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Analysis of the deep-seated gravitational slope deformations over Mt. Frascare (Central Italy) with geomorphological assessment and DInSAR approaches
Geomorphology 201 (2013) 281–292
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Analysis of the deep-seated gravitational slope deformations over Mt. Frascare (Central Italy) with geomorphological assessment and DInSAR approaches C. Tolomei a,⁎, A. Taramelli b, M. Moro a, M. Saroli a,c, D. Aringoli d, S. Salvi a a
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy ISPRA Institute for Environmental Protection and Research, via Vitaliano Brancati, 60, Rome, Italy c DICeM-Civil and Mechanical Engineering Departement, University of Cassino and Southern Lazio, 03043 Cassino, Italy d School of Environmental Sciences, University of Camerino, via Gentile III da Varano, Camerino, Italy b
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 2 May 2013 Accepted 2 July 2013 Available online 10 July 2013 Keywords: DGSD Differential SAR interferometry Remote sensing Central Apennine
a b s t r a c t A quantitative and innovative DGSD (deep gravitational slope deformation) assessment method that used integrated remote sensing was tested in the central Apennine mountain range (Italy). The movement rate was calculated for selected test areas using the differential SAR interferometry small baseline subset (SBAS) technique. The selected test areas were previously identified by interpreting both aerial photos and using 32 ERS radar images taken between 1993 and 2000. More than 15 cm of cumulative surface displacement occurred across the Podalla DGSD along the satellite line of sight (LoS). Moreover, the displacement time series showed non-linear deformation rates, which included periods of accelerated movement correlated with strong rainfall. The high estimated Podalla DGSD activity indicates that this type of study should be conducted to monitor the evolution of this phenomenon. In addition, the DInSAR movement rate can be used to improve mapping and identify DGSDs in specific landscapes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since the 1940s, various authors have observed morphologic evidence of mass movement of entire slopes. These movements occurred over areas that were larger than landslides and were caused by gravity (Dal Piaz, 1936; Ampferer, 1939; Stini, 1941). In the 1950s and 1960s, the concept of “depth creep” was developed to explain this surface evidence. In addition, the first definition of this phenomenon was supplied (Terzaghi, 1950; Jhan, 1964; Ter-Stepanian, 1966; Beck, 1968; Zischinsky, 1969; Nemcock, 1972; Radbruch-Hall, 1978). Over the last 20 years, several studies regarding DGSD phenomena have highlighted the differences between these phenomena and typical large landslides, which are characterized by a slide surface that is not well defined following DGSD (Savage and Varnes, 1987; Chigira, 1992). Recently, the term DGSD has been used to indicate slope movements on high relief-energy hillslopes with an upper portion that is characterized by tensional structures. These tensional structures are comparable to the entire slope regarding size, and have little displacement relative to the slope itself (Cendrero and Dramis, 1996; Agliardi et al., 2001; Goudie, 2004; Baron et al., 2005; Kellerer-Pirklbauer et al., 2010). Trenches, double ridges and counter slopes provide superficial evidence for the mass movement of scarp edges. Sags, cambers and widespread landslides provide ⁎ Corresponding author. E-mail address: [email protected] (C. Tolomei). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.07.002
evidence for compressional stress deformation in the lower portion of slopes (Hutchinson, 1988; Saroli et al., 2005; Galadini, 2006; Moro et al., 2007, 2009, 2011). Deep-seated gravitational slope deformations are generally subdivided into the following two types (Goudie, 2004): 1) sackung (or rock flow) and 2) lateral spreading (or rock spreading): 1) The sackung DGSD is a rock flow that bulges from depths and creeps along very high and steep slopes (Zischinsky, 1969; Hermann et al., 2000). In this case, a constant uplift (with a high catchment discharge at the bottom of the slope) and an isotropic and continuous faulting or jointing of a coherent lithotype are required. Surface evidence for a brittle ruptured middle portion of the slope is characterized by plastic behavior (Radbruch-Hall, 1978) that increases the volume of the bulging landform at the bottom of the slope. 2) When a coherent and thick layer of rock overlies a weaker lithotype with a very gentle dip angle and when the rock masses are affected by tectonic residual stress, lateral mass movement may occur towards the outside and bottom of the slope. These phenomena are clearly linked to active faults and tectonic settings (Dramis and Sorriso-Valvo, 1994; Galadini, 2006; Moro et al., 2007, 2009, 2011, 2012). First, uplift and subsidence may produce enough energy to cause gravitationally driven slope failures. Furthermore, tectonics also produce aligned fault surfaces or joints along the slope and can act as triggering planes for deep mass creep.
