Monitoring of the large slow Kahrod landslide in Alborz mountain range (Iran) by GPS and SAR interferometry

Engineering Geology 100 (2008) 131–141

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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e n g g e o

Monitoring of the large slow Kahrod landslide in Alborz mountain range (Iran) by GPS and SAR interferometry M. Peyret a,⁎, Y. Djamour b, M. Rizza a, J.-F. Ritz a, J.-E. Hurtrez a, M.A. Goudarzi b, H. Nankali b, J. Chéry a, K. Le Dortz a,1, F. Uri a a b

GM: Université de Montpellier 2, Géosciences Montpellier, CNRS, UMR 5243, 34095 Montpellier, France NCC: National Cartographic Center, Tehran, Iran

A R T I C L E

I N F O

Article history: Received 16 November 2007 Accepted 18 February 2008 Available online 7 June 2008 Keywords: Landslide GPS DInSAR Surface displacement Rainfall Kahrod Alborz Iran

A B S T R A C T In this study, we quantify and analyze the spatial and temporal evolution of the surface displacement of Kahrod landslide located in the center of Alborz range (Iran) within the Haraz valley. This landslide represents a threat for this main drainage axis and its numerous infrastructures. We present three sets of displacement vectors based on GPS technique. An 8-benchmark network has been surveyed four times on a 1-year period basis. It provides accurate information on the rate of displacement within the landslide, and addresses the problem of the mechanical resistance of a small hillock, down slope, under the stress imposed by the landslide. Then, this network is densified (57 marks) and measured twice in 6 months using a rapidstatic approach. This yields to a dense description of surface deformation over the whole landslide. Finally, a 1-year time series of permanent GPS recordings is presented and compared to rainfall. Furthermore, we analyze Envisat radar differential interferograms (DInSAR) spanning the same period as permanent GPS. These geodetic data allow to precisely determine the limits of the current sliding zone and to describe the spatial and temporal evolution of surface displacement. The combination of geodesy and field observations leads to a precise description of the past and present kinematics behavior of Kahrod landslide. The chaotic nature of the sliding mass suggests a first catastrophic landslide in a first episode. During the period of observation, the landslide appears to deform quite steadily, and the evidence of short-term correlation between rainfall and deformation amplitude needs to be confirmed by future measurements. Carrying on the acquisition of GPS and InSAR data within the sliding mass but also within adjacent bedrock should give fundamental information with regards to major activation processes (river sapping, water seeping, earthquakes, or failure within the frontal hill of bedrock) and their potential consequences. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Landslides are superficial processes occurring over a wide variety of spatial and temporal scales in many mountainous regions. Depending on their spatial distribution, and the frequency and magnitude of movement, landslides have various effects on the environment. Studies including evaluation of slope failures location and estimation of their activity are extremely important. Indeed, landslides have been proved to be the dominant erosion process in active mountain range, responsible for the long-term geomorphologic evolution of landscapes (Burbank et al., 1996; Hovius et al., 1997, 2000; Korup, 2005). Moreover, landslides are major destructive natural events striking civilian urban settlements and infrastructures, causing serious damage, loss for life and property worldwide every year. One of the most striking example is that of Iran, where landslides cause several ⁎ Corresponding author. E-mail address: [email protected] (M. Peyret). 1 Now at Université Pierre et Marie Curie — Paris 06, UMR 7072, F-75005 Paris, France. 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.02.013

tens of deaths per year. Natural causes, such as rainfall or earthquake, as well as man-made disturbance are triggering factors for land sliding. Therefore, especially in regions where the risks associated to landslide hazard are high, inventory techniques and surveying methods are strongly needed in order to precisely evaluate the location and the size of slides and to estimate the characteristics of their displacement. Such an accurate inventory of slope instabilities is a prerequisite for analyzing the forces that control the spatial and temporal patterns of slope movement, and for geomorphologic hazard assessments. Surveying of surface movement can be performed with the Global Positioning System (GPS) (e.g. Gili et al., 2000; Malet et al., 2002; Mora et al., 2003) or conventional monitoring techniques, such as theodolites, electronic distance meters, tiltmeters (e.g. Angeli et al., 2000), thus providing accurate results. Perfect for a precise description of landslide activity over a specific site, this conventional monitoring is neither suitable to regional scale investigations, nor to reveal spatial heterogeneities of mass movements. Complementary to point-based measurements obtained from conventional monitoring networks,

