| Methods and Results
|WHAT DO WE KNOW ?
Compared with other
ground failure phenomena like liquefaction, earthquake-induced landslides
have been studied relatively little. This can be explained by several
The usual procedure
for assessing the landslide risk is to locate the known landslides,
to define the major factors controlling the slope failures, and
to extrapolate the results to the rest of the area. Landslide distribution
maps are commonly drawn from aerial photo interpretation, remote
sensing analysis and geological observations. The basic factors
influencing landslide distribution are slopes, geology, hydrogeology,
anthropological factors (land use) and seismic ground motion. A
detailed analysis of individual landslides is necessary to check
and to specify the main factors before the results can be applied
to the whole area.
Landslides are caused
by a large number of processes and come in a wide variety
of forms and types of movement (rockfalls, rockslides, earth
slides, earth flows...).
Slope failures depend
on both aseismic factors (geology, groundwater conditions,
topography, and human activity) and earthquake parameters
(amplitude and duration of strong ground motion), making the
problem very complex.
by earthquakes do not necessarily occur during the shaking.
In fact, the ground movements can occur quite some time after
the seismic activity.
landslides frequently occur in terrain already affected by
static slope failures.
with a 3D model
Remote sensing imagery
provides synthetic views of the studied areas at different scales.
These views are the best tools for studying large tectonic events
combined with individual slope movements. Their processing and interpretation
are combined with the study of seismic and topographic data in digital
format in a Geographic Information System (GIS).
The topography is
also represented here in digital format files known as Digital Elevation
Models (DEMs), that can be used to compute slopes, shaded relief
images and three-dimensional views as block diagrams.
Example of Issik Kul basin and ranges, Resurs
1 image draped on a DEM
|On land scale
A picture of the land
The whole country
is covered by RESURS 1 multispectral data at 160 m ground resolution.
This allows the publication of colour composites as 1:1,000 000
and 1/500, 000 scale image maps. These image maps are useful for
defining the general geologic and structural context and plotting
all the observations.
Landsat MSS images provide an intermediate ground resolution of
57 m; they have been published as 1/200 000-scale colour composites.
The different images
are combined to enhance compositional contrasts, such as in basement
lithology, weathering products and alluvial deposits, and detection
of active or passive geological structures.
RESURS 1 colour composite of Kyrgyzstan,
studied sites (Courtesy of Novosibirsk Regional Center of Geoinformation
Technology, Russian Academy of Sciences, Siberian Branch)
North of lake Issik Kul, Landsat MSS
© U.S. Geological Survey
On regional scale
A picture of the area: Chon Kemin
The geological structures
and landslides of the Chon Kemin region are best expressed on SPOT
panchromatic imagery at 10m ground resolution.
1/50 000 DEMs of Chon
Kemin surface rupture area, rockslides and soft sediment landslides
associated with the earthquake of 1911 have been processed together
with SPOT P imagery and a historical map of surface ruptures and
The regional scale
GIS includes digital geological maps that provide structural and
lithological information and will contribute to the geotechnical
definition of rock bodies. Slope maps derived from medium-scale
DEMs will be included in the risk assessment.
SPOT mosaic of the Chon Kemin river area, structural and morphologic
from archive and field observations. The red lines represent structures
the blue ones are landslides in soft sediments. ©CNES, Distribution
Slope map of Suusamyr area
On local scale
A picture of one detail: Bielogorka
A DEM of Bielogorka rockslides has been built
by interpolating the topographic contours from a 1/25000-scale map.
The results allowed the construction of block diagrams where the
orthorectified aerial photo was draped on the cartographic DEM.
These data will be used with a DEM of the
area before the landslides to reconstruct the geometry of the events.
Perspective view of the Bielogorka landslides
for field work
of the existing landslides has been performed to provide detailed
information on the geometry of the landslides, the kind of movement
(fall, slide, topple, lateral spread, bulging, etc.) and the direction
of motion and height of the displaced soil. This information will
be used for a comprehensive classification of landslides, which
will allow comparison and modelling of the landslide map with the
geological, structural and neotectonic context.
investigations were performed at two sites in the northern Tian
Shan. Inverting electrical tomography profiles gave detailed information
on the geometry of the near surface geology (up to a depth of 30m).
The aim of the
project is to compare the landslide distribution map with all the
others, including the topographical and geological maps of the region.
From this analysis, a correlation will be made between landslide
location and the different factors (geology, slope, hydrogeology,
peak acceleration, etc.).
From these results and the detailed study
of specific slope failures, the key parameters controlling the landslide
occurrence in the Tian Shan region will be pointed out and the landslide
risk will be evaluated by extrapolating the results to the whole
area. The final output will be a landslide susceptibility map, which
will be of direct use for planners.
Though this study is limited to the Republic
of Kyrgyzstan, many areas of the earth present problems of landslides
triggered by earthquake and can be studied by similar techniques.
Distribution of recent earthquakes in 1999
(courtesy of USGS)