Host laboratory: Seismic Risk Assessment Section (BERSSIN)
Beginning of the thesis: October 2016
Student name: Jessica THOMAS
Subject descriptionThe Upper Rhine Graben (URG) is the central
part of the Central European Rift System and one of the most seismically
active regions in Western Europe, north of the Alps. This graben is
delimited by faults, which are supposed to be active. The objective of
the thesis is the analysis of the fault system activity to figure out
its seismotectonic parameters useful to estimate the associated seismic
hazard. This is a challenging task because of the numerous critical
facilities and highly populated cities. A first scientific challenge is
the recognition and characterization of the fault activity in such a
highly human-modified landscape and subject to erosional/depositional
processes, both competing the growth of fault-controlled relief. To
eventually determine the long-term deformation rate and the earthquake
history, a key topic is the analysis of the sedimentary record along the
faults which might have registered those deformation episodes.
The
objectives during the first stages of the thesis, before investigating
the sedimentary record, are 1) to analyze in detail the morphology with
high-resolution Digital Elevation Models (DEM) and 2) to perform a
detailed geophysical survey. The geomorphological analysis provides a
first order fault location, its lateral extension and significance:
interesting spots have already been found out and guided our first
geophysical investigations. These consisted in Ground-Penetrating Radar
(GPR) and Electrical Resistivity Tomography (ERT) soundings which aim at
imaging the sedimentary layers geometry and anomalies that could
correspond to faults. Proven indicators for faults in GPR profiles are
displacement of reflectors or changes in dip angle, reflection angle and
pattern or signal intensity. The ERT proofed very useful for the
revision and verification of features found in GPR profiles. The GPR has
limitations in case of high soil water content and there is a tradeoff
between achievable spatial resolution and investigation depth. Several
faults described in former studies (e.g. geological map of
Baden-Württemberg, GeORG, 2012 and Nivière et a1., 2008) could be
verified, but partially with deviations in the exact location, possibly
related to accuracy and density of the data sets, as well as the
geophysical and interpolation methods that were employed. For defining
an exact location for further trenches (planned for 2018),
investigations with percussion drilling will be conducted to verify if
the GPR and ERT anomalies are fault-related features rather than being
caused by erosion.