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In the last 20 years, many studies have identified the presence of DGSD phenomena in the central Apennine mountains, particularly along the calcareous mountain ranges (Farabollini et al., 1995; Aringoli et al., 1996). However, most of these studies found strong evidence of DGSDs in terrigenous rocks (Genevois and Tecca, 1984; Crescenti et al., 1994; Calabresi et al., 1995; Dramis et al., 1995; Baron et al., 2005). Most results contained field evidence (geomorphologic and stratigraphic) for the activity between the Pliocene and the Pleistocene and the maximum relief along slopes (Ambrosetti et al., 1982). All of the presented studies identified some morphologic evidence from the tectonic and lithologic structures of the Apennines (Galadini, 2006; Moro et al., 2007, 2009). Thus, we chose to study the central Apennines in the Umbria and Marche regions of central Italy (Fig. 1). The use of different satellite remote sensing approaches is a new and potentially useful way to identify the presence of DGSD phenomena (Hilley et al., 2004; Saroli et al., 2005; Ambrosi and Crosta, 2006; Taramelli and Melelli, 2009). Thus, the development of DGSD detection methods that use remote sensing data has become an important
topic of research in the last decade (Allievi et al., 2003; Catani et al., 2005; Moro et al., 2007, 2009; Melelli and Taramelli, 2010; Moro et al., 2011). Unfortunately, when treated separately, the characteristics of this slope instability phenomenon were not comparable to the acquisition parameters of different space-borne missions, such as spatial resolution, revisiting time and view geometry (Stramondo et al., 2005; Moro et al., 2007). Therefore, these results were not as good as the results obtained using other geophysical applications (Antonello et al., 2004; Stramondo et al., 2007). However, the use of integrated remote sensing methods could provide more flexibility regarding its operation, the acquisition of morphometric parameters for spatially distributed information and its independence relative to different acquisition conditions. In addition, remote sensing methods could eliminate the drawbacks that are related to different satellite platforms (Antonello et al., 2004). Recently the potential advantages of combining synthetic aperture radar (SAR) and aerial photography to analyze surface changes and classify DGSDs were investigated (Melelli et al., 2007).
Fig. 1. Study area. The rectangle shows the area of Figs. 5 and 9 (Frascare Mountain).
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SAR collects high-resolution radar echo maps of the Earth's surface from satellites (Bürgmann et al., 2006). In addition, SAR measures the backscattering coefficient of the surface with a pixel size of 10 m or less. Specifically, SAR measures the phase of the signal echoed by each ground pixel as an incoherent sum of the return echoes from the pixel's targets. Thus, SAR is strongly sensitive to surface changes. DGSDs modify observed scenarios and their electromagnetic behavior, which enables the detection of these changes by comparing the SAR images in terms of both amplitude and phase. Improving classical differential SAR interferometry (DInSAR) has expanded the application of this technique. For example, this technique can be used to detect very slow deformations and provide ground velocity and displacement measurements across a time series with sub-centimeter accuracy (Ferretti et al., 2000, 2001; Berardino et al., 2002; Lanari et al., 2004; Salvi et al., 2004). Here, an innovative approach for improving DGSD detection and analysis was tested. This approach was based on the multiscale application of DInSAR and aerial photography. We estimated the DGSD signature of the central Apennine ridge (terrigenous fractions) by using integrated analysis. We selected the NE slope of Mt. Frascare as a test area to study classical geological, geomorphological and advanced DlnSAR techniques for deformation measurements. This area contained the most impressive geomorphological evidence and the greatest velocities, which were measured across the study area with the interferometry technique. The paper is organized as follows. The interpretations of the aerial photographs were outlined to define the topographic analyses that were used to characterize the DGSD evidence in the central Apennine ridge (Italy). Next, the large-scale gravitational phenomena were identified by their morphological and geological descriptions at the individual study sites (detailed in the next section). The succeeding section describes the application of the DInSAR method. Finally, detailed analysis of the results and the displacement time series of the investigated area were used to explain the main DGSD characteristics
