5th Swiss Geoscience Meeting, Geneva 2007
Pleistocene tectonic activity in NW Switzerland from
seismotectonic and geomorphologic evidence
Fraefel Marielle*, Schmid Stefan M.*, Densmore Alexander L.**
*Institute of Geology and Paleontology, Bernoullistrasse 32, CH-4056 Basel
(Marielle.Fraefel@unibas.ch)
**Department of Geography, Durham University, South Road, Durham DH1 3LE UK
Evidence for Quaternary tectonic activity in NW Switzerland is mainly given by
the observed earthquake activity and geodetic measurements. However, low
deformation rates and the corresponding relatively low seismicity make the
definition of the present-day state of stress and the localisation of deformation
difficult. The missing sediment record between Middle Miocene and Middle
Pliocene times, as well as the difficulty in dating the existing Miocene and Plio/Pleistocene sediments further impede the characterisation of the youngest
deformation in this area.
We present results of a study which employs seismological and
geomorphological data to extend the knowledge of recent tectonic activity in
NW Switzerland. Earthquake fault-plane solutions and the spatial distribution of
hypocentres are analysed in order to determine areas characterized by different
styles of deformation and to find evidence of actively deforming structures. The
faulting styles derived from fault-plane solutions suggest that the regional stress
regime changes from E to W. In addition to these „natural“ earthquakes, we
studied the fault-plane solutions of a cluster of earthquakes induced by fluid
injections related to a geothermal energy project (http://www.geothermal.ch/).
Even though these events were triggered, they allow conclusions to be drawn
about the state of stress in the Rhine Graben. The record of triggered
earthquakes points to a local maximum stress direction of SE-NW, as has also
been determined for this region by stress inversion based on the fault plane
solutions of natural earthquakes.
The morphology of fluvial terraces and the characteristics of river longitudinal
profiles can both be helpful to detect accumulated local or regional deformation.
The Pleistocene sedimentary terraces of the river Aare provide marker horizons
which are able to reveal potential young deformations in this region. They are
defined and analysed using a high-resolution digital terrain model (2m/pixel) in a
GIS. In order to also study the effect of spatially varying uplift rates on rivers
without a terrace system, we quantify the variations of the river morphology
(longitudinal profile) on the basis of a 25m resolution digital elevation model.
The resulting pattern of physiographic indicators (steepness and concavity
index) suggests ongoing subsidence of the Upper Rhine Graben as well as
uplift in the Folded Jura.