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airborne and satellite remote sensing methods offer synoptic views of displacements along active landslides and surrounding terrain (Mantovani et al., 1996). These techniques do not require any previous benchmarking. Moreover, they allow obtaining results in the past epochs when archives exist. Among remote sensing techniques, radar interferometry (InSAR or DInSAR), which is an all weather technique, has proved to be effective to monitor slow ground deformations and has demonstrated its capabilities in landslide monitoring and detection (Gabriel et al., 1989; Massonnet and Feigl, 1998; Squarzoni et al., 2003). Here, we investigate the displacement field of a large (0.7 km2) landslide located near Kahrod village, along the Haraz River, central Alborz, Iran (Fig. 1). Composed with blocks and brecciated sandstone probably created by an initial mountain collapse, the sliding mass is likely to deform due to the sapping of Kahrod River, a small tributary of the Haraz River. A failure of this landslide would cause strong damage to the village (200 inhabitants), to the Haraz road and its numerous infrastructures, and could disrupt Haraz River. Now, it happens that tension cracks affect the conglomerate cover of a small mound of in-situ bedrock, located at the toe of the landslide. Thus, in order to comprehend the mechanical behavior of this landslide, it is essential to precisely quantify the spatial and time distribution of its surface deformation. In order to characterize it, the present work is

based on the combination of geomorphologic field observations and geodetic measurements (GPS and DInSAR). The aims of the work presented in this paper were (i) to determine the spatial limits of the present active Kahrod landslide, (ii) to quantify the spatial distribution of surface deformation by means of GPS and DInSAR techniques (iii), to analyze the time evolution of surface displacement, and (iv) to get a better understanding of the main causes of this slide motion. After a brief reminder of the central Alborz tectonic setting, we first describe the landslide from a geological and morphological point of view. Then, geodetic measurements of surface displacement are presented: at first for determining the spatial distribution of surface displacement, then for analyzing in detail its time evolution. We analyze the influence of rainfall on landslide activity, and finally, we discuss the likeliest causes of activation of the movement and the associated risk for Kahrod and Haraz valleys. 2. Tectonic, geological and morphological characteristics of the Karhod landslide The Kahrod landslide is located in the Alborz mountain range in northern Iran (Fig. 1). This mountain range surrounding the SouthCaspian basin is known for its active and strong tectonic activity with

Fig. 1. Location of Kahrod landslide (yellow star). It is located north of Damavan volcano, on the western side of Haraz valley. This landslide threatens the road that links Tehran to Amol, one of the main communication axes (dashed black lines) across the Alborz range. Main left-lateral strike–slip and thrust faults (red lines) are from Ritz et al. (2006). Focal mechanisms of large earthquakes are from McKenzie (1972), Jackson et al. (2002) and Tatar et al. (2007). Seismicity (M N 4) between 1973 and 2005 (yellow circles), and since 2005 (green circles) is from USGS earthquake catalog. Meizoseismal area of 1957 Sanghechal earthquake (white dashed line) is from Ambraseys and Melville (1982). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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several destructive earthquakes in the past (Berberian and Yeats, 1999). Structural and seismological data for the Alborz show that the deformation is partitioned along range-parallel thrusts and left-lateral strike–slip faults (Jackson et al., 2002; Allen et al., 2003). A recent GPS study shows that south–north shortening across the Alborz occurs at 5 ± 2 mm/yr and that the left-lateral shear across the overall belt has a rate of 4 ± 2 mm/yr (Vernant et al., 2004). However, a slight component of extension is associated to some large strike–slip faults such as the Mosha fault (Ritz et al., 2006). Compiling seismological data and paleo-seismological data show that Alborz can be struck by large earthquakes with magnitude reaching Mw 7 or more and characterized by various fault kinematics, from pure compression on the edges of the belt, to left-lateral strike– slip-normal inside the range (Ritz et al., 2006). These earthquakes are liable to trigger landslides as observed for instance in the Taleghan valley during the 958 historical earthquake (Berberian and Yeats, 1999), or in the vicinity of June 20, 1990, Rudbar earthquake (Shahrivar et al., 2006) and June 22, 2002, Avaj earthquake (Mahdavifar et al., 2006). But not all the landslides observed in the range can be undoubtedly associated with earthquakes. The case of Kahrod landslide is complicated because it represents a mass movement that is clearly reactivated from an earlier landslide phenomenon. It is difficult to tell whether the first landslide was earthquake-triggered but it is clear that this first mass movement has been catastrophic as shown by the avalanche-type rock pile of the collapse material. Indeed, a slow landslide would have likely preserved the original structure of the slid material. On July 2nd, 1957, Kahrod Village has been partially destroyed by Sangechal earthquake (Mw 7.3) which epicenter was located about 40 km to the northeast (Fig. 1). 12 fatalities were recorded in Kahrod and numerous rockfalls affected the epicentral region (Ambraseys and Melville, 1982;