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and how the approach can be directly used to quantitatively detect these phenomena. 2. Geology and geomorphology of the study area Most mass wasting studies on the central Apennine ridge have focused on quantifying shallow landslides, flow erosion debris (Melelli and Taramelli, 2004) and their effects on landscape functions on the human and geologic time scales (Galadini, 2006). The presence of a shallow landslide near a lake dam in the Mt. Frascare area suggests that DGSDs potentially played an important role in shaping the region (Dramis and Sorriso-Valvo, 1994; Aringoli et al., 1996). The study area is located at the top of the Sibillini Mountain thrust next to the Lower Lias–Upper Eocene calcareous formations, which contain Oligocene marls that were formed during the Tortonian–Lower Pliocene compressive tectonic phase (Calamita, 1990). The calcareous units form a complex folded setting with a northward axial depression (direction of the axes N170°). These units are dissected by faults in the following directions: Apenninic (N150°–170°), anti-Apenninic (N30°–50°) and N–S (Fig. 2). The Apenninic faults are related to the compressive tectonic phase (Lower Pliocene) and were reused as normal faults during the Pleistocene, which lowered the entire structure to the west. The anti-Apenninic faults are related to the same compressive phase, but they have more complex kinematics. For example, the main movement was mainly inverse (left-transpressive) and resulted from a more recent reactivation of the normal faults (right-transtensive). The N–S fault system is characterized by right-transtensive kinematics and an en-echelon geometry that was generated during the reactivation (Upper Pliocene) of the Sibillini Mountain thrust by a gravitational tectonic mechanism. The area is crossed by an important shallow (approximately 200 m deep) thrust plane that strikes NE and dips approximately 10° to the NW (Fig. 2). The thrust is superimposed on calcareous formations that occur on the more plastic marly formations. The complex litho-
Fig. 2. Geological and geomorphological sketch. 1) calcareous formations; 2) marly formations; 3) slope waste deposits; 4) slides; 5) flows; 6) rock falls; 7) strata attitudes; 8) normal faults (a) and thrusts (b); 9) tectonized zones; 10) main edges of structural scarps; 11) trenches; 12) structurally conditioned downcutting streams; and 13) the DSGSD upper limit.
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structural setting was affected during the Quaternary period by extensional tectonics and isostatic uplift, which generated new discontinuities (Ambrosetti et al., 1982; Dramis et al., 1995) and renewed previous discontinuities. In this area, photogeological analysis allowed us to identify different morphological features that indicated DGSDs, such as double crest lines, scarps, counterslope scarps, slope-parallel trenches, fractures, open fissures and small depressions that were linearly aligned to the NE (Fig. 3). We determined that the Podalla DGSD (hereinafter PD) is a sackung, which moves towards the NW, is controlled by NE-striking, and has an NW-dipping sliding plane that reaches a
depth of approximately 500–600 m and evolves in a shear zone at these depths. The top of the sackung is limited by the presence of arcuated trenches (Figs. 3 and 4). From a geomorphologic point of view, the folded structures in the study area, which were mainly composed of calcareous rocks, were eroded during the Pliocene period (almost exclusively during continental conditions) to produce a low relief erosional surface. This ancient landscape was deeply deformed and incised, which began during the Lower Pleistocene (Ciccacci et al., 1985; Coltorti and Dramis, 1995). This process and an important anti-Apenninic tectonic line formed the Fiastrone River Gorge, which borders the study area
Fig. 3. Photogeological interpretation. 1) slope waste deposits; 2) slides; 3) flows; 4) rock falls; 5) DSGSD upper limit; 6) strata attitudes; 7) normal faults (a) and thrusts (b); 8) tectonized zones; 9) main edges of structural scarps; 10) trenches; and 11) structurally conditioned downcutting streams.
