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Titel |
Neotectonic fault structures in the Lake Thun area (Switzerland) |
VerfasserIn |
Stefano C. Fabbri, Marco Herwegh, Fritz Schlunegger, Christian Hübscher, Benedikt J. Weiss, Cedric Schmelzbach, Heinrich Horstmeyer, Kaspar Merz, Flavio S. Anselmetti |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250131875
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Publikation (Nr.) |
EGU/EGU2016-12324.pdf |
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Zusammenfassung |
Strong historic earthquakes (i.e. intensities I0 ≥ V) in Switzerland are well documented
by the earthquake catalogue of Switzerland ECOS-09 (e.g. Frutigen, 1729 AD,
Mw=5.2, I0=VI). Many of these strong events can be recognized paleoseismically by
large subaquatic, earthquake-triggered mass movements that occur frequently in
Swiss Lakes. Some of these represent the occasional occurrence of even stronger
earthquakes (i.e. Mw ∼6.5) in the Alpine region (Strasser et al., 2013), which are
expected to produce noticeable surface ruptures. However, convincing evidence for
Quaternary displacements with offset surface expressions have scarcely been found (e.g.,
Wiemer et al., 2009). Applying a multi-disciplinary approach, this study presents
potential candidates for such faults in the larger Lake Thun area at the edge of the
Alps.
The overdeepened basin of Lake Thun is situated at the northern Alpine front, which
extends orthogonally to the general strike direction of the Alpine nappe front. The northern
shoreline is predominantly shaped by the front of the Subalpine Molasse, which is in strong
contrast to the south western shore built by the structurally higher units of the Middle and
Lower Penninic nappes. This pattern with obvious differences of both lake sides suggests a
major fault along the lake axis and high tectonic activity during nappe emplacement, i.e.
from Eocene times throughout the Late Miocene. The area is dominated today by a
strike-slip stress regime with a slight normal faulting component (Kastrup et al.,
2004).
As part of a multi-disciplinary study, attempting to find potential neotectonically active
fault structures in the Lake Thun area, a 2D ground penetrating radar (GPR) survey was
conducted. The aim of the GPR survey was to link observations from a multichannel
reflection seismic survey and a multibeam bathymetric survey carried out in Lake Thun with
findings in a nearby gravel quarry revealing suspicious deformation features such as rotated
gravel clast as well as significantly offset horizons.
The GPR data reveal the occurrence of several morphologic depressions from gypsum
cones and clearly dipping reflections. The reflection seismic data set shows prominent
reflections, characteristic seismic facies and a few sets of normal and reverse faults in the
north western part of the lake basin within the glacio-lacustrine deposits that may point to a
transpressional strike-slip regime. A first neotectonic analysis links these prominent
lake floor features with geomorphologic patterns from the surrounding landscape,
pointing to a potential candidate for a fault that is active in the Quaternary period. |
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