by Allison Mills Friday, October 23, 2015
“Modern Germany is not known for its earthquakes,” says Christian Brandes, a geoscientist at the University of Hannover in Germany. The country, after all, is in the middle of a tectonic plate, he says, away from any plate boundaries or other features that would cause tectonic strain to build up in underground faults.
Yet intraplate seismicity does happen: The 2011 magnitude-5.8 earthquake in Virginia was one example from North America. And Northern Europe has historically experienced sizable intraplate earthquakes as well, such as a 1612 earthquake along the Osning Fault in Germany that is estimated to have reached VI or VII on the Modified Mercalli Scale. In a recent study published in Geology, Brandes and his colleagues set out to find the mechanism of these mysterious European earthquakes.
The researchers looked at historical earthquakes recorded by people over the past 1,200 years as well as ones that occurred between about 20,000 and 12,000 years ago in the Quaternary Period. They found a common explanation for both groups of quakes: Brandes says the tremors were likely reverberations from the last ice age, when the Scandinavian Ice Sheet stretched south to North Central Europe before beginning to retreat about 20,000 years ago.
The immense weight of an ice sheet causes the underlying lithosphere to sink. When the glacial ice recedes and the weight is removed, the lithosphere slowly rises back up in a process called isostatic rebound. In the United States, this process is still happening along parts of the Eastern Seaboard and in the Great Lakes region following the retreat of the Laurentide Ice Sheet.
“If you load a ship, it lowers into the water and, if you unload it, the ship rises again. This is much like the lithosphere” when it comes to glacial loading and unloading, Brandes says. And because the crust is contiguous and not made up of isolated portions, the downward push from the ice sheet spreads throughout the region, distributing stress in the lithosphere over long distances, he says.
In fact, the ice sheet — which extended into northern Germany at its maximum extent — even piled stress onto the crust in parts of Europe never reached by the ice, such as France and farther south in Germany. To better understand the effects of the ice sheet and its retreat on faults in North Central Europe, Holger Steffen, a geodesist at the Swedish National Land Survey and one of the study’s co-authors, used models to gauge crustal deformation. He focused on crustal stress related to the ice sheet and then connected that deformation to the crustal response near known faults in the region. The model findings were compared to historical records of earthquakes, along with data collected by Brandes on paleoearthquakes gleaned from records of sand volcanoes — cones of sand that form when water-saturated sediments are ejected from the ground during quakes.
The comparison showed that glacial unloading and isostatic rebound could have provided the small amount of stress needed to nudge some faults in the region to break. The model simulations were also consistent with earthquake records across North Central Europe.
Isostatic rebound as a driving mechanism for intraplate seismicity isn’t a new idea, says Eric Calais, a geoscientist at École Normale Supérieure in Paris. But the team made headway in connecting field data and model data to support the notion.
“Running the models and making the calculations is one part,” he says. “Corroborating the results with actual data is another, and this team has collected a sizable dataset.”
Calais says he thinks isostatic rebound may be becoming less and less influential on Northern European quakes, and that its effect on modern seismicity is perhaps negligible. Still, he says, understanding the causes of past earthquakes is key to anticipating modern ones.
“For Europe, it is particularly relevant because a lot of our energy is coming from nuclear power plants,” Calais says. “We need to understand those conditions around power plants in terms of seismic hazards.”
In ongoing efforts, Brandes, Steffen and their colleagues plan to further develop the models they’ve used, and expand their field dataset. In particular, Brandes says he wants to dig deeper into the Quaternary quakes that shook the region, including trying to unearth more information from northern Germany’s old sand volcanoes.
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