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Fig. 4. Three dimensional view and simplified cross-section for Mt. Frascare. Vertical exaggeration = 3×. Highlighted typical DGSD features are trenches, scarps, and surficial slides.
to the north (Coltorti et al., 1996). Along the base of this small river in the northwest area, valley incision created steep slopes at the channel margin. Similarly, steep sections were observed during our field investigations along the northern slope of Mt. Frascare (Fig. 5). Different patterns were observed between the NW and NE river profiles. Profiles 1 and 2 show obvious knickpoints while profiles 3 and 4 show more regular patterns with only minor deviations (Fig. 5). A steep composite slope in the lower part characterizes the area that contains rivers 1 and 2 with a convex shape to the west and concave shape to the east. The central portion of the slope has a low-gradient topography (Fig. 6) that is affected by a set of N50°-trending scarps and small incisions that isolate elongated blocks with nearly flat surfaces (Fig. 5). The tip of the triangle connects the headwaters of rivers 1 and 2 with the flat summit of Mt. Frascare by a steep scarp. Upslope of this scarp and inside a belt that has a width of approximately 600 m, several near-parallel trenches outcrop at the mesoscale (Fig. 7). These trenches vary in length from a few to 800 m, width from 50 cm to N20 m, and depth up to 10 m. In addition, repeated field surveys showed that these fractures are subject to constant evolution. Very fresh-looking scarps, up to 50 m long and 50 cm wide, were first surveyed in 1995 (Aringoli et al., 1996). In the following years, the increasing length and width, opening up to few cm per year, were observed (Fig. 8). In fact, the wider area around the triangular block is subject to structural control by (mainly) the N60° tectonic trend, as shown by the alignment of the main incisions and scarps (Figs. 3 and 5). The main boundary condition that constrains the re-use of these fractures and joints by gravitational forces is the deep incision of the Fiastrone gorge. Other important factors that may control the deformation rates include the direction of the slope with respect to structural trends and the presence of marl layers that were locally isolated by previous tectonics (Figs. 3 and 5). The central triangular block that is described above provides evidence for the largest deformation. This block has been identified as PD. 3. DInSAR data analysis and active ground deformation measurements The differential interferometric SAR technique was used to investigate the current deformation rates. In fact, the classical differential SAR Interferometry is a technique that uses synthetic aperture radar image data (usually collected by space sensors) to calculate ground surface movements that occur between two satellite passes over the
same area. This technique is based on the radar concept. The phase of the radar signal that is returned to the satellite conveys quantitative information regarding changes in the sensor-to-ground distance (range) that are caused by surface deformation (Bϋrgmann et al., 2000). By subtracting the phase of the two images and then simulating and subtracting the phase contributions due to topographic relief (a DEM is needed for this), a differential interferogram is formed. This interferogram contains the ground deformation signal that occurred between the two passes (Massonnet and Feigl, 1998). The accuracy of the surface movements that are measured by the DInSAR technique depends on various factors such as atmospheric effects, orbital effects, stability of ground scatterers, and unwrapping errors. However, in favorable cases, the accuracy of the displacement is less than 1 cm (Hanssen, 2001). In the last decade, this technique was improved by the development of a multi-temporal DInSAR approach, which allows users to obtain displacement time series and surface mean velocities data at accuracies of ~ 1 mm yr−1 (Casu et al., 2006). Here, we used the small baseline subset (SBAS) algorithm (Berardino et al., 2002). With this algorithm, we processed SAR data acquired by the ERS-1/2 satellites of the European Space Agency (ESA) to estimate the displacement time-series and the mean velocities of the coherent ground areas. This method has been used to accurately measure ground deformation for a variety of applications, including urban subsidence (Stramondo et al., 2007), volcanic eruptions (Pritchard and Simons, 2004), post-seismic and inter-seismic monitoring (Hunstad et al., 2009), and analyses of gravitational phenomena (Catani et al., 2005; Saroli et al., 2005; Ambrosi and Crosta, 2006). The SBAS algorithm uses a large number (several tens) of radar images to reduce various noise components of the DInSAR interferograms, which increases the accuracy of the displacement measurements (Casu et al., 2006). Initially, the operator defines the acceptable temporal and orbital separations that occur between each of the DInSAR interferogram images to be generated. Next, a consistent number of differential interferograms are generated with a DEM of comparable resolution and are unwrapped to generate actual displacement maps. Then, the reference pixel, for which all calculated displacements and velocities will be referenced, is chosen. The unwrapped phases are inverted with the singular value decomposition (SVD) technique, and the displacement time series for each image (at a given date) are retrieved for each pixel that has a coherence value greater than the fixed threshold. The SVD method is used to compute the matrix pseudoinverse to solve the over-determined linear
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Fig. 5. Rivers and their profiles along the northern slope of Mt. Frascare. A) Location of the rivers. The areas marked as UT, PD and AB are discussed in the text. The locations of knickpoints (np) are marked in green. B) River profiles. Streams 1 and 2 have prominent knickpoints.
