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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
323
DOI: 10.4408/IJEGE.2013-06.B-30
USING SATELLITE RADAR IMAGES TO INVESTIGATE
THE INFLUENCE OF TECTONIC SETTING ON LARGE SLOPE INSTABILITIES
A
nnA
DE AGOSTInI
(*)
, M
ASSIMO
BARBIERI
(**)
, A
lESSIO
CANTONE
(*)
, M
ARcO
DEFIlIPPI
(**)
,
l
OREnzO
FlAIM
(*)
, M
ARIO
FlORIS
(*)
, l
ucA
GAnDOlFO
(*)
, R
InAlDO
GEnEVOIS
(*)
,
P
AOlO
PASQuAlI
(**)
& P
AOlO
RIccARDI
(**)
(*)
University of Padua - Department of Geosciences - Via Gradenigo, 6 - 35131 Padova, Italy
(**)
Sarmap S.A., Cascine di Barico, 10 - 6989 Purasca, Switzerland
INTRODUCTION
Nowadays, many paper were published regarding
the relevance of InSAR methods in landslides charac-
terisation (H
IllEy
et alii, 2004; c
OlESAnTI
& W
ASOWSkI
,
2006; W
ASOWSkI
et alii, 2008; c
AScInI
et alii, 2012).
Differential Interferometric SAR techniques (DInSAR),
are a good approach to detect and characterise instability
phenomena, because the use of a multi acquisition stack
of images lets to reduce the limitations typical of SAR
Interferometry (i.e. atmospheric distortions or temporal
de-correlation). Using these multi temporal techniques
allows not only the estimation of discrete deformation
events, but also the evolution of displacements.
At regional scale DInSAR techniques allow de-
tecting new landslides and improving the delimita-
tion of landslide-prone areas. At local scale, DInSAR
methods are an useful tool to rapidly recognise por-
tions of unstable area with different displacement rates
or to create a twenty years long displacement velocity
dataset, where a complete field monitor is lacking.
The main aim of this paper is to verify if PS and
SBAS data are able to detect unstable areas and to record
displacement rates and directions, in order to define the
influence of tectonic setting on landslide evolution.
PREZZO LANDSLIDE
The study area is located in the Giudicarie Val-
ley (south-west sector of the Trentino-Alto Adige Re-
gion, NE Italy) were many instability phenomena are
present. The paper is focus on Prezzo landslides, which
ABSTRACT
The paper focuses on identification of tectonic
setting on large slope instabilities in the area of Giu-
dicarie Valley, located in the south-western sector of
the Trentino-Alto Adige region (northern Italy), with
particular reference to those phenomena inducing
a high level of risk. The study area, located in the
Southern Alps, shows a complex structural setting,
mainly influenced by the south Giudicarie Line and
its associated steep to subvertical faults WNW-dip-
ping. Tectonic setting strongly influences the evolu-
tion of slopes, it may be considered one of the main
cause of the onset of current instability phenomena.
Geological, geomorphological, structural and geo-
mechanical field investigations were conducted on
Prezzo landslide area (Giudicarie Valley) in order
to investigate the relationship between the tectonic
setting and the instability. Advanced DInSAR tech-
niques, such as Permanent Scatterers (PS) and Small
Baseline Subset (SBAS) algorithms, were applied to
ERS 1-2 (1992-2010) and ENVISAT (2003-2010)
satellite RADAR images, to detected displacement
areas related to geological structural elements which
could influence the Prezzo landslide development.
Results show that PS and SBAS data interpretation
is an helpful tools in investigating the influence of
tectonic settings on landslide processes.
K
ey
words
: DInSAR, tectonic setting, landslides
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A. DE AGOSTINI, M. BARBIERI, A. CANTONE, M. DEFILIPPI, L. FLAIM, M.FLORIS, L. GANDOLFO, R. GENEVOIS, P. PASQUALI & P. RICCARDI
324
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
The sliding surface depth is between 40 and 110 m,
with a maximum thickness in the middle sector of the
landslide. The total volume is 20 million m
3
.
