D-BAUG - Departement Bau, Umwelt und Geomatik - ETH Zürich

Highlights ▪ Geo-, Structural- and Environmental Data
Gornergletscher and Gornersee Simulation of geodynamic processes
‘Icequakes’: Seismic activity on Gornerglet-
scher during Gornersee outburst floods.
During the summers 2004, 2005, 2006 and 2007, the Laboratory
of Hydraulics, Hydrology and Glaciology (VAW) installed
high-density seismometer networks on Gornergletscher
in order to study the yearly drainages of the
nearby ice-marginal lake Gornersee. Among the record of
the nearly 200,000 'icequakes' most seismic events are
consequences of crevasse opening near the surface.This is
supported by moment tensor inversions which show that
source mechanisms are generally tensile dislocations.
However, we also identified about one dozen of near-surface
icequakes whose moment tensors represent solid evidence
for shear fracturing within alpine glacier ice. Icequakes
near the glacier bed or at intermediate depths
make up only a small fraction of the recorded events.Their
source mechanisms are generally tensile dislocations, as indicated
by a detailed analysis of several icequake clusters.
Qualitative waveform characteristics indicate that other
basal icequake sources may also exist.
Subglacial water pressure beneath Gornergletscher is
strongly influenced by the daily surface melt cycle and the
drainage of Gornersee.We analyzed how subglacial water
pressure influences basal fracturing by comparing the activity
of basal icequakes to fluctuations in the water table
in boreholes drilled to the glacier bed. The results show
that basal icequakes occur mainly during low or falling
water pressure periods (Fig. 1). This indicates that basal
fracturing occurs when the glacier couples to its bed after
periods of water-enhanced basal sliding.
66 ▪ D-BAUG Annual Report 2009
Geodetic measurements provide impor-
tant boundary conditions for the simula-
tion of geodynamic processes.
by Fabian Walter / VAW by M.D. Müller, A. Geiger, H.-G. Kahle / IGP
The understanding of geodynamic processes such as the
deformation of the Earth's crust along plate boundaries is
of great importance for seismic hazard analysis. Stress accumulation
in the lithosphere and related deformations
depend on a multitude of geophysical parameters. Many of
them are not directly measurable and therefore only approximately
determined.
Numerical simulation enables the reverse engineering of
geodynamic processes and their calculation for a well-defined
parameter range. A comparison of developed models
and used parameters with geodetic and geologic observations
yields a plausibility assessment.
For the seismically very active region of the Eastern
Mediterranean and especially for Greece GPS measurements
provide detailed information about recent crustal
deformations. Additionally, seismic and geologic observations
are available. Using the Finite Elements method it is
attempted to develop tectonic models in such a way that
their kinematic patterns concur with existing observations.
First models incorporate only the most active seismotectonic
structures.They yield promising results and are going
to be improved in an iterative process (Fig. 2, 3).