equation system. At this step, the orbital residual and topographic errors are estimated before subtracting. Finally, the short-term atmospheric contribution is removed by double filtering in time and space. From our SAR data processing, only the pixels with an interferometric coherence of more than 0.7 (over 30% of the total number of interferograms) were considered to be reliable (Burgmann et al., 2000). Thus, for each retained pixel of 80 × 80 m, a time series of the ground displacement was calculated for each image. All displacements are relative to the reference pixel (or area) and are assumed to be stable in the image (Burgmann et al., 2000) and along the sensor line of sight (LoS). We applied the SBAS technique to a data set of 32 ERS1–2 images that were acquired from the ascending pass, track 401, and frame 858
between 1993 and 2000 (Tables 1 and 2). To generate the DInSAR couples, we imposed a maximum orbital separation of 300 m to reduce spatial decorrelation, and a maximum temporal distance of 1200 days between two passes to limit the temporal decorrelation effects. By using these constraints, a maximum of 73 interferograms were generated. Of these interferograms, 16 were excluded due to their scarce coherence (leaving 57 usable interferograms). We used the SRTM DEM for topography subtraction (http://www2.jpl.nasa. gov/srtm — Farr et al., 2007) and processed a spatial subset of the images in the Mt. Frascare area that contained pixels of 80 × 80 m. The obtained ascending mean velocity map is shown in Fig. 9, where the image pixels are symbolized as points. Due to high relief, diffuse
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Fig. 6. Counterslope between the Fiastrone River and Mt. Frascare with different trench types (at the top and in the mid part of the slope) in the SE–NW direction.
vegetation cover and scarce rock outcrops, the temporal coherence is relatively low in this area. Only a few pixels reached the minimum coherence level of 0.7. As previously mentioned, the displacement time series values were calculated relative to a given pixel that was located in a stable area. This reference area was selected near the summit of Mt. Frascare, where no geomorphological evidence of long-term gravitational processes was found (Fig. 9). To reduce the uncertainty, we averaged the time series that were available for the reference area. Thus, all of the displacement time series and ground velocities discussed below are relative to that average. The mean ground velocities shown in Fig. 9 were calculated from the time series by assuming a linear deformation between 1993 and 2000. They represent the general ground deformation pattern. However, the time series also contains some non-linear components, as demonstrated by the change of slope in the time series in Fig. 10. Surface movements measured by the DInSAR techniques always include scalar measurements along the satellite's LoS (Burgmann et al., 2000). In our case (ERS imagery), the LoS is vertically inclined by approximately 23° and looks eastward from the ascending orbit (i.e., a line running N13°W) (Fig. 9). Fig. 9 indicates that the ground in the central and upper portions of the PD moved away from the satellite at rates of up to 3–4 cm yr−1
between 1993 and 2000. However, areas located outside the mapped DGSD limits were relatively stable during this period. Based on the field observations and the general geological setting, we hypothesize that the main horizontal component of DGSD movement was perpendicular to the main slope (Fig. 3). In addition, we hypothesize that negligible deformation occurred along the slope. These horizontal movements, which occurred nearly parallel to the ascending orbit (Fig. 9), cannot be resolved in the DInSAR interferograms because they only caused very small changes in distance between the SAR antenna and the ground (Burgmann et al., 2000). Therefore, we assume that the displacement that resulted from our SBAS analysis represents the projection in the vertical component LoS of ground deformation. In addition, we can calculate the actual vertical displacement or velocity by dividing the LoS value by 0.9, i.e., the cosine of the local incidence angle. 4. Discussion We investigated the geomorphological evidence of a DGSD (PD) in the study area at increasingly larger scales. The lower triangular block that corresponded to the PD showed well-developed gravitational landforms. The parallel NE–SW scarps in the central part of the block are up to 25 m tall and consist of several dislocated and
Fig. 7. Trenches near the top of Mt. Frascare. Greater trenches are mainly occupied by debris and vegetation. However, small recent trenches (on the right) are less open, not filled by debris, and do not contain vegetation.