The houses of Prezzo Village and the county road
show deformations and splits whereas water supply
network presents losses and damages. The phenom-
enon is monitored by Trento Province Geologic Serv-
ice through GPS, topographic stations, piezometers
and inclinometers. Additional field investigations
were carry out in order to better define the geologic
and geomorphologic settings of the unstable area.
DEM derived and LIDAR data were also used.
GEOLOGICAL AND STRUCTURAL SET-
TINGS
The geology of the Prezzo area is characterized by
Triassic marine and coastal deposits (Fig. 2). The Angolo
Limestone Formation is composed of flat-lying dark-grey
fine limestone, marly limestone and siltstone. This For-
mation outcrop in the north flank of landslide. The Prezzo
Limestone, is composed of black marly limestone with
involves the medium-basal part of eastern flank of Mt.
Melino. It is located between an altitude of 510 and 980
m and presents one main scarp on the top, with related
morphologic structures (such as cracks and trenches)
and two minor scarps on right and left boundary (Fig.
1). The phenomenon is an old landslide, reactivated,
that involves silty deposits, with blocks of marly-cal-
careous and dolomitic rocks. The unstable area is 380
m wide and 1150 m long with average slope of 20°.
Fig. 1 - Location of study area
Fig. 2 - Geological map and sections of Prezzo area (modified from B
argossi
et alii, 2012)
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USING SATELLITE RADAR IMAGES TO INVESTIGATE THE INFLUENCE OF TECTONIC SETTING ON LARGE SLOPE INSTABILITIES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
325
slope instability phenomena, sometimes evolving into
catastrophic landslides (P
ERnA
, 1974; B
ASSETTI
, 1997).
The study area is tectonically bounded to the South
by the South Giudicarie Line, whereas a NE-SW trend-
ing fault, probably associated to the South Giudicarie
Line, limits the area to the North. The Triassic succes-
sion of the Prezzo area is folded forming a synclinal-
type fold. The northern sector shows a moderate (30°-
45°) SE dipping stratification, while in the southern one
the beds dip towards N-NE with medium angle (35°-
55°). The fold axis, which is located in the central part
of the slope, gently dips towards E-NE (Fig. 3).
Geomechanical surveys were carried out to investi-
gate the mechanical behaviour of the rock masses inside
and outside the area affected by the slope instability .
Besides the stratification, other three sets of mechani-
cal discontinuities cut the rock masses. Sets K1 and
K2 steeply (70°÷75°) dip respectively towards SW and
W, while K3 is linked to the southern stratification, but
showing a higher angle (65°÷70°) (Figg. 4a and 4b).
The increase of fracturing is related to an expectable
lowering of the rock masses quality of all the forma-
tions from the areas outside to those inside the landslide
body. Geomechanical surveys have also provided two
centimetric clayey marls intercalations. Buchenstein For-
mation and Wengen Formation are present in the lower
part of landslide and dipping towards N-NE; the first one
is composed of dolostone, limestone, marlstone inter-
bedded with volcanoclastic deposits and chert nodules,
the second one is a silicoclastic-carbonatic unit made of
sandstone and marlstone interbedded with tuff layers.
The presence for long periods of Pleistocene broad
glacial areas strongly influenced the characteristics of
the Quaternary sediments. The slopes of Giudicarie
Valley are mostly covered by glacial deposits and more
rarely by moraine. The valley bottom is covered with
large alluvial fans that rest on the flat alluvial plains.