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Tchalenko et al., 2007). However, no specific information can be related to Kahrod landslide. Nevertheless, it is noticeable that the former Kahrod cemetery, located within the southern toe of the landslide, has been abandoned at about that time. A recent track bordering the Kahrod River cuts through this ancient cemetery. This suggests that the Sangechal earthquake probably induced a significant reactivation of Kahrod landslide. Nowadays, the Kahrod landslide is characterized by a slow, seemingly continuous in time, displacement of material made of blocks and brecciated sandstone and shale Jurassic in age. It occurs within the Shemshak sedimentary formation (Fürsich et al., 2006), which exhibits fine to coarse sandstones with some conglomeratic deposits and marl interbedding. The present-day active landslide extends from altitude 1200 m to 1600 m (Fig. 2). The average slope orientation is N110°E with a mean slope of about 25°. Lateral fracture zones, mainly oriented N110°E, clearly delimit its spatial extend almost everywhere. The mapping of the landslide from aerial pictures, digital elevation models (DEM) and field observations shows clearly that the present extension of the landslide is smaller than its initial size, as determined from the scarps surrounding its basin extension (Figs. 2 and 3). In place sandstone beds surrounding the sliding mass are heavily fractured and globally dipping 40° to the South (Fig. 3). These structural surfaces correspond to the landslide surface of rupture on the north-west part of the landslide, but fail to explain its geometry elsewhere. The hypothesis of an initial catastrophic landslide is also supported by a stratigraphic log performed within an alluvial terrace located at the toe of the landslide, in the Kahrod River bed. It displays a sharp thin clay layer (about 20 cm) interbedded between stream deposits. This suggests that a dam has likely been created consecutively to a large landslide. The depth of the shear surface is not obvious to determine on this large and complex site. The extrapolation of lateral scarps geometry

Fig. 2. Perspective view (towards southwest) of a Digital Elevation Model centered on Kahrod landslide. The present deforming area (light pink) stretches from an altitude of 1200 m to 1600 m. The mean slope is about 25°. The Kahrod River undercuts the central toe of the landslide. The northern toe expansion towards Haraz Valley is blocked by a fixed hillock which displays tension cracks within its conglomeratic upper layer. The lateral steep slopes are the morphological mark of an initial catastrophic landslide. The Kahrod village is located just below the southern limit of the landslide. Within the landslide, there is a radio communication station (RTS) where the permanent GPS station is installed.

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Fig. 3. Geomorphologic map of the landslide. Coordinates are in km (UTM projection system, zone 39). The crests surrounding the present sliding zone delimit the initial, likely catastrophic, landslide. The lithology of the Shemshak formation is coarsely dipping 40° to the south. Some sandstone beds seem to be preserved within the landslide, at mid-height. The upper part of the landslide is characterized by a series of plurimetric scarps. The southern toe (A) seems decoupled from the rest of the landslide by 2 fracture zones. The central toe is characterized by a large “niche” of detachment (B) undercut by the Kahrod River. The northern toe expansion towards the Haraz Valley is blocked by a small hillock (heavy black dashed line) which exhibits tension cracks on its southern end.

allows estimating a depth that reaches about 70 m in certain places. Therefore, we estimate the volume of the original destabilized material to about 80 × 106 m3, while the remaining sliding material would be limited to about 15 × 106 m3. The upper part of the landslide (altitude higher than about 1450 m) is made of chaotic material within a marl groundmass (Fig. 4a). Conspicuous lateral fracture zones are oriented N110°E (Fig. 4b). This area is characterized by a series of plurimetric scarps with back-tilted slopes (Fig. 4c), giving evidence of a major rotational component of the landslide. Many tension cracks have also been mapped (Fig. 3). The intermediate zone (altitude approximately ranging from 1350 to 1450 m) exhibits a clear partition between southern and northern parts. The former simply displays a significant slope increase with respect to the upper part of the landslide. Yet, the latter is characterized by the predominance of large chaotic blocks (Fig. 4d). However, some conglomerates and sandstone beds seem to be preserved in structure and orientation within this groundmass. This tends to prove that the sliding mechanism is locally translational. The precise location of the northern fracture zone delimiting the sliding mass becomes more difficult to determine than at higher elevation. The lower part exhibits a large “niche” of detachment in its central part, undercut by the Kahrod River (Figs. 3 and 4e). It seems that the river incision has not reached the surface of rupture yet. This suggests that the present movement is, at least partially, controlled by the sapping of the Kahrod River. Two fracture zones, oriented N100°E, separate this intensely deforming central zone from the southern toe of the landslide, close to the village, which is not affected by Kahrod River (Fig. 3). This discontinuity suggests that the deformation rate for this southern limit is significantly lower, which would mitigate the risk for the Kahrod Village. Lastly, north of the toe undercut by the Kahrod River, the landslide looks locked or slowed down against a piece of bedrock damming the landslide down slope, just on the leftbank of the Haraz River. Large tension cracks (about 5-m high and 20m long, with N100° and N40° orientation) affect the southern end of

