Kaspar Merz

Rock Glacier Degradation and Instabilities in the European Alps: A Characterisation and Monitoring Experiment in the Turtmanntal, CH

Global climate change is impacting sensitive alpine cryogenic regions, through slope instabilities in rocks and soils. Significant temperature increase at the air-ground surface interface may be accompanied by increased rainfall, more extreme storms and additional severe rise in mean global temperatures in the coming decades, enhancing risk of mass movement hazards to human life and infrastructure. Rock glaciers and degrading permafrost on steep Alpine slopes are particularly susceptible to warming and phase change in either massive or interstitial ground ice, which may lead to release of water, accelerated motions, initiation of landslides and instabilities. Accumulated failure in soil elements, determined on artificial frozen specimens of rock glacier materials at temperatures below 0 °C, is linked to these processes at field scale. A geophysical and geotechnical field characterisation and monitoring experiment is being conducted on a rock glacier that is undergoing thermally induced creep and growth of thermokarst. Preliminary investigations are described in this contribution.

A new 3‐D thin‐skinned rock glacier model based on helicopter GPR results from the Swiss Alps

Mountainous locations and steep rugged surfaces covered by boulders and other loose debris are the main reasons why rock glaciers are among the most challenging geological features to investigate using ground-based geophysical methods. Consequently, geophysical surveys of rock glaciers have only ever involved recording data along sparse lines. To address this issue, we acquired quasi-3-D ground-penetrating radar (GPR) data across a rock glacier in the Swiss Alps using a helicopter-mounted system. Our interpretation of the derived GPR images constrained by borehole information results in a novel “thin-skinned” rock glacier model that explains a concentration of deformation across a principal shear zone (decollement) and faults across which rock glacier lobes are juxtaposed. The new model may be applicable to many rock glaciers worldwide. We suggest that the helicopter GPR method may be useful for 3-D surveying numerous other difficult-to-access mountainous terrains.

Neotectonic fault structures in the Lake Thun area (Switzerland)

Strong historic earthquakes (i.e. intensities I0 ≥ V) 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.

Geophysical Characterization of the Furggwanghorn Rock Glacier, Switzerland

Degradation of alpine permafrost due to changing mean annual air temperatures can act as trigger for landslides and other ground instabilities. For a better understanding of the underlying thermo-hydromechanical processes an interdisciplinary research project has been set up. An extensive geophysical and monitoring campaign was carried out on rock glacier in the Turtmann valley, Canton Valais, Switzerland over the last two years to investigate its internal structure. We employed seismic refraction tomography, electrical resistivity tomography and ground-penetrating radar. Additionally seven boreholes were drilled to a depth of 25m and equipped with temperature sensors and inclinometers. Results from the seismic tomography show a lateral very heterogeneous zone below an active layer of 3-4m thickness. The bedrock depth could not be detected over large parts of the profiles. On the electrical tomograms we can clearly distinguish between ice-free zones at the front and the flanks of the rock glacier and an ice-rich zone in the central part. Several internal shear horizons could be identified on the radar profiles. Most of them could be tracked over several profiles. Deformation measurements in a nearby borehole show that the horizon at about 15m depth is currently active.