The tectonic regional setting is characterized by
the presence of the Giudicarie Fault which represents
the tectonic boundary between the Southern Alps and
the Austroalpine domain. Due to its location, oblique
to the Southern Alps chain, the Giudicarie belt shows
a very complex structural setting, mainly character-
ized by steep to subvertical WNW-dipping transpres-
sive faults and moderate dipping N to NNE-trending
reverse faults (D
OGlIOnI
& B
OSEllInI
, 1987; P
IcOTTI
et
alii, 1995). This complex structural setting influences
the evolution of slopes, causing the onset of large scale
Fig. 3 - Tectonic and geomorphologic settings of Prezzo unstable area
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A. DE AGOSTINI, M. BARBIERI, A. CANTONE, M. DEFILIPPI, L. FLAIM, M.FLORIS, L. GANDOLFO, R. GENEVOIS, P. PASQUALI & P. RICCARDI
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
important parameters to classify the mechanical behav-
iour of rock masses in terms of both Joint Compressive
Strength (JCS) and Geological Strength Index (GSI)
(D
EERE
& M
IllER
, 1966; M
ARInOS
& H
OEk
, 2000).
The values of JCS, (Fig. 4c) show how the rock mass
strength decreases starting from the stable areas of the
slope, to the inside of the landslide body. GSI values
highlight the weakness behaviour or rock masses in-
side the landslide body and in the areas approaching the
fold axis (Fig. 5). The presence of a rock masses highly
fractured is also confirmed by the boreholes executed
by Geological Service (data not reported in this paper).
The Angolo Limestone shows, inside the landslide
body , average values of JCS around 25÷35 MPa and
GSI value of 35, whereas, far from it, the JCS increases
up to 70÷130 MPa and the GSI is around 45-50, both
in the northern and southern limb. More northerly, this
trend is not observed and both JCS and GSI decrease,
probably due to the proximity to the NE-SW trending
fault. This trend could not be assessed for the other for-
mations due to the lack of outcrops.
FIELD DATA
Several field instruments were installed on Prezzo
landslide area and 13 boreholes were executed by lo-
cal Geological Service, to improve the knowledge
Fig. 4 - Stereoplots of geomechanical discontinuities outside (a) and inside (b) the landslide body (S(x) stratification K(x) sec-
ondary discontinuities). JCS data outside and inside landslide body (c)
Fig. 5 - GSI Values outside and inside the landslide body
Fig. 6 - Location of field instruments install by Trento
Geological Service
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USING SATELLITE RADAR IMAGES TO INVESTIGATE THE INFLUENCE OF TECTONIC SETTING ON LARGE SLOPE INSTABILITIES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
327
response of objects with time, cause by atmospheric
phenomena or anthropic changes or vegetation growth.
Satellite look angle (θ) of 23.3° (for ERS and ENVISAT
images) and right side-looking acquisitions mode are
the responsible for geometric distortions effects. Layo-
ver effect is present where slopes inclination exceed the
look angle, producing a strong image distortions, which
prevent the correct signal interpretation. Shadow effect
is present in areas that are not illuminated by the radar
signal. These effects in relationship with aspect and in-
clination of slopes need to be taken in account before
starting an investigation of mountainous area, because
slope instability processes could be on area affected by
layover or shadow effects.
Another important limit of DInSAR approach in
the study of landslide is the measurements of displace-
ments along the Line of Sight (LOS). Therefore, if the
direction of displacements is not along LOS, the satel-
lite will measure only a part of real velocity.
INTERFEROMETRIC ANALYSIS
To analyze the influence of structural setting on the
evolution of slopes, the Earth Observation data (provided
by European Space Agency, ESA) were used. Advanced
DInSAR techniques, Permanent Scatterers (PS) and
Small Baseline Subset (SBAS) algorithms, were applied
to ERS 1-2 (1992-2010) and ENVISAT (2003-2010) sat-
ellite RADAR images to detected displaced areas.
The Prezzo area is affected by geometrical distortion
(layover effect) in descending acquisition mode. Moreo-
ver, the ERS ascending dataset has many corrupted and
unprocessable images. So the obtained displacement
velocity dataset is provided by 43 ENVISAT images
acquired in ascending mode, spanned from November
2004 to September 2010, with revisiting time of 35 days.