this hillock (Figs. 3 and 4f). Thus, the landslide spreads along this abutment towards north (Fig. 4g). From a hydrological point of view, many decanting zones can be found all over the landslide. But very few springs exist, and a single pond has been found on the upper part of the landslide despite the survey occurred after a long-term rainy period (Fig. 3). The sliding mass exhibits a globally high porosity with very limited superficial water content. Thus, Kahrod landslide is a complex one with regards to its history and its spatial heterogeneity. In order to go further in the understanding of the mechanism that controls this landslide, we use GPS and InSAR techniques to quantify the spatial and time evolution of the present surface deformation. 3. Spatial distribution of surface displacement 3.1. GPS analysis 3.1.1. Semi-permanent network In June 2003, 8 GPS benchmarks have been installed on the site of Kahrod (Fig. 5). The sliding zone is instrumented from lower to upper part with 3 monitoring points (K6, K7 and K8). Five other control points have been placed outside the landslide, where no deformation is expected: either far away from the landslide (K5: Kahrod village, K1: Haraz road) or close to the landslide on the small hill that seems to block the landslide expansion towards Haraz valley (K2, K3 and K4). Large fissures reveal the importance of stress over that hillock. It seems to play a major role with respect to the landslide stability and the threat it constitutes for the Haraz valley. All these benchmarks allow for antenna forced centering ensuring millimetric accuracy of the GPS antenna set up. Marks K1, K5, K6, K7 and K8 have been drilled on sandstone blocks while all others are on concrete bases built to overcome the absence of competent boulders. This small GPS network has been measured in June 2003, June 2004, August 2005 and November 2006. We used 8 Trimble 4000SSi

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Fig. 4. Pictures of the landslide. a) Chaotic material within a marl groundmass. b) Lateral fracture zone that delimits the southern limit of the sliding mass, on the upper part of the landslide. c) One scarp close to the head of the landslide. It is about 10 m high and exhibits back-tilted slope. This expresses a local rotational behavior. d) Large chaotic blocks on the northern side, at mid-height of the landslide. e) Facing view of the bottom escarpment, undercut by the Kahrod River, in the central part of the toe. f) Tension cracks observed at the top of the hillock blocking the landslide expansion towards Haraz valley. g) View towards South of the northern toe. The yellow line depicts the limit of the landslide. The sliding mass (right) spreads along the blocking hillock (left). h) Picture of the permanent GPS station and the rain-gauge: the GPS antenna is in the foreground, the rain-gauge in the background. Both receivers are in the safe box.

receivers. The average session duration is 8 h. The data have been processed with GAMIT/GLOBK software (Herring, 2002; King and Bock, 2002). We have removed the regional velocity vector estimated at Abali station, 30 km southwest of Kahrod (Djamour, 2004). Processing separately each pair of campaigns allows pointing out the steady state behavior of all benchmarks. All velocity fields are totally similar. Hence, velocity vectors integrating all measurements are presented in Table 1. Fig. 5 displays their horizontal and vertical components. The associated 95% confidence ellipses indicate the reliability of these velocity estimates.

This velocity field reveals a quasi uniform downwards motion of the 3 monitoring points located within the landslide. Displacements occur along the steepest local slope by as much as 25 cm/yr. Benchmarks located outside of the landslide remain stable during the 4-year observation time, except for K4 which has significantly and steadily moved horizontally towards the south by as much as 2 cm/yr. Because of the direction of this velocity vector which is perpendicular to landslide displacement, this can be a warning of a possible impending rupture of that fixed abutment. Moreover, fresh fissures’ opening occurs precisely in this direction. However, benchmark K4 is

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Fig. 5. GPS horizontal (a) and vertical (b) velocity fields estimated over 3 years (June 2003–November 2006). Background image is our shaded high-resolution DEM. Each benchmark has a steady velocity vector over the 3 periods of measurement. Benchmarks K6, K7 and K8, which are located within the sliding mass (yellow dotted line), move down slope with similar rates (about 25 cm/yr). Only the vector directions slightly change in concordance with local slope. Benchmarks K2, K3 and K4 are located on the crest of a small abutment that seems to block the landslide expansion towards Haraz valley (dotted light red line). Large fissures reveal the importance of stress over that hillock. K2 and K3 confirm the stability of this block whereas K4 suggests a significant deformation of its southern part. However, K4 benchmark is installed in a concrete base built at the extremity of the crest. This place shows evidence of significant erosion process that is likely to be the origin of this 2.0 cm/yr displacement vector. As expected, benchmarks K1 and K5 (Kahrod village) appear to be fixed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

located in a concrete base built on the southernest limit of this ridge and we cannot rule out any erosion origin to this displacement. Future GPS campaigns should provide important indication for distinguishing erosion process from landslide-related compression. 3.1.2. Rapid-static network Previous GPS network needs to be densified within the landslide in order to address the spatial variability of surface deformation. Moreover, 3-dimensional interpretation of radar interferometric phase will require a-priori information about local deformation direction. Hence, 57 holes have been drilled on blocks of metric size essentially embedded in the sliding mass (only 3 of them are located just outside the deforming area). They are distributed all over the landslide, with higher density on the upper part where many scarps and tension cracks have been listed. The northern part of the intermediate zone has not been measured since its access is quite uneasy.

