ORIGINAL RESEARCH published: 11 August 2021 doi: 10.3389/feart.2021.670532 Active Faulting in Lake Constance (Austria, Germany, Switzerland) Unraveled by Multi-Vintage Reflection Seismic Data S.C. Fabbri 1*, C. Affentranger 1, S. Krastel 2, K. Lindhorst 2, M. Wessels 3, Herfried Madritsch 4, R. Allenbach 5, M. Herwegh 1, S. Heuberger 6, U. Wielandt-Schuster 7, H. Pomella 8, T. Schwestermann 8 and F.S. Anselmetti 1 1Institute of Geological Sciences and Oeschger Centre of Climate Change Research, University of Bern, Bern, Switzerland, 2Institute of Geosciences, Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 3Institut für Seenforschung der LUBW, Langenargen, Germany, 4National Cooperative for the Disposal of Radioactive Waste (NAGRA), Wettingen, Switzerland, 5Federal Office of Topography Swisstopo, Wabern, Switzerland, 6Department of Earth Sciences, ETH Zürich, Zürich, Switzerland, 7Landesamt für Geologie, Rohstoffe und Bergbau Baden-Württemberg, Freiburg i. Br., Germany, 8Department of Geology, University of Innsbruck, Innsbruck, Austria Probabilistic seismic hazard assessments are primarily based on instrumentally recorded Edited by: and historically documented earthquakes. For the northern part of the European Alpine Francesco Emanuele Maesano, Istituto Nazionale di Geofisicae Arc, slow crustal deformation results in low earthquake recurrence rates and brings up the Vulcanologia (INGV), Italy necessity to extend our perspective beyond the existing earthquake catalog. The Reviewed by: overdeepened basin of Lake Constance (Austria, Germany, and Switzerland), located Chiara Amadori, within the North-Alpine Molasse Basin, is investigated as an ideal (neo-) tectonic archive. University of Pavia, Italy Alessandro Maria Michetti, The lake is surrounded by major tectonic structures and constrained via the North Alpine University of Insubria, Italy Front in the South, the Jura fold-and-thrust belt in the West, and the Hegau-Lake *Correspondence: Constance Graben System in the North. Several fault zones reach Lake Constance S.C. Fabbri [email protected] 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 Specialty section: catalog of potentially active fault zones, we compiled an extensive 445 km of multi-channel This article was submitted to fl Structural Geology and Tectonics, re ection seismic data in 2017, complementing a moderate-size GI-airgun survey from a section of the journal 2016. The two datasets reveal the complete overdeepened Quaternary trough and its Frontiers in Earth Science sedimentary infill and the upper part of the Miocene Molasse bedrock. They additionally Received: 21 February 2021 complement existing seismic vintages that investigated the mass-transport deposit Accepted: 23 July 2021 Published: 11 August 2021 chronology and Mesozoic fault structures. The compilation of 2D seismic data allowed Citation: investigating the seismic stratigraphy of the Quaternary infill and its underlying bedrock of Fabbri SC, Affentranger C, Krastel S, Lake Constance, shaped by multiple glaciations. The 2D seismic sections revealed 154 Lindhorst K, Wessels M, Madritsch H, Allenbach R, Herwegh M, fault indications in the Obersee Basin and 39 fault indications in the Untersee Basin. Their Heuberger S, Wielandt-Schuster U, interpretative linkage results in 23 and five major fault planes, respectively. One of the major Pomella H, Schwestermann T and fault planes, traceable to Cenozoic bedrock, is associated with a prominent offset of the Anselmetti FS (2021) Active Faulting in Lake Constance (Austria, Germany, lake bottom on the multibeam bathymetric map. Across this area, high-resolution single Switzerland) Unraveled by Multi- channel data was acquired and a transect of five short cores was retrieved displaying Vintage Reflection Seismic Data. fi Front. Earth Sci. 9:670532. signi cant sediment thickness changes across the seismically mapped fault trace with a doi: 10.3389/feart.2021.670532 surface-rupture related turbidite, all indicating repeated activity of a likely seismogenic Frontiers in Earth Science | www.frontiersin.org 1 August 2021 | Volume 9 | Article 670532 Fabbri et al. Active Faulting in Lake Constance 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. Keywords: coring, earthquakes, seismic hazard, seismic stratigraphy, active faults, glacial overdeepening, turbidites, Molasse basin INTRODUCTION evidence in active fault systems within Switzerland have been reported (Ustaszewski et al., 2007; Kremer et al., 2020), such as In intraplate environments with low crustal deformation rates the identified active normal fault near the eastern edge of the such as in the North Alpine Front