The PS density is high over the Prezzo village,
where the houses represent a good Persistent Scatterers
(122 PS were found on an area of 0.35 km
2
), but outside
the village, where trees and grass land prevailed, the PS
density is very low (coherence threshold = 0.6). Also
the Distributed Scatterers of the SBAS analysis are lo-
cated on the Prezzo village. Both, PS and SBAS data
distributions, show a clear identification of the unstable
area on Prezzo village (Fig. 7).
In Figure 7, orange points show the maximum rate
of movement detected by the two techniques (more than
10 mm/year with a maximum of 20 mm/year for the
PS data) , whereas the green ones identify a stable area,
of geology of the unstable slope and monitoring the
unstability (Fig. 6). There are 14 GPS point (some
of them since 1995 and other since 2005), 10 incli-
nometers since 2002 and 4 piezometers since 2004.
GPS network reveals a deformation velocity of 3 to 6
cm/y. The inclinometers show a sliding surface depth
between a minimum of 27 m at the base of slope and
a maximum of 110 m in the middle part of landslide.
Rainfall data from 2004 is available on the website
of Trento Province weather forecast. But this dataset
isn’t complete because of some missing or not vali-
date data in 2005, 2006, 2008 and 2009. However we
compared the displacement velocity data from DInSAR
processing with the rainfall data to find correlation be-
tween rainfalls regime and displacement of landslide.
DIFFERENTIAL INSAR METODOLOGY
Interferometric phase (phase difference between
two SAR images) is the sum of many contributions:
the topographic one is associated to the relationship
between phase and topography, the displacement one
is due to the land movements (subsidence, landslides,
earthquake), the atmospheric one is due to the variation
of atmosphere characteristics, and the noise one due to
instrumental properties. Comparing two SAR images
is possible exploit the phase contribution due to land
movements, knowing the other phase components and
subtracting them from the total phase difference.
The advanced DInSAR techniques include Small
Baseline Subset (SBAS) (B
ERARDInO
et alii, 2002) and
Persistent Scatterers (PSInSAR) (F
ERRETTI
et alii, 2001)
algorithms. These approaches involve the use of multi-
ple acquisitions stacks (large SAR temporal series). The
concept of combining InSAR information from a large
number of SAR images (thanks to repeated satellite pass-
es on the same area), allowing the solution of a deforma-
tion time series, has been introduced by F
ERRETTI
et alii
(2000), B
ERARDInO
et alii (2002), l
AnARI
et alii (2004) and
l
AuknES
(2004). Using a multi temporal technique allows
to estimate both, discrete deformation events and evolu-
tion of displacements. SBAS method exploits the spatially
distributed coherence, instead of estimating the coherence
exclusively on local scatterers. In fact, PS methods is fo-
cus on the analysis of point targets, instead SBAS one is
intended to the analysis of distributed targets.
The main limits of InSAR techniques are tempo-
ral decorrelation and geometrical distortions. Temporal
decorrelation is due to changes of electro-magnetic
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A. DE AGOSTINI, M. BARBIERI, A. CANTONE, M. DEFILIPPI, L. FLAIM, M.FLORIS, L. GANDOLFO, R. GENEVOIS, P. PASQUALI & P. RICCARDI
328
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
with displacements rate close to zero mm/year (Line of
Sight displacements). SBAS processing shows displace-
ment velocity values similar to the PS one, although the
SBAS values are lower than the PS ones. Furthermore,
the direction of displacements is towards the direction of
the fold axis and of maximum slope (E-NE), and its sub-
stantially parallel to LOS of ascending satellite track, so
the real displacement velocity is substantially recorded.
Figure 8 shows the LOS displacements for two
PS: PS 290 is located in the middle of unstable zone
and shows a high annual velocity rate, while the PS
248 has a low annual velocity rate and it is located in
the stable area on the southern part of landslide. Also
the monthly rainfalls, from 2004 to 2010 are reported
in figure 8 to describe the correlation between main
rainy periods and main displacements of the landslide.
In fact, a more in depth study of the rainfalls dataset
evidences that the main displacement periods (indi-
cate with pink arrows) follow the peaks in the rainfall
trend (blue continuous line in Fig. 8).
