Abstract
Probabilistic seismic hazard assessments are primarily based on instrumentally recorded and historically documented earthquakes. For the northern part of the European Alpine Arc, slow crustal deformation results in low earthquake recurrence rates and brings up the necessity to extend our perspective beyond the existing earthquake catalog. The overdeepened basin of Lake Constance (Austria, Germany, and Switzerland), located within the North-Alpine Molasse Basin, is investigated as an ideal (neo-) tectonic archive. The lake is surrounded by major tectonic structures and constrained via the North Alpine Front in the South, the Jura fold-and-thrust belt in the West, and the Hegau-Lake Constance Graben System in the North. Several fault zones reach Lake Constance such as the St. Gallen Fault Zone, a reactivated basement-rooted normal fault, active during several phases from the Permo-Carboniferous to the Mesozoic. To extend the catalog of potentially active fault zones, we compiled an extensive 445 km of multi-channel reflection seismic data in 2017, complementing a moderate-size GI-airgun survey from 2016. The two datasets reveal the complete overdeepened Quaternary trough and its sedimentary infill and the upper part of the Miocene Molasse bedrock. They additionally complement existing seismic vintages that investigated the mass-transport deposit chronology and Mesozoic fault structures. The compilation of 2D seismic data allowed investigating the seismic stratigraphy of the Quaternary infill and its underlying bedrock of Lake Constance, shaped by multiple glaciations. The 2D seismic sections revealed 154 fault indications in the Obersee Basin and 39 fault indications in the Untersee Basin. Their interpretative linkage results in 23 and five major fault planes, respectively. One of the major fault planes, traceable to Cenozoic bedrock, is associated with a prominent offset of the lake bottom on the multibeam bathymetric map. Across this area, high-resolution single channel data was acquired and a transect of five short cores was retrieved displaying significant sediment thickness changes across the seismically mapped fault trace with a surface-rupture related turbidite, all indicating repeated activity of a likely seismogenic strike-slip fault with a normal faulting component. We interpret this fault as northward continuation of the St. Gallen Fault Zone, previously described onshore on 3D seismic data.
Highlights
In intraplate environments with low crustal deformation rates such as in the North Alpine Front area (e.g., Houlié et al, 2018), current probabilistic seismic hazard assessments are primarily based on historically documented and instrumentally recorded earthquakes (Wiemer et al, 2009), especially since documentation of active faults is sparse
A new drilling technique with an advanced piston coring tool was tested in Lake Constance reaching a subsurface depth of 24 m (Harms et al, 2020), expanding significantly previous coring limits
Since lithological and sedimentological information from cores is lacking for the deeper sections, the stratigraphic and lithologic interpretations are mainly based on seismic facies analysis and seismic stratigraphy
Summary
In intraplate environments with low crustal deformation rates such as in the North Alpine Front area (e.g., Houlié et al, 2018), current probabilistic seismic hazard assessments are primarily based on historically documented and instrumentally recorded earthquakes (Wiemer et al, 2009), especially since documentation of active faults is sparse. Large efforts have been made to overcome the limited time span of historically documented and instrumentally recorded earthquakes in the Alps by the investigation of secondary evidence such as earthquake-triggered subaquatic mass movements and their related turbidites (Becker et al, 2002; Schnellmann et al, 2002; Monecke et al, 2004; Kremer et al, 2015b; Sammartini et al, 2021) and small-scale in situ deformation features (e.g., liquefaction structures, micro-faults, mushroom-like intrusions) in lake sediments (Rodriguez-Pascua et al, 2000; Monecke et al, 2004; Schnellmann et al, 2006) Despite this convincing off-fault evidence suggesting several strong earthquakes and several distinct phases of increased activity between 300–600, 1,400–1700, 2,200–2,500, 3,000–3,600, 6,200–7,000 and at around 9,500–9,900 calibrated years before present in the Alpine realm, there is a general absence of known seismogenic fault structures with clear surface ruptures supporting these observations (Kremer et al, 2020). The predominantly accumulative character and high preservation potential of Lake Constance (Austria, Germany, and Switzerland, inset Figure 1) may serve as an ideal laboratory to unravel existing fault structures and lake-bottom offsets
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