will have another bigger earthquake,” Niemeijer said at the time. “But it is difficult to make a statement about that: if I say that now and tomorrow there is a bigger earthquake, I was wrong. Because that chance remains.”
On Thursday night it became once again clear how difficult earthquakes are to predict, says Niemeijer, the morning after the third strongest earthquake ever recorded in Groningen. “It is of course not the largest, but this is still surprising. I don’t think there is a scientific model that would have predicted such a large earthquake. We have stopped removing gas for a while, so the driving force for earthquakes is actually not increasing.”
KNMI
Although the size of this earthquake was surprising, it was known that earthquakes would continue to occur for the time being – even after the gas tap was turned off. “The number of earthquakes is decreasing and so are the heavier ones, but we knew that the risk of these types of heavy earthquakes would continue to exist,” says seismologist Läslo Evers of the KNMI. “The KNMI has always argued that.”
Decades of gas extraction have caused the pressure in sandstone layers to drop, causing the layers to collapse. The resulting tension on the more than a thousand fault lines in Groningen can be discharged with a shock, like an earthquake. But even the most advanced models cannot accurately predict how big such a quake is and when exactly it will occur.
If an aftershock is larger than the main shock, it becomes the main shock
For the time being, there is a chance of aftershocks. More than five hours after the first shock with a magnitude of 3.4, the KNMI measuring stations registered the first aftershock. “It had a magnitude of 2.1,” says Evers. “That is clearly noticeable and can lead to damage. In general, an earthquake can be felt roughly from magnitude 1.5.” Aftershocks are less severe than the original shock, but even that does not provide any guarantees, Evers emphasizes. “It is a matter of definition: if an aftershock is larger than the mainshock, then that becomes the mainshock and the previous shock then becomes a foreshock.”
Evers received a call from colleagues on Thursday night after the first quake, as is usual with larger earthquakes. The KNMI has approximately seismological measuring stations in Groningen, which transmit the smallest vibrations directly. Institute employees monitor these signals around the clock and a relatively heavy earthquake like Friday night also triggers an automatic notification. “Then the seismologist on duty will do the further analysis.”
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Devastating
It is then too late to warn residents or take measures. In countries such as China and Japan, seismologists can sometimes provide early warning, minutes before the devastating major shock. “That could be just enough to stop a high-speed train and shut down critical infrastructure,” says Niemeijer. “People can then look for a safe place.”
In Groningen, those kinds of warnings are impossible, “because the energy travels across the entire region in a few seconds,” says Evers.
Geologist André Niemeijer said this summer that he could not imagine that such a large earthquake would follow
Compared to natural earthquakes abroad, those in Groningen are small. However, the damage can be extensive because they occur closer to the surface. Quakes in Groningen occur at a depth of approximately three kilometers, while natural earthquakes are much deeper.
“Because this earthquake has a magnitude of 3.4 at a depth of three kilometers, the ground movement is greater than if it were a 3.4 at a depth of, for example, ten kilometers,” says Niemeijer. “And the most important factor for the damage is not the force, but the ground movement.”
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