A calibrated multi-scale 14 ka lacustrine earthquake record from the Eastern Alps, Austria

Knowledge about the seismic history of a region provides the foundation of seismic hazard assessment. In slowly deforming regions, such as the Eastern European Alps, interseismic periods of the largest earthquake on a fault segment typically exceed the time covered by instrumental (~100 years) and historical (~1000 years) records, potentially leading to inaccurate seismic hazard analysis. To fill this knowledge gap, lacustrine sedimentary sequences are increasingly used. They can archive past earthquakes with local seismic shaking of intensities > ~V (EMS-98) as subaqueous landslides, turbidites and in-situ sediment deformation, and can provide long (> 10 ka) and continuous paleoseismic records.

We investigated Wörthersee, a large lake (~19 km²) with several subbasins located in the Austrian state of Carinthia, close to the Slovenian and Italian border. Although situated in an intraplate setting, this region has experienced several devastating historically and instrumentally recorded earthquakes with intensities ranging from V to IX, e.g. in AD1348 (M_w ~7; possibly the largest historical earthquake in the Alps), AD1511 (M_w 6.9), AD1690 (M_w 6.5), AD1857 (M_w 5) and AD1976 (M_w 6.4). Based on the sedimentary imprint of these well-documented earthquakes, we derived (i) seismic intensity thresholds for the different subbasins of Wörthersee (ranging from intensity VI to IX) and (ii) scaling relationships between the thickness of turbidites and seismic intensity.

Here, we apply the above-mentioned ground motion indicators to eight long (up to 11 m) sediment cores retrieved from different subbasins and establish the first calibrated and multi-scale dataset from the Eastern European Alps. The sediment cores, which cover the last ~14 ka, were analysed, correlated, and dated by visual inspection, high-resolution XRF scanning and numerous radiocarbon ages. 44 stratigraphic levels of turbidites, deposited synchronously in different lake basins and therefore inferred to indicate a seismic trigger, were identified. Our data show a sudden increase of recorded earthquakes at 2.8 ka BP. This coincides with a change in lake sedimentation from calcite-rich to more easily remobilised organic sediments, suggesting a shift in the lakes’ sensitivity to record seismic shaking. In the organic-rich unit, the mean recurrence interval of strong (I > VI) earthquakes is 250 ± 30 years (9 events), whereas in the calcite-rich unit, long recurrence intervals (640 ± 30 years; 11 events) are observed. At 13-12.6 ka, at 3.5-3.3 ka, phases of enhanced regional seismicity are recorded. Similar paleo-seismicity patterns indicating group-fault clustering have been documented in the Swiss Alps.

The inferred aperiodic to strongly bursty recurrence behaviour (calculated mean burstiness ranges from -0.05 to 0.25, depending on intensities and time interval) suggest the potential for elevated hazard after large earthquakes – a factor not considered yet in hazard calculations in the Eastern Alps. The intensity-frequency distribution of earthquakes derived from our data for the last 2.8 ka suggests that the hazard curve provided by the Geophysical Service of Austria underestimates the seismic hazard. Our dataset therefore highlights the importance of long and well-calibrated multi-scale paleoseismic datasets in intraplate settings and constitutes an important contribution to hazard assessment in the South-Eastern European Alps.