This area will be instrumented during the next campaigns. Each hole is 10 cm deep in order to guarantee the verticality of the rod that supports the GPS antenna. The planimetric location of the antenna is expected to be constrained to less than the formal error of 1 cm. Following the rapid-static GPS method, the rover receiver recorded 5 min sessions on each point with a time sampling of 2 s. The base station was installed on K5. This network has been measured in November 2006 and May 2007 with a Trimble Z-Max receiver. Data have been processed with Trimble Geomatics Office software. The relative positioning accuracy with respect to K5 has been estimated to about 0.3 cm in planimetry and 0.6 cm in elevation. Displacement vectors are presented in Fig. 6. First, it is worth noting that displacement estimated close to K6, K7 and K8 benchmarks are in agreement with semi-permanent velocity vectors. Moreover, despite the short duration of recording, the associated 95% confidence ellipses indicate the high reliability of these measurements.

M. Peyret et al. / Engineering Geology 100 (2008) 131–141 Table 1 Velocity vectors (east, north and vertical components, respectively) and their corresponding 1σ uncertainties, inferred from semi-permanent GPS measurements spanning June 2003 to November 2006 period Stations

K1 K2 K3 K4 K5 K6 K7 K8

Veast

Vnorth

Vup

dVeast

dVnorth

(mm/yr)

(mm/yr)

(mm/yr)

(mm/yr)

(mm/yr)

(mm/yr)

−0.3 −0.8 −0.9 9.1 −0.1 230.1 242.5 165.8

0.9 −0.1 −0.6 −23.0 0.3 − 49.8 −97.3 −162.3

−6.4 4.8 1.9 2.7 −1.0 −91.5 −97.3 − 111.2

1.27 1.39 1.31 1.60 1.28 1.61 1.30 1.27

1.29 1.44 1.35 1.66 1.32 1.66 1.33 1.29

2.43 5.18 3.05 4.50 2.63 3.98 2.67 2.13

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rotational components on the upper part of the landslide. The rupture surface at depth is probably complex. 3.2. Interferometric analysis

dVz

All stations located outside the landslide reveal displacement beneath the 1σ uncertainty threshold (K1, K2, K3 and K5), except K4 which is likely affected by erosion process, the sapping of Kahrod River or stress imposed by landslide motion. Stations within the landslide moves downwards along the local slope by as much as 25 cm/yr.

Surprisingly with respect to field observation, this displacement field is globally very homogeneous, particularly on the upper part of the landslide. A 40° clockwise rotation occurs between the head and midheight of the landslide. In the lower part, displacement mainly occurs towards the Kahrod River confirming its major activation role. About 5 m before the contact between the landslide and the hillock that prevents the expansion of the landslide towards the Haraz Valley (benchmark 2), the displacement vector is oriented towards the hillock and still has a large value (10 cm over 6 months). Further north, displacement orientation changes from east to north. The landslide spreads along this blocking abutment. Finally, in agreement with field observation, the point located in the southeast part of the landslide (benchmark 1) reveals to be fixed. This area seems to be totally decoupled from the body of the landslide. Vertical displacement is less obvious to interpret. Large values (about 10 cm over 6 months) can be noticed close to the head and at mid-height where significant slope discontinuity exists. This vector field suggests a predominant translational type of deformation with some likely

Radar interferometry (InSAR) is a technique which measures the phase difference between two radar images taken from two slightly different positions (e.g. Massonnet and Feigl, 1998; Bürgmann et al., 2000). The geometric contribution to this phase difference results from orbital changes, topography and changes in the satellite line-of-sight distance to the ground. Interferometric phase is coherent if dielectric properties and spatial distribution of individual targets within one pixel remain steady over the 2 acquisitions. This is why, in the case of a catastrophic slope failure, InSAR may only provide precise DEM (a few meters vertical precision for a decametric ground resolution) before and after the event, allowing DEMs comparison and landslide mapping (e.g. Kimura and Yamaguchi, 2000; Arturi et al., 2003; Pesci et al., 2004). By contrast, in the case of a slow creep process, one can use a DEM for removing the topographic contribution for constructing differential interferograms (DInSAR) attaining the surface deformation signature (e.g. Fruneau et al.,1996; Rott et al.,1999; Vietmeier et al.,1999; Berardino et al., 2003; Squarzoni et al., 2003). In cases of significant temporal decorrelation, analysis of permanent scatterers in SAR interferometry may reveal successful in restoring deformation information (Ferretti et al., 1999, 2000; Colesanti and Wasowski, 2006; Farina et al., 2006). Beside temporal decorrelation mainly caused by vegetation coverage, erosion or destructive event, DInSAR faces several limitations (Zebker and Villasenor, 1992). First of all, due to the satellite oblique line-of-sight, wide mountainous areas are affected by shadowing and overlay effects. Still in steep slopes zones, some topographic residue is likely to remain if the DEM is not accurate enough, especially at high frequencies. Finally, atmospheric delay may change between the two passes and contribute coherently to interferometric phase rotation (Zebker et al., 1997). A spatially uniform change in atmospheric conditions leads to range delay variations inversely proportional to surface elevation. Yet, much more heterogeneous tropospheric artifacts may exist in interferograms.

