Satellite SAR interferometry for wide-area slope hazard detection and site-specific monitoring of slow landslides

Data acquired by satellite-borne Synthetic Aperture Radar (SAR) systems can be profitably used for the detection and quantification of slow mass movements, provided that the interferometric analysis and data interpretation is guided by field knowledge and comprehension of slope failure/ground deformation mechanisms. The Permanent Scatterers (PS) technique, which overcomes several limitations of conventional SAR differential interferometry (DInSAR) applications in slope instability studies, is capable to generate high precision ground displacement data which can be integrated with landslide inventory maps for both wide-area and site-specific hazard assessment. On an individual slope scale, where sufficient ground truth may be available, this kind of information can potentially represent a valuable quantitative input for warning purposes. To demonstrate the operational applicability of PS interferometry we present the results from a 9year monitoring of a large landslide complex in the Liechtenstein Alps. The study area is situated in the westernmost part of the Eastern Alps, near the main Alpine suture zone developed along the Austroalpine/Penninic boundary. The local tectono-stratigraphic relations in the landslide area are shown in Figure 2. The bedrock of the middle-lower slopes is constituted by Late Cretaceous and Late Cretaceous-Earliest Tertiary flysch units known as Vaduzer and Triesen Flysch (Allemann, 2002). The upper slopes contain deformed units arranged in tectonic nappes. A variety of rocks is present including Perm-Trias age sandstones, undifferentiated flysch and breccias, and Late Cretaceous flysch and chalks. The bedding is irregular, but counter-slope dips predominate. A vast hillslope area is mantled by Quaternary age superficial deposits, which include considerable amounts of coarse materials of rock fall and rock slide origin. Talus materials are present in the upper slope area, at the base of steep rock scarps. Moraine deposits crop out extensively along the middle slopes of the valley. Finally, large alluvial fans are present at the slope base near the northern and southern lateral margins of the Triesenberg-Triesen landslide. 2.2 The Triesenberg-Triesen landslide The Triesenberg-Triesen hillslope is covered by a large landslide complex (Figs 1, 2), which has an area of approximately 4.2 km, the depth on the order of 100 m and estimated volume of 500 million cubic meters (Allemann, 2002). The origin of the slide may date back to the retreat of the Rhine valley glacier, which took place over 10,000 years ago. Today the toe of the landslide complex is distant a few hundred meters from the Rhine River and results unaffected by fluvial erosion. The overall landslide slope is around 18.5°, but there are several local slope breaks (Fig. 2). Two main streams follow lateral flanks of the landslide, whereas the surface water drainage within the slide mass is irregular. There appear to be several mass movements superimposed on the main slide mass and this adds some complexity to the mechanism of motion. Nevertheless, the inferred basal slip geometry indicates that the movements are predominantly translational (Fig. 2). There are several evidences of the ongoing activity of the landslide. These include the documented damage to the roads and to several buildings, especially those located in Triesenberg. Furthermore, surface movements were measured during the topographic campaigns in the 1980’s and the GPS surveys in 1990’s (Frommelt AG, 1996). The inclinometer monitoring demonstrated the presence of deformations occurring at depths varying from several to about 20 m (GEOTEST AG, 1997). All the ground control data indicate the occurrence of displacements with average velocities between 1 and 4 cm/yr. 3 SPACE-BORNE SAR INTERFEROMETRY