How rethinking the formation of the Alps can improve earthquake risk assessment

An alternative mechanism for the formation of the Alps could help improve the assessment of earthquake risks. The key is in the tectonic plates detaching and uplifting rather than bulldozing, say Swiss researchers.

For a long time, geoscientists have assumed that the Alps were formed when the Adriatic plate from the south collided with the Eurasian plate in the north. A few years ago, however, new geophysical and geological data led ETH geophysicist Edi Kissling and Fritz Schlunegger, a sediment specialist from the University of Bern, to express doubts about this theory.

Kissling and Schlunegger pointed out that the topography and altitude of the Alps have barely changed over the past 30 million years, and yet the trench at the site of the Swiss Plateau has continued to sink and the basin extended further north. This leads the researchers to believe that the formation of the Central Alps and the sinking of the trench are not connected as previously assumed.

They argue that if the Alps and the trench indeed had formed from the impact of two plates pressing together, there would be clear indications that the Alps were steadily growing, explains a statement. That’s because, based on the earlier understanding of how the Alps formed, the collision of the plates, the formation of the trench and the height of the mountain range are all linked.  

Uplifting, not bulldozing

The behaviour of the Eurasian plate provides a possible new explanation. Since about 60 Ma ago, the former oceanic part of the Eurasian plate sinks beneath the continental Adriatic microplate in the south. By about 30 Ma ago, this process of subduction is so far advanced that all oceanic lithosphere has been consumed and the continental part of the Eurasian plate enters the subduction zone.

This denotes the begin of the so-called continent-continent collision with the Adriatic microplate and the European upper, lighter crust separates from the heavier, underlying lithospheric mantle. Because it weighs less, the Earth’s crust surges upwards, literally creating the Alps for the first time around 30 Ma ago. While this is happening, the lithospheric mantle sinks further into the Earth’s mantle, thus pulling the adjacent part of the plate downwards.

New model confirms lift hypothesis

To investigate the lift hypothesis, Luca Dal Zilio, former doctoral student, has now teamed up with Kissling and other ETH researchers to develop a new model.

According to Kissling, the model is an excellent way to simulate the uplifting processes that he and his colleague are postulating. The model is based on physical laws. For instance, the Eurasian plate would appear to subduct southwards. In contrast to the normal model of subduction, however, it doesn’t actually move in this direction because the position of the continent remains stable.

Rethinking seismicity

In addition, the model simulates the occurrence of earthquakes, or seismicity, in the Central Alps, the Swiss Plateau and below the Po Valley. “Our model is the first earthquake simulator for the Swiss Central Alps,” says Dal Zilio.

The advantage of this earthquake simulator is that it covers a very long period of time, meaning that it can also simulate very strong earthquakes that occur extremely rarely.

Current earthquake statistics tend to underestimate such earthquakes. The new simulations therefore improve the assessment of earthquake risk in Switzerland.

Photo credit: barnyz / CC BY-NC-ND 2.0

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