Fig. 6. Displacement field estimated by 2 rapid-static GPS surveys over a 6-month period. The network on the upper part of the landslide is dense since many scarps suggested heterogeneous distribution of deformation. By contrast, the northern central part of the landslide has no benchmark since it is not easily accessible. This displacement field is very homogeneous. It reveals a uniform N110° direction in the upper part of the landslide. The southern toe, decoupled from the rest of the landslide by 2 fracture zones, is fixed (benchmark B1). The central toe moves towards Kahrod River. Just north of the “niche” of detachment, benchmark 2 reveals a significant displacement perpendicular to the contact between the blocking hillock (dotted light red line) and the landslide. Finally, the northern toe rotates along this blocking abutment. Coordinates are in km (UTM projection system, zone 39). Background image is the 10 m resolution shaded DEM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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In order to precisely map the Kahrod landslide limits and its associated deformation field, we used DInSAR technique applied to Envisat ASAR images. We programmed acquisitions in ascending orbits, track 714, IS2 mode (mean incidence angle of 23°). This geometric configuration guarantees that expected surface deformation is nearly in slant range. The scalar product between the unit vector along the satellite line-of-sight and the direction of displacement ranges from about 0.6 for most of the sliding mass to almost 1 for the northern toe. A set of 15 images have been acquired from October 9th, 2005, until March 18th, 2007, with one image each 35 days. Interferometric processing requires the perpendicular baseline between two orbits to be lower than about 600 m, otherwise interferograms are decorrelated. All pairs spanning 35 days except 3 meet this requirement. We processed Single Look Complex (SLC) images provided by the European Space Agency (ESA), with a 20 m resolution in azimuth and ground-range directions, using Diapason software (CNES, 1997). We used precise Envisat orbits estimated from DORIS instruments for removing orbital phase signature. In order to remove the topographic phase signature and to geo-reference the differential interferograms, we used a 5 m resolution photogrammetric DEM provided by the National Cartographic Center of Tehran (NCC), sub-sampled to 20 m. Cross analysis of all the differential interferograms reveals that no fringe system is characterized by phase migration proportional to perpendicular baseline. Hence, topographic residue is estimated to be lower than noise level (typically one fifth of a fringe within the landslide). The main fringe patterns at regional scale have their origin in atmospheric changes between the two passes. However, they are closely correlated with topography. A simple linear estimation of the tropospheric delay variation with respect to altitude suffices to simulate, and consequently remove, most of this fringe pattern. We a posteriori check that interferometric phase is constant (to within the level noise) over the surrounding fixed areas. An example of differential interferogram is presented in Fig. 7a. Coherence is very good for the 35-day interferograms since the Kahrod landslide is free of any significant vegetation. This spatial and temporal coherence guarantees that a standard InSAR processing will permit to measure the ground surface displacement without the need to develop a PS-InSAR approach. During the winter period, snow coverage was very sparse and, apparently, induced neither any coherent phase rotation nor any higher loss of coherence. However, interferometric coherence decreases very much with time making the interpretation of interferograms spanning longer periods quite difficult. Fortunately, the amplitude of surface motion is well suited to a 35-day delay of acquisition (about one fringe). Slopes oriented towards the satellite which produce the overlay effect have been masked.

First of all, the high similarity of all the interferograms suggests a global steady rate of surface deformation over time, whatever the season. Most of the landslide limits are clearly revealed by significant phase discontinuity. It establishes clearly that the current active sliding process only affects a part of the mountain side delimited in the topography by large scarps. This is compatible with a likely ancient large catastrophic landslide whose smaller residue would be the only current active process. These limits fully agree with field observations. However, InSAR fails to map this limit, on the one hand close to the blocking hillock which is affected by overlay effect, and on the other hand on the central northern part of the landslide. Indeed, in this latter zone phase, discontinuity is very close to one fringe which makes interpretation ambiguous. The existence of a likely non-deforming zone at mid-height of the landslide, on its northern side, is not supported by field observation. The interferometric phase is globally homogeneous over the whole landslide, except on its central northern part where deformation increases, and on its northern toe where deformation evenly decreases. Let us notice that, as suggested by field observation and rapid-static GPS surveys, DInSAR confirms that the southern part of the toe, close to the village, is fixed. Drawing profiles across the landslide allows for a more detailed spatial analysis of phase changes (Fig. 7b). Profile AB longitudinally samples the landslide from its head towards its central foot, in the bottom escarpment undercut by Kahrod River. All 35-day interferograms show roughly the same pattern. First, phase discontinuity always occurs at the top of a series of scarps identified as the head of the currently deforming zone. Its amplitude corresponds to a ground displacement along the satellite line-of-sight by as much as 20 mm. Then phase decreases slightly about 500 m down slope till the mid-part of the landslide where it gets back to the same value as on the upper part of the slide. No attenuation or acceleration can be noticed at the foot of the slide. The projection of K6, K7 and K8 velocity vectors onto the satellite line-of-sight is in agreement with interferometric phase. In order to interpret interferometric phase information in terms of deformation, one needs first to unwrap it. Indeed, phase is known modulo 2π radians and needs to be unwrapped in order to recover the continuity of surface deformation. We used SNAPHU software (Chen and Zebker, 2002) with the constraint that unwrapping must be limited to within the landslide limits as mapped by field observation. This constraint allows eliminating the northern mid-height ambiguity. Because InSAR is a one-dimensional distance technique measurement, interferograms only provide displacement projected on the satellite line-of-sight. So, recovering 3-dimensionnal surface deformation requires a-priori information on its direction. We interpolated the orientation vectors field derived from the rapid-static GPS surveys. The homogeneity of the measured vector field suggests that this interpolated