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C. Tolomei et al. / Geomorphology 201 (2013) 281–292 Table 2 List of ascending images used.
Fig. 8. Fresh crack widening up to a few cm per year. The crack has opened more than 0.5 m.
elongated flat-tops and tilted blocks of up to 1 km long and 500 m wide (Fig. 9). A less pronounced deformation can be observed in the upper trench area (hereinafter UT), where the fractures are smaller (up to a few meters) and are closer together. The strong extension that affects the UT acts approximately perpendicularly to the steep slope, which represents the scarp of the upper PD. This result suggests that the recent trench development corresponds with a continuously decreasing DGSD. at least in its upper portion. To obtain a better representation of ongoing deformation, we averaged the DInSAR displacement time series over five areas (Fig. 9). These areas correspond to the following homogeneous blocks, which were defined by geomorphological analysis (Figs. 3, 4, 5, and 7): 1) the flat area upslope of the UT zone, where no fresh scarps are observed; 2) the proper UT zone where the recent fracture field is present; 3) flat and 4) tilted terraces in the central portion of the PD; and 5) a stable area west of the UT zone which shows no sign of recent deformation.
Table 1 Specifications of ERS1 and 2 satellites. Satellite Launch date Altitude Revisiting cycle Acquisition time
ERS-1
17 July 1991 800 km 35 days 21.16 in ascending orbit 9.40 in descending orbit Orbit inclination 98.5° inclination orbit Look angle 23 deg look angle towards right Band/wavelength C/5.8 cm Frame dimension 100 × 100 km SAR pixel size 20 × 4 m
ERS-2 21 April 1995 800 km 35 days 21.16 in ascending orbit 9.40 in descending orbit 98.5° inclination orbit 23 deg look angle towards right C/5.8 cm 100 × 100 km 20 × 4 m
Acquisition date
Sensor
23/07/1993 02/04/1995 07/05/1995 17/07/1995 25/09/1995 07/01/1996 08/01/1996 17/03/1996 18/03/1996 21/04/1996 22/04/1996 01/07/1996 18/11/1996 07/04/1997 12/05/1997 16/06/1997 21/07/1997 29/09/1997 08/12/1997 16/02/1998 06/07/1998 10/08/1998 14/09/1998 23/11/1998 21/06/1999 29/08/1999 08/11/1999 16/01/2000 17/01/2000 01/05/2000 23/10/2000
ERS1 ERS1 ERS1 ERS2 ERS2 ERS1 ERS2 ERS1 ERS2 ERS1 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS1 ERS2 ERS1 ERS2 ERS2 ERS2
By averaging the displacements, we reduced the data uncertainty and estimated the local variability of the deformation. The large displacement error bars in Fig. 10 indicate the presence of internal deformation. In all areas, the average time series exhibited a non-linear temporal trend with periods of more stable deformation and of increased ground velocity (Fig. 10). The same variations, albeit with variable intensities, are present in all of the time series, suggesting that the variations originated from a common cause but were different due to local responses. When we calculated the mean velocities before and after the sharp change that occurred in November 1999 (Fig. 10), we observed strong acceleration in the unstable areas (Table 3). Moreover, while the velocities prior to August 1999 were similar, a greater acceleration was observed during the second period in the upper zones (1 and 2) than in the central blocks of the PD (zones 3 and 4). These results indicate larger gravitational deformation rates on the UT relative to the central PD. In addition, a rather sharp transition occurs between the stable ground velocities in the SW area and the rapid deformations in the UT and PD. This finding suggests the presence of a structural discontinuity that is oriented in the NW and SE direction along incision no. 1 (Fig. 5). Based on the geomorphological analysis and the current patterns of ground deformation, the entire Mt Frascare NW slope can be partitioned into three different sub-regions that are characterized by different activity rates. These sub-regions include the central triangular block including its upslope extension in the UT, the southwestern area and the northeastern area. The central triangular block corresponds to the PD as defined in Section 2. Here, the surfaces of the tilted blocks show considerable ongoing vertical deformation (~− 2 cm yr−1, Fig. 9). The flat and tilted surfaces in this area are likely relics of the same erosional surface that formed the upper portion of Mt. Frascare. Because the two surfaces now differ in elevation by 80 to 100 m, gravitational deformation occurred over a time frame of between 4000 and 5000 years (this is only an estimation because deformation rates are not constant).
