Geophysical investigations of large landslides in the Carnic Region of southern Austria

The area under investigation for the past two decades is in the vicinity of the Gailtal lineament, which is the most dominant tectonic feature of the eastern Alps of southern Austria. An area of about 8 km(2) is in a state of constant instability, as documented by movement of road tracks of several centimetres per year. Geotechnical and surveying techniques have been used to measure these movements in the past but without solving the problem of t he mechanism of these failure processes. Geophysical methods (seismic refra ction, geoelectrics, and electromagnetics) were applied in order to determi ne the validity of one of the discussed movement models. In-situ velocity m easurements were used to identify different lithologies beneath surficial t alus deposits. The thickness of these talus deposits, of about 4-30 m, foun d by seismic refraction clearly demonstrates that huge 'blocks' (i.e. more or less undisturbed lithologic units) within the talus/debris ale in close contact with the basement. This basement, which shows lower seismic velocit ies in different Darts combined with low electric resistivities, is obvious ly strongly disturbed by different failure surfaces. The different gliding velocity of the blocks and the talus/debris deposits leads to a geological model in which huge rock blocks move slowly in relation to the disintegrati ng basement, whereas the talus/debris deposits move over the surface of the se blocks at a higher velocity. The interpretation of these landslide studi es is not a straightforward analysis. It is a complex problem with a comple x solution, including all information from geotechnical, geophysical, and s urveying investigations. (C) 2000 Elsevier Science B.V. All rights reserved .