Geologists from the University of Innsbruck have discovered a tectonically active earthquake fracture zone in the Achensee, the largest lake in Tyrol. This had triggered past strong earthquakes, the scientists announced on Monday. During profile measurements in the Achensee, the geologists accidentally came across an unusual terrain step in the subsurface of the lake. The analyzes by the earthquake experts from the Institute of Geology finally revealed the active fracture zone.
For the first time, the team discovered a tectonic fracture zone within the Alps that caused prehistoric strong earthquakes and delivered an archive of strong earthquakes from the past 17,000 years for the Tyrolean lowlands. “The earthquakes below the Achensee took place within the last 17,000 years, but most recently around 8,300 years ago”, explained Patrick Oswald, doctoral student in the working group for sediment geology at the Institute of Geology at the University of Innsbruck and main author of the recently published study and added: “Im From a geological point of view, this fracture zone is therefore considered to be tectonically active, since strong earthquakes have occurred within the last 10,000 years ”.
“Terrain level” in the lake
It was possible to show that the “terrain step” in the Achensee is a tectonic fracture zone that was created by offsets during earthquakes. The scientists could have estimated a total of three strong earthquakes with a moment magnitude of around 6 to 6.5 in the sediment layers of the lake at this point of rupture.
In addition to the strong earthquakes below the lake in the Tyrolean lowlands, the geologists said they found evidence in the form of underwater landslides and collapsing parts of the coast for a further eight earthquakes. These quakes very likely occurred in the wider area around the Achensee, but were either too weak to create offsets on the surface, or possible traces on land were blurred by erosion or human changes.
“Better understanding”
The discovery of the fracture zone is of particular importance for a better understanding of the tectonic history of the Alps. “This enables us to present a first paleoseismological archive for the Tyrolean lowlands, where no information on prehistoric earthquakes was previously available. The frequency of earthquakes is similar to that in the Tyrolean Oberland, ”emphasized Jasper Moernaut from the Institute of Geology.