Calibrated long-term lacustrine paleoseismic records from Carinthia (Austria): implications for earthquake hazard in the south-eastern Alps

In formerly glaciated intraplate settings with moderate seismicities, such as the Eastern Alps, recurrence intervals of strong earthquakes (M_w >6) typically exceed the short time span of instrumental (~100 years) and historical (~1000 years) data. To assess the seismic hazards and draw conclusions about the role of postglacial isostatic rebound on earthquake recurrence in these regions, lakes have been increasingly used as natural seismographs over the last two decades.

We present paleoseismic records from three glacigenic lakes (Wörthersee, Millstätter See, and Klopeiner See) situated at the south-eastern rim of the Alps, Austria. This region, although located in an intraplate setting, has experienced several devastating historically and instrumentally recorded earthquakes with intensities ranging from V to IX (EMS-98) in our study area, e.g., in AD1348 (M_w ~7; possibly the strongest historical earthquake in the Alps), AD1511 (Mw 6.9), AD1690 (Mw 6.5), AD1857 (Mw 5) and AD1976 (Mw 6.4).

The lakes were investigated with multibeam bathymetry and a very dense grid of reflection seismic profiles (~1.3, 3.5, and 8 kHz; 640 km in total). Numerous short (~1.5 m; ~80 cores) and long (~up to 14 m; 22 cores) sediment cores were retrieved from all lakes and their respective subbasins and were independently dated (varve counting in the last ~1000 years, radiocarbon, and ²¹⁰Pb/¹³⁷Cs dating). This spatially and temporally high-resolution approach allows to construct a complete picture of the sedimentary imprint of strong earthquakes in these lakes.

The geophysical data image an archive of multiple simultaneous subaqueous landslides. In the sediment cores, which cover the last 7500 years in Millstätter See and reach back into the Late Glacial in Wörthersee and Klopeiner See, these landslides are represented as turbidite deposits (from mm to m-scale) interspersed in the partly finely laminated background sediments. By comparing the seismic intensities of the well-documented historical earthquakes to the spatial distribution of sedimentary imprint in the lakes, we revealed the earthquake recording thresholds (EQRT) of different depositional areas. Most of the sites record local intensities ≥VI. In shallow basins with low sedimentation rates, however, the EQRT is significantly higher, either solely recording the AD1348 event (VIII-IX) or showing no evidence of seismic shaking at all. Contrastingly, sites close to alluvial fans in Wörthersee also record the AD1857 and the AD1976 earthquakes (V½). Quantification of the earthquake-related deposits (e.g., cumulative turbidite thickness, percentage of depositional areas recording an event) shows a linear size-scaling relationship with the respective intensities. This provides us with a tool to constrain the local seismic intensity of prehistoric earthquakes.

Our data show that the AD1348 earthquake generated the strongest earthquake shaking in the study area for the entire Holocene. Generally, the seismicity peaked in the Late Glacial, around ~3.5 ka, and in the last ~1000 years, whereas the early- to mid-Holocene was a relatively calm period. Other paleo-earthquake studies from both the Fennoscandian Peninsula and the Swiss Alps show a similar seismicity pattern, suggesting that seismicity in the Alps is governed by postglacial rebound rather than tectonically induced stress.