Fig. 7. a) Example of a 35-day Differential Envisat radar Interferogram. Some atmospheric artifacts residue can be noticed in the vicinity of the landslide. Their influence on the phase pattern within the landslide is negligible. Deformation is roughly homogeneous except on the northern part at mid-height of the landslide, where deformation increases, and down slope, in the northern toe, where deformation evenly decreases. Masked zones correspond to slopes oriented towards the satellite (west), inducing overlay effect on interferograms. Projection uses UTM coordinates in km. b) Interferometric phase along the longitudinal (AB) profile. GPS displacement vectors estimated on the semi-permanent network are projected onto the satellite line-of-sight (blue circles). They fully agree with InSAR measurement.

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Fig. 8. 3D deformation map estimated by stacking all the 35-day interferograms. We interpolated the rapid-static GPS displacement vectors field in order to obtain the direction of deformation for each pixel. The amplitude of deformation is roughly homogeneous over the whole landslide, except, on one hand, at its head and in the “niche” of detachment incised by the Kahrod River where deformation increases significantly, and on the other hand, in the northern toe where deformation evenly decreases.

field is close to reality, even though this approximation is undoubtedly wrong in some places (especially far from GPS benchmarks). Major information derived from this InSAR analysis is that deformation does not seem to change significantly with time over the analyzed period. Based on this, we averaged all 35-day interferograms to infer surface deformation map. Besides, it allows minimizing the remaining atmospheric artifacts. We projected this phase information back onto the direction provided by the interpolated rapid-static GPS displacement field. We then obtained the deformation map presented in Fig. 8.

The overall motion is estimated to about 30 cm/yr. This is in good agreement with GPS measurements, though not spanning the same time interval. The amplitude of deformation is roughly homogeneous over the whole landslide, except, on the one hand, at its head and in the “niche” of detachment incised by the Kahrod River where deformation increases significantly (up to 45 cm/yr), and on the other hand, in the northern toe where deformation evenly decreases to about 20 cm/yr. To be complete, we must mention that, thanks to the high quality of our DEM, this DInSAR study succeeded in detecting 2 other

Fig. 9. a) Permanent GPS time series (North, East and Vertical components) between June 2006 and May 2007. Time evolution of position is very stable. The mean velocity is in agreement with other GPS measurements in the central lower part of the landslide. b) Black error bars represent the GPS time series without the linear trend. Blue line is the cumulated rainfall. Red vertical lines locate the GPS time-series discontinuities. These discontinuities always occur just a few days after some rainfall events. However, this time series is too short and discontinuities not significant enough to establish certainly any correlation between rainfall and landslide activity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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landslides, unregistered by Iranian institutes. They are located along the Penjab Valley about 5 km more to the North. Their mechanical behaviors are likely similar to the one of Kahrod. All processed interferograms, similarly to reiterated GPS surveys, attest that no significant change affected the Kahrod landslide during the time of observation. However, the time sampling of both techniques is coarse (from month to year). 4. Temporal evolution of surface displacements In order to address the detailed temporal evolution of the landslide activity, we set up a permanent GPS station (Trimble NetRS receiver) within the radio-communication platform located close to K6 point (RTS on Figs. 2, 4h), in the central lower part of the landslide. A rain-gauge has also been installed so that any influence of rainfall on landslide activity can be detected. Data have been collected since June 2006, with a 30 s time sampling. They have been processed with GAMIT software. The time series are presented in Fig. 9a. The mean velocity vector is similar to the one estimated for K6. Despite the heavy rainfall (about 1000 mm) recorded during this period, no major discontinuity affects the time series. However, removing the linear trend allows discerning (in the planimetric components) several possibly significant discontinuities (Fig. 9b). Although any strong correlation cannot be established, we can notice that these discontinuities always occur just a few days (from 1 to 20 days) after significant precipitations. This is notably the case between December 2006 and February 2007. Furthermore, it seems that the deformation before August 2006 significantly differs from the one occurring later. The displacement rate is slightly higher (by about 20%) and could be interpreted as the ground response to the heavy precipitations of June 2006, although the lack of GPS data before that date prevents us from being conclusive. Finally, let us remark that snow melt is expected to play a minor role during this time of observation since snow coverage has been very limited in time and space. This 1-year time series suggests that no external force induced any major change on landslide activity during that time interval. Indeed, neither excessive precipitation, nor any earthquake with magnitude higher than 2 in the vicinity of Kahrod, may have significantly affected the Kahrod landslide behavior. However, the plausible correlation with precipitation that we mentioned let us suspect some critical ground response to any exceptional pluviometric event. 5. Interpretation and conclusion The present deforming area mapped by InSAR with a decametric resolution is totally correlated with lateral scarps mapped on the field. This delimitation is confirmed by 3 stable points belonging to the rapidstatic network. Deformation initiates on a place where field observation reveals successive major scarps that define the head of the landslide. The present sliding mass is the residue of a former catastrophic landslide that has not yet been drained by Kahrod and Haraz Rivers. Downslope, a fracture zone indicates that the southern toe is decoupled from the central part of the toe whose behavior seems to be controlled by the Kahrod river incision. Not only InSAR and GPS rapidstatic measurements confirm this interpretation, but also they indicate that this southern zone does not deform at all. This may mitigate the risk upon the village. Although affected by significant temporal and spatial decorrelation, radar interferometry provides a global and dense view of the deforming zone, and allows quantifying accurately the surface motion induced by the Kahrod landslide. The use of an interpolated displacement field obtained from rapid-static GPS surveys was necessary to have access to the 3-dimensional deformation. The amplitude of deformation fully agrees with the one estimated by all the GPS approaches. Surface deformation is roughly homogeneous over the whole landslide. However, higher rates of deformation affect the head of the landslide and the bottom escarpment incised by the Kahrod River.

