Past earthquakes have caused large landslides in the eastern Alps

IMAGE: In sediment cores of 8 meters long, geologists found different types of earthquake effects. The photo shows Plansee in Reutte, Tyrol, Austria. Show more

Credit: Jasper Mornout

The many steep valleys of the European Alps show remnants of large avalanches, during which hundreds of millions of cubic meters of rock are precipitated, crumble and affect everything in their path. “For most of these, we still don’t know why they happened, because these rockslides happened long before the beginning of written history in the area about 1,000 years ago,” says Patrick Oswald, a PhD student in the university’s geology department. From Innsbruck and lead author of the study. “Strangely, many of these ancient rock slides occurred together in groups, which means that they are found in small areas and have a somewhat similar age.” This mysterious pattern has puzzled researchers over the past decades and sparked some intense discussion. Some experts suggest that sudden climate changes could degrade rocky slopes toward unstable conditions, while others believe a strong earthquake is the main driver. It is known that such problems are difficult to solve, because the subject of study – the rocky slope – has collapsed and cannot be investigated anymore. Therefore, the research team decided to change perspective and search underwater for answers to these questions. “The different layers of sediments deposited year after year at the bottom of our lakes and oceans provide long-term information about climatic and environmental conditions, but they also record the disturbances caused by strong earthquake vibrations that occurred long ago, in prehistoric times,” says Michael Strasser, chair of the Geology Working Group. Sedimentology at the Department of Geology and the Austrian Basic Facility for Basic Scientific Analysis at the University of Innsbruck.

Severe prehistoric earthquakes

Geoscientists have focused on two of Tirol’s largest rock slides, the Tschirgant and Fernpass. “Instead of investigating the remains of these landslides,” Jasper explains, “we dug into the muddy sedimentary archives at the bottom of the Piburgersee and Plansee lakes in the area and looked for specific traces that could inform us of powerful earthquakes.” Moernaut from the Department of Geology, project lead for this “Tyrol on Shaky Slopes” study. “By comparing the reconstructions that have occurred in earthquakes and rockslides over the past 10,000 years, we can assess whether or not these are related to each other.” By applying the latest techniques, such as subsurface lake hydroacoustic profiling or computer tomography of sediment cores of 8 meters in length, researchers found two different types of earthquake effects in sediments: Strong seismic vibration has deformed surface sediments at the bottom of lakes and also caused many occurrences. From underwater mudslides. By radiocarbon dating of the organic matter in the nuclei, this research has discovered ten prehistoric earthquakes over the past 10,000 years. The Earth’s vibrations associated with these earthquakes were stronger than those that struck the region in the past ~ 1000 years. “Through a painstaking evaluation of historical earthquake reports and a comparison with sedimentary fingerprints in the lakes, we estimated the size of the earthquake to be between M5.5 and 6.5,” says Christa Hammerl, a historical seismologist at the Austrian Central Institute of Meteorology and Dynamics. “Since earthquakes in the eastern Alps occur at a depth of only a few kilometers, such earthquakes can cause significant damage to infrastructure and landscapes.”

Rock slip clusters induced by earthquakes

Remarkably, the ages of two unusually strong earthquakes coincide very well with multiple large rock earthquakes about 3,000 and 4,100 years ago. The coincidence of the era allowed geologists to conclude that intense seismic vibrations eventually triggered avalanches at these times, thus resolving the debate over the cause of rockslides in the region. Since then, the area has not been hit by any major rock slides or such extraordinarily strong earthquakes. The results also indicate that a close succession of at least five severe earthquakes preceded the collapse of the rockslip about 3,000 years ago. “Therefore, we suggest that seismic vibration not only leads to avalanches, but can also lead to a gradual degradation of rock slopes toward the critical tipping point,” explains Michael Strasser. “With all this new information, the challenge now is to implement it in order to better assess the risk of future earthquakes and rockslides in the densely populated alpine valleys. Adequate mitigation of such a small probability but high-impact events is a major challenge. It helps to better understand the occurrence of earthquakes and provide basic information for assessing future earthquake and rockslide risks.

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This research is supported by the European Regional Development Fund, Interreg VA Italy-Austria 2014-2020 (ITAT3016-Armonia project), the Tyrolean Science Fund (UNI-0404-2151 project), the Austrian Science Fund FWF (project P30285-N34) and the Austrian Academy of Sciences ÖAW (ESS-IGCP-project S4LIDE-Austria).

the post:

Seismic control of large prehistoric landslides in the Eastern Alps. Patrick Oswald, Michael Strasser, Krista Hammerle, Jasper Murnout. Nature Communications 2021. DOI: 10.1038 / s41467-021-21327-9 https://doi.org/10.1038/s41467-021-21327-9

Links:

Sedimentary Geology Working Group of the Geology Department: https://www.uibk.ac.at/geologie/sediment/

Austrian Central Institute of Meteorology and Geodynamics: https://www.zamg.ac.at/cms/de/geophysik/erdbeben

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