Abstract
The Eastern European Alps are characterized by slow active deformation with low- to moderate seismicity. Recurrence rates of severe earthquakes exceed the time span of historical documentation. Therefore, historical and instrumental earthquake records might be insufficient for seismic hazard assessment and high-quality paleoseismic data is required. However, primary geological observations of postglacial fault activity are scarcely found, because major faults are buried below thick sedimentary sequences in glacially overdeepened valleys. Moreover, high erosion rates, gravitational slope processes and penetrative anthropogenic landscape modification often obscure geomorphic features related to surface ruptures. Here we present one of the rare paleoseismic data sets showing both on-fault evidence as subaqueous surface ruptures and off-fault evidence as multiple coeval mass-transport deposits (MTDs) and megaturbidites within a single high-resolution seismic-stratigraphic framework of the inner-alpine lake Achensee. Co-occurrence of on-fault and off-fault paleoseismic evidence on three stratigraphic levels indicates seismic activity with inferred moment magnitudes MW ∼6–6.5 of the local, lake-crossing Sulzgraben-Eben thrust at ∼8.3 ka BP and twice in Late Glacial times. Additional eight stratigraphic levels with only off-fault paleoseismic evidence document severe seismic shaking related to the historical MW ∼5.7 earthquake in Hall (CE 1670) and seven Holocene earthquakes, which have exceeded a local seismic intensity of ∼VI (EMS-98) at Achensee. Furthermore, we discuss natural and methodological influencing factors and potential pitfalls for the elaboration of a subaqueous paleoseismic record based on surface ruptures and multiple, coeval MTDs.
Highlights
The European Alps are a slowly but actively deforming orogen with an overall convergence rate of 1–2 mm/year between the counter-clockwise rotating Adriatic plate and a stable European plate (Métois et al, 2015)
Historical and instrumental earthquake records might be insufficient for seismic hazard assessment
For several parts of the Alps, advanced understanding of severe earthquake recurrence has been gained from long Holocene paleoearthquake records established in lake sediments (e.g., Beck et al, 1996; Kremer et al, 2017; Rapuc et al, 2018)
Summary
The European Alps are a slowly but actively deforming orogen with an overall convergence rate of 1–2 mm/year between the counter-clockwise rotating Adriatic plate and a stable European plate (Métois et al, 2015). Lacustrine paleoseismology takes advantage of the often high-resolution, continuous and accurately datable sedimentary succession in lakes, while being sensitive for seismic shaking and able to preserve sedimentary imprints of earthquakes (Strasser et al, 2013) Such off-fault paleoseismic evidence, when documented at multiple lake sites and interpreted to be coeval, can be used to evaluate epicentral locations and magnitudes of paleoearthquakes by constraining intensity threshold conditions at each lake site and applying empirical seismic attenuation relation in a grid-search approach (Strasser et al, 2006; Kremer et al, 2017). The causative source fault for the majority of severe earthquakes in the Alps remains not constrained
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