At the northern landslide base, wide fissures in the conglomerate layer overlaying the bedrock on the blocking abutment give evidence of significant (present or past) deformation under the stress imposed by the sliding mass. A critical question is to know whether these fissures extend within the bedrock or not. Rapid-static surveys show evidence of high amplitude displacement, perpendicular to the contact between the sliding mass and this hillock, even very close to it. This obstacle to the landslide expansion towards Haraz Valley induces a rotation of the landslide spread towards north. On that hillock, interferograms do not reveal any deformation but they are partially affected by overlay. By contrast, two GPS velocity vectors are steady while the third one, located on the southern flank of this ridge, indicates a significant deformation perpendicular to landslide motion. Since the contact between the landslide and this hillock takes place on the bedrock, we interpret the large fissures as former tension cracks, at the time when most of the original destabilized material had not been discharged yet by the river. The present-day deformation revealed by GPS may be attributed to erosion or lateral incision of Kahrod River rather than landslide stress. This interpretation needs to be confirmed by complementary GPS measurements and a more detailed analysis of fissures development and interrelationship between the area affected by Kahrod River sapping process and the sliding mass itself. The crucial questions are to know whether this outcrop of bedrock can resist through time, and whether a strong earthquake or heavy rainfall could trigger a catastrophic acceleration of the landslide with potential consequences: destruction of infrastructures in a new mega catastrophic debris flow further down in the valley when the dam would break. The time evolution of surface deformation estimated by semipermanent GPS campaigns on a 1-year time scale, or by InSAR with data spanning 35-day periods, does not exhibit significant changes. Since it is demonstrated that heavy rainfalls or snowmelt may induce significant changes in displacement rates and consequently increase the likelihood of slope failure (Iverson and Major, 1987; Coe et al., 2003), we have compared GPS data for a permanent station located within the landslide, with rain-gauge recordings. For the May 2006–May 2007 time period, the GPS time series reveals to be quite steady. This indicates that no major external forcing (climatic or seismic) induced any significant change on landslide activity. However, small discontinuities affect the GPS time series. They occur just a few days after significant rainfall. This likely short-term correlation between rainfall and surface deformation needs to be confirmed with longer recordings. It would indicate that exceptional heavy rains could trigger some major catastrophic change on the landslide activity. Kahrod is located within a very active tectonic region. The initial landslide has probably been activated by a large earthquake, and we can suppose that the 1957 Sangechal earthquake significantly accelerated it. C14 and Be10 dating of samples taken on well-preserved surfaces of rupture of the initial landslide, or within a sharp clay layer of the Kahrod river terrace should provide important clues to Kahrod landslide history. However, during the time of observation, no earthquake with magnitude higher than 2 occurred in the vicinity of Kahrod. Yet, any correlation between seismicity and landslide activity still needs to be demonstrated. In the absence of major precipitation or earthquake, the main active process seems to be the progressive removing of the rock mass through the sapping effect of Kahrod River. This interpretation mitigates the risk of any future catastrophic evolution of this landslide. It is nevertheless a fact that Kahrod landslide is a large unstable mass located in an active seismic zone, and threatening Kahrod village and the Haraz Valley. Hence, it is important to complement the data presented in this study with longer time series in order to establish the likely correlations between the landslide activity and external forces (tectonic, seismic, climatic or hydrologic). Acknowledgments We thank the European Space Agency for delivering Envisat SLC images. We also thank two anonymous reviewers for their very

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