This study proposes a new concept for upscaling local information
on failure surfaces derived from geophysical data, in order to develop
the spatial information and quickly estimate the magnitude and intensity
of a landslide. A new vision of seismic interpretation on landslides
is also demonstrated by taking into account basic geomorphic information
with a numeric method based on the Sloping Local Base Level (SLBL).
The SLBL is a generalization of the base level defined in geomorphology
applied to landslides, and allows the calculation of the potential
geometry of the landslide failure surface. This approach was applied
to a large scale landslide formed mainly in gypsum and situated in
a former glacial valley along the Rhone within the Western European
Alps. Previous studies identified the existence of two sliding surfaces
that may continue below the level of the valley. In this study. seismic
refraction-reflexion surveys were carried out to verify the existence
of these failure surfaces. The analysis of the seismic data provides
a four-layer model where three velocity layers (<1000 ms(-1), 1500
ms(-1) and 3000 ms(-1)) are interpreted as the mobilized mass at
different weathering levels and compaction. The highest velocity
layer (>4000 ms(-1)) with a maximum depth of similar to 58 m is interpreted
as the stable anhydrite bedrock. Two failure surfaces were interpreted
from the seismic surveys: an upper failure and a much deeper one
(respectively 25 and 50 m deep). The upper failure surface depth
deduced from geophysics is slightly different from the results obtained
using the SLBL, and the deeper failure surface depth calculated with
the SLBL method is underestimated in comparison with the geophysical
interpretations. Optimal results were therefore obtained by including
the seismic data in the SLBL calculations according to the geomorphic
limits of the landslide (maximal volume of mobilized mass = 7.5 x
10(6) m(3)).