HR: 14:00h
AN: NS53A-01 INVITED     [Abstracts]
TI: Detecting Fracture and Fault Zone Within an Unstable Mountain Slope in the Swiss Alps
AU: * Green, A
EM: alan@aug.ig.erdw.ethz.ch
AF: ETH Zurich, ETH Zurich Institute of Geophysics, Zurich, CH-8707, Switzerland
AU: Maurer, H
EM: Maurer@aug.ig.erdw.ethz.ch
AF: ETH Zurich, ETH Zurich Institute of Geophysics, Zurich, CH-8707, Switzerland
AU: Spillmann, T
EM: Spillmann@aug.ig.erdw.ethz.ch
AF: ETH Zurich, ETH Zurich Institute of Geophysics, Zurich, CH-8707, Switzerland
AU: Heincke, B
EM: Heincke@aug.ig.erdw.ethz.ch
AF: ETH Zurich, ETH Zurich Institute of Geophysics, Zurich, CH-8707, Switzerland
AB: Risks associated with sudden mountain-slope failures are escalating as a result of (i) rapidly expanding population centers, lifelines, and other critical infrastructure within mountain valleys and (ii) increases in exceptional climatic events and accelerated melting of alpine permafrost due to global warming. Accordingly, there is a need to implement suitable mitigation measures in the form of early warning systems and protective barriers. To design effective barriers, comprehensive knowledge of the locations and volumes of unstable rock is essential. To address these issues, we have been investigating an unstable mountain slope situated above the largest rockslide in recent Swiss history. We have further developed and applied a number of geophysical techniques at the study site. Innovative data processing schemes that included f-x-y deconvolution, topographic migration and semblance migration were required to obtain meaningful images of shallow- to steep-dipping fracture zones from 3-D surface ground-penetrating radar data. Critical additional details on the locations and geometries of the steep-dipping fracture zones were provided by single-hole radar data acquired in three moderately deep boreholes. Tomographic inversions of a comprehensive 3 D seismic refraction data set delineated a huge volume of very low quality crystalline rock with ultra-low to very low P-wave velocities of 500- 2700 m/s. These values were astonishingly low compared to the average horizontal P wave velocity of 5400 m/s determined from laboratory analyses of intact rocks collected at the site. The extremely low velocities extended to more than 35 m depth over a 200 x 150 m area that encompassed an actively moving segment of the mountain slope and a large part of the adjacent stationary slope. Finally, a specially designed microseismic network detected 223 microearthquakes during a 31-month monitoring period. These events, which had moment magnitudes of -2 to 0, were concentrated within 50-100 m of the surface in two zones, one that followed the recent rockslide scarp and one that spanned the volume of highest fracture zone/fault density.
DE: 1835 Hydrogeophysics
DE: 1859 Rocks: physical properties
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly