Mapping Rainfall-Triggered Slidequakes and Seismic Landslide-Volume Estimation at Heumoes Slope

Applying the method of nanoseismic monitoring in field campaigns between 2005 and 2008 at the Heumoes landslide in the Austrian Alps, we discovered and located fracture processes with seismic local magnitudes (M-L) between -2.2 and -0.7. The creeping Heumoes slope consists of weak sediments (loamy scree and glacial till) and moves an average of a few centimeters per year at the surface. The seismic detection of single fracture processes or initial stress relief resolves the creeping movement of the slope into discrete rupture episodes. The spatial distribution of fractures is concentrated in parts of the slope with higher deformation rates at the surface. The temporal occurrence correlates with rainfall events and reinforces the assumption of a rainfall-triggered slope movement. In addition, we observed weak local earthquakes (M-L similar to 2) with a distance of similar to 10 km to have an influence on fracture generation at the slope. Furthermore, an average thickness of similar to 20 m (and a maximum of >30 m) for the unstable sediment cover of Heumoes slope was determined by refraction seismic techniques along several seismic profiles. The slope's total volume is estimated to be 107 m(3); its mass is approximately 1.6 x 10(7) Mg. The determination of the landslide volume is essential for further hazard assessment of the unstable Heumoes slope.