Alps: New findings about earthquake history

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Sampling at Lake Wörthersee The subsurface of Lake Wörthersee was sampled from t
Sampling at Lake Wörthersee The subsurface of Lake Wörthersee was sampled from this 8-meter-high platform in 2018. © Michael Hilbe

A team of geologists from the University of Innsbruck examined the sediments of Carinthian lakes for traces of past earthquakes. The results show that the earthquake of 1348 caused the strongest shaking in the Carinthian region since the end of the last cold period. Earthquakes with potential building damage are rare there, but can occur in temporal clusters.

As historical records show, Carinthia was shaken by one of the strongest known earthquakes in the Alpine region in 1348. It is certain that the quake caused parts of Villach’s local mountain Dobratsch to fall, damming the Gail River to form a lake and thus leading to the flooding of some localities. The epicenter of this earthquake and the actual extent of the damage in Carinthia is, however, a matter of disagreement among historians as well as seismologists. Some researchers assume an epicenter near the Austrian-Italian border, but some authors locate the earthquake up to 50 km southwest in Friuli. This area was also affected by earthquakes up to a magnitude of 6.4 in 1976 and 1977, when 989 people lost their lives.
However, for an accurate assessment of the current earthquake hazard in the densely populated central region of Carinthia, precise knowledge of the epicenters of historical events and the frequency of strong shaking is essential. A team of geologists from the University of Innsbruck, in collaboration with geologists and geophysicists from the University of Bern and ETH Zurich, as well as historian Christa Hammerl and seismologist Stefan Weginger from GeoSphere Austria (formerly ZAMG), therefore searched for evidence of past earthquakes in Lake Wörthersee and Lake Millstätter See. "Strong tremors that cause at least minor building damage on land trigger mudslides in the lakes. We located the deposits of these submarine landslides using acoustic methods and then selectively took sediment cores up to 12 meters long to determine their age," says Christoph Daxer, a doctoral student in the Sedimentary Geology Group at the Institute of Geology at the University of Innsbruck.

14,000 years of earthquake history

In the process, the researchers realized that the traces of the 1348 quake in both Lake Wörthersee and Lake Millstätter See are immense. "In both lakes, more than five million cubic meters of lake mud were mobilized at that time. We also found deposits associated with other historically known earthquakes in 1201, 1511, 1690 and 1857. Interestingly, however, the 1976 earthquake left hardly a trace," Daxer explains. "These findings confirm that the intensities, i.e. the strength of the shaking on site, were noticeably higher during the 1348 quake in the Carinthian region than those of 1976. Consequently, with a similar magnitude, the epicenter of the 1348 quake should have been much closer to the lakes than that of 1976." However, the samples taken extend far beyond historical time periods, as Jasper Moernaut, assistant professor at the Institute of Geology at the University of Innsbruck, points out: "In Lake Wörthersee, we can trace the earthquake history back to about 14,000 years before today. 44 earthquakes during this period were strong enough to leave their traces in the lake. Since the volume and number of mudflows correlate with the intensity of an earthquake, we have a measurable quantity in the geological archive to determine the intensities of prehistoric earthquakes as well. The good news is that one as strong as that of 1348 probably last occurred about 11,500 years ago, at about the exit of the last cold period."

Assessment of dangers for the future

The researchers* are also using the data to review existing earthquake hazard assessments, which form the basis for building design, among other things, as Michael Strasser, head of the Sedimentary Geology Group at the Institute of Geology and the Austrian Core Facility for Scientific Drill Core Analyses at the University of Innsbruck, explains, "This hazard assessment is based on seismological data and the results of historical earthquake research, which roughly cover the last 1,000 years. However, the accuracy of these models is difficult to verify. With the earthquake data from the lakes, however, we can do just that, since it goes far back in time and is thus completely independent of the instrumental and historical data." It turned out that the current hazard assessment, at least in the Wörthersee area, reflects the long-term earthquake hazard well. "According to our data, the probability of a shaking of intensity level VII or higher on the European Macroseismic Scale - where cracks in the walls can at least be expected - is five to six percent over the next fifty years," Moernaut said. But the data also show that the frequency of earthquakes has not been constant locally, Daxer adds: "There have been longer phases with few earthquakes, which were then interrupted by many strong earthquakes in short succession. At the moment, we seem to be in a phase of increased earthquake activity, which must be taken into account when calculating earthquake hazard."

Daxer, C., Huang, J.-J.S., Weginger, S., Hilbe, M., Strasser, M. and Moernaut, J. (2022a) Validation of seismic hazard curves using a calibrated 14 ka lacustrine record in the Eastern Alps, Austria. Sci. Rep., 12, 19943. ’022 -24487-w

Daxer, C., Ortler, M., Fabbri, S.C., Hilbe, M., Hajdas, I., Dubois, N., Piechl, T., Hammerl, C., Strasser, M. and Moernaut, J. (2022b) High-resolution calibration of seismically-induced lacustrine deposits with historical earthquake data in the Eastern Alps (Carinthia, Austria). Quat. Sci. Rev., 284, 107497.