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Fig. 9. Ascending velocity map of the Fiastra area. Five areas shown by yellow outlines are where the average time series were obtained (Fig. 10).
Geomorphological investigations indicate that in the UT zone of the PD, the gravitational deformations began more recently. This may be due to the fragile response of the pre-existing NE–SW fractures to slope disequilibrium from the movement of the PD block. This zone has the highest vertical velocity (up to 10 cm yr−1) and different deformation mechanisms from that of the PD (toppling). Additional evidence regarding the strong influence of the NE–SW structural discontinuities that control landform development in this area is provided by the analysis of river profiles. As shown in Fig. 5A, streams 1 and 2 cross the structural trend at a high angle and streams 3 and 4 follow nearly the same trend. Only streams 1 and 2 have well developed knickpoints (Fig. 5B) that are aligned with one of the main scarps in the PD block (Fig. 5A), which may represent the same deformation surface. Finally, the western portion of the lower PD slope and the presence of a more advanced block (AB in Fig. 5A) with a prominently convex slope, which is separated from the PD main body by a large and well developed trench (Figs. 4 and 5A), suggests strong ongoing deformation. Unfortunately, no coherent DInSAR pixel is available to measure the ground velocity at this site. In the second area, located SW of the PD, the drainage network indicates the presence of a structural control based on the NE–SW fracture pattern. However, no marked gravitational landforms exist and the current ground velocities indicate relative stability (Fig. 5A). In the third area east of the PD, the dominant landforms include two nearly parallel streams (3 and 4 in Fig. 5A) that cause strong localized erosion. The development of these landforms is controlled by the same NE–SW structural trend. The slopes in this area do not indicate recent or historical large mass movements. In addition, the main stream profiles do not contain evident knickpoints. We interpreted the different behaviors of the three areas based on the complex interactions of various factors. The rapid deepening of
the Fiastra River was the first factor that we considered, which caused the valley slopes to move laterally and progressively, resulting in toe unloading. Given the curvature of the Fiastra valley, the main-slope stress direction in the central block (the PD and UT) was favorably oriented perpendicular to the extension of the NE–SW joint set. In the other two sectors, the angle was reduced to 30° (Fig. 5A). In addition, the slope of the SW sector has lower relief and inclination than the other two sectors. Although the NE sector has the highest relief and steepest slope inclination, the joint set was rapidly exploited by erosion, which effectively reduced the gravitational stress. Another potentially important control factor regarding the development of the PD is the shallow thrust (see Section 2) that outcrops to the east, which likely reaches a depth of 200 m beneath the PD block. This outcrop potentially served as a detachment surface that was favorably oriented with the PD block movement (NNW). However, we hypothesize that this outcrop was related to the different behavior between the more rigid limestone formation and the more plastic underlying marly units. Finally, we searched for a possible correlation between the patterns that were observed in the displacement time series, the patterns that were observed in the precipitation time series, the temporal variations of the lake level and seismic activity. Based on the 8-year SAR data, the displacement rate variations (Fig. 10) were not correlated with the main oscillations of the reservoir level (Fig. 11). We compared the total precipitation rates (rain + snow) with 3-month averages and a deformation time series (Fig. 11B). A strong temporal correlation between higher precipitation values and the beginning of acceleration events occurred (Fig. 11B). The largest discrepancy occurred for the acceleration episode that started at the end of 1997. This acceleration period was correlated with the Colfiorito seismic event (see below). On September 26, 1997, two earthquakes with magnitudes of 5.8 and 6.0 occurred on the Colfiorito plain, which is approximately
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Fig. 10. InSAR time series of the surface displacements of the five areas. Data for the reference area are also shown. Error bars represent the standard deviations around the average.