area (e.g., Houlié et al., 2018), Upper Rhine Graben, identified by geomorphological and current probabilistic seismic hazard assessments are primarily geophysical analysis, supplemented by six trenches at two based on historically documented and instrumentally recorded different sites (Meghraoui, 2001), or the roughly 4 km long earthquakes (Wiemer et al., 2009), especially since strike-slip fault crossing the shoreline of Lake Thun (Fabbri documentation of active faults is sparse. The earthquake et al., 2017). This is mostly due to three factors, 1) high catalogue of Switzerland (e.g., Fäh et al., 2011) builds such a erosion rates in the Alpine region and strong landscape basis for hazard assessment, covering a time span of roughly modifications by glacio-fluvial processes in the Alpine 1,000 years with ∼20,000 events. The limited temporal coverage foreland, 2) pervasive anthropogenic landscape modification of instrumentally recorded and historically reported events (Ustaszewski et al., 2007) and 3) a minimum threshold of motivates the approach to use primary and secondary moment magnitude that needs to be exceeded in order to paleoseismic evidence, known as on- and off-fault records, create surface rupture (Wells and Coppersmith, 1994; Stirling respectively, and to expand the time span of the existing et al., 2002). The first two factors can be addressed by focusing on database. Such “paleoseismic” data aim at documenting almost erosion free environments, such as perialpine lakes. The prehistoric earthquakes throughout the Holocene and Late latter factor requires imaging the subsurface spatially densely to Pleistocene, reflecting recent tectonic activity and various depths, so that the investigated time span is expanded to characterizing location, timing, and size of events (McCalpin several thousand years. This guarantees the required vertical and and Nelson, 2009). lateral resolution, increasing the likelihood of discovering the In recent years, large efforts have been made to overcome the roots of high-magnitude events offsetting lacustrine sediments. limited time span of historically documented and instrumentally However, the confirmation of on-fault evidence is often solely recorded earthquakes in the Alps by the investigation of possible via a multidisciplinary approach combining information secondary evidence such as earthquake-triggered subaquatic from geophysical (e.g., seismic, electromagnetic, radar data), mass movements and their related turbidites (Becker et al., geomorphological (bathymetric, lidar, satellite imagery) and 2002; Schnellmann et al., 2002; Monecke et al., 2004; Kremer geological (outcrop, core, geochemical analysis) independent et al., 2015b; Sammartini et al., 2021) and small-scale in situ datasets (Fabbri et al., 2017; Oswald et al., 2021). Hence, only deformation features (e.g., liquefaction structures, micro-faults, few study sites in lacustrine settings showed on-fault paleoseismic mushroom-like intrusions) in lake sediments (Rodriguez-Pascua evidence documenting active faulting in the Alpine realm, such as et al., 2000; Monecke et al., 2004; Schnellmann et al., 2006). Lake Garda (South of the Alps in northern Italy) with mass- Despite this convincing off-fault evidence suggesting several movement deposits and seismo-turbidites close to interpreted strong earthquakes and several distinct phases of increased active tectonic deformations (Gasperini et al., 2020), Lake Le activity between 300–600, 1,400–1700, 2,200–2,500, Bourget (northwestern French Alps) with Riedel-like expressions 3,000–3,600, 6,200–7,000 and at around 9,500–9,900 calibrated of Quaternary deformation in the Holocene sediments related to years before present in the Alpine realm, there is a general onshore strike-slip faults (de La Taille et al., 2015), or inner-alpine absence of known seismogenic fault structures with clear Lake Achensee (eastern Austrian Alps) with offset postglacial surface ruptures supporting these observations (Kremer et al., sediment infill unraveled by multiple, coeval mass wasting 2020). Their identification is absolutely vital though to minimize deposits (Oswald et al., 2021). The predominantly uncertainties associated with the calculated recurrence time of accumulative character and high preservation potential of Lake strong earthquakes. In recent years, paleoseismological research Constance (Austria, Germany, and Switzerland, inset Figure 1) has provided a wealth of information about paleoseismicity may serve as an ideal laboratory to unravel existing fault (Michetti et al., 2005), but is often related to the successful structures and lake-bottom offsets. identification of