DISCUSSION AND CONCLUSIONS
The Prezzo landslide is an example of how a
complex structural setting influences the gravitational
evolution of slopes (Fig. 2 and Fig. 3). The structural
analysis shows the role played by the syncline fold as
the main predisposing factor of the slope instability,
thanks to its E-NE trending fold axis oriented favour-
ably for wedge failure. In fact, the lower values of the
JCS and GSI index inside the landslide body and near
the syncline axis point out the great fracturing and
weakness of the rock masses (Fig. 4 and Fig. 5).The
advanced DInSAR approach allowed to improve
the identification of the unstable area at Prezzo vil-
Fig. 7 - PS and SBAS data on Prezzo landslide area
Fig. 8 - LOS displacement value for a red point (PS 290)
and for a green one (PS 248)(see Fig.7). Rainfall
data are plotted to investigate relationship between
rainfalls regime and main displacement rates
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USING SATELLITE RADAR IMAGES TO INVESTIGATE THE INFLUENCE OF TECTONIC SETTING ON LARGE SLOPE INSTABILITIES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
329
instability. It is important to notice that comparison be-
tween field displacement and PS or SBAS data is not
easy due to the different acquisition geometry (LOS
vs. flow direction) and the different temporal coverage.
But, thanks to E-NE directions of landslide displace-
ments, essentially parallel to ascending LOS, the Prezzo
landslide is a good test area to study the applicability of
interferometric data on landslide study. It follows that
PS, SBAS and GPS numeric data slightly differs from
each other, but we notice that each methods clearly
identify the active and stable zones. Moreover because
the flow direction is determined by the syncline axis
direction, we can use only the ascending LOS velocity
data. Otherwise, both, ascending and descending im-
ages should be needed to exploit displacement vectors.
Regarding DInSAR techniques, PS and SBAS
data interpretation jointly with geologic, geomorpho-
logic and structural analyses, and with field investiga-
tions (e.g. GPS displacement data), are helpful tools
in landslide identification and characterization. The
twenty years of interferometric data allow the defi-
nition of a long displacements dataset permitting to
retrace the history of land movements, also when a
lage with respect to the GPS measurement. The latter
are distributed only along the northern and southern
side of the landslide body allowing a first identifica-
tion of the landslide area. The PS and SBAS data are
distributed on the overall Prezzo village, permitting
a better definition of the distribution of activity. As a
consequences the integrations of PS, SBAS, GPS and
geologic/geomorphologic data well define the sharp
separation between the active and the stable zones,
improving the identification of different displacement
areas of the two landslide sectors (Fig. 7 and Fig. 9).
Furthermore, the displacement velocity dataset
from 2004 to 2010 obtained with the PS and SBAS
processing ENVISAT images, allowed to determine the
annual displacements rate and to compare it with the
GPS data (Fig. 9). In fact GPS maximum annual rate of
movement (2004-2010 time interval) is 3 cm/y, for PS
is 2,1 cm/y and for SBAS is 1 cm/y.
So, the conclusions regarding the data suggest that
the field and interferometric analyses indicate an active
central area, along the fold axis, whereas the southern
sector of the landslide body is quite stable, suggesting
again an influence of the structural setting on the slope
Fig. 9 - Correlation between GPS and PS data. The direction of the arrows corresponds to the landslide flow direction; the
length is intensity of the displacement. The light yellow points represent the GPS which velocity is zero
background image
A. DE AGOSTINI, M. BARBIERI, A. CANTONE, M. DEFILIPPI, L. FLAIM, M.FLORIS, L. GANDOLFO, R. GENEVOIS, P. PASQUALI & P. RICCARDI
330
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
gering factors of movements; whereas the high weak-
ness of rock masses (low GSI and JCS near the fold
axis), due to the complex tectonic setting, is the most
likely the predisposing factor.
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monitor system is lacking or incomplete.
Finally, we also compare main rainy periods with
main displacements periods pointing out, in a qualita-
tive way, that rainfall could be considered as the trig-
Statistics