20 km from our study area. We observed a minor positive anomaly in the displacement time series in September 1997 (Fig. 10) for all of the areas that were strongly deformed (Fig. 9). Therefore, we cannot exclude the possible transient reactivation of the PD by local ground accelerations from the Colfiorito earthquake (Moro et al., 2007). 5. Conclusions This study led to the following conclusions. 1) Ground deformation rates were measured with the DInSAR SBAS technique and 32 ERS images that were acquired between 1993 and 2000. Over 15 cm of cumulated vertical surface displacement was detected across the PD. In addition, the highest deformation rates occurred where very fresh-looking extensional fractures occurred in the Upper Trench zone. Table 3 The mean ground velocity (mm y−1) for the areas shown in Fig. 9 and two different periods.
Area Area Area Area Area
no. no. no. no. no.
1 2 3 4 5
7/23/1993 to 8/29/1999
7/23/1993 to 8/29/1999
~0 −15.9 −14.3 −14.0 −15.6
~0 −67.7 −95.7 −59.4 −52.3
The 8-year displacement time series indicates the presence of non-linear deformation rates with 1–2 year periods of more stable behavior and accelerated movement. Additional investigations are needed to explain the causes of the acceleration events. However, we determined that the acceleration events were not caused by fluctuations in the Fiastra lake level. The observed correlation between these acceleration events and strong precipitation can be explained by the positive effect of pore water pressure on deformation rates (Terzaghi, 1950; Forlati et al., 2001; Kilburn and Petley, 2003). In addition, our results suggest that the short-term dynamic stresses imposed during the earthquakes (even at relatively long distances, such as 20 km) potentially triggered the release of gravitational strain that had accumulated in the deep-seated slope deformations (Moro et al., 2007, 2009, 2011, 2012). 2) The Mt. Frascare case study provided strong evidence for a DGSD in terrigenous rocks, as indicated by most of the reported evidence. The PD and UT areas should be closely monitored due to the rate of activity in these areas. Additional studies are needed to quantify the displacement rates across a current time series with SAR images. 3) This combined and multiscale approach between DInSAR and aerial photograph interpretations may have additional positive implications. The application of this method to different areas that are characterized by the presence of different DGSD typologies and an evident meteorological event could allow the separation of gravity from tectonic contributions.
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Fig. 11. DInSAR displacement, reservoir water level and average precipitation. (A) Time series of water level between 1993 and 2001 in response to the Fiastra dam storage. (B) Total precipitation (rain + snow) based on three-month averages (gray diamonds) and deformation (black diamonds).
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Analysis of the deep-seated gravitational slope deformations over Mt. Frascare (Central Italy) with geomorphological assessment and DInSAR approaches C. Tolomei a,⁎, A. Taramelli b, M. Moro a, M. Saroli a,c, D. Aringoli d, S. Salvi a a
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy ISPRA Institute for Environmental Protection and Research, via Vitaliano Brancati, 60, Rome, Italy c DICeM-Civil and Mechanical Engineering Departement, University of Cassino and Southern Lazio, 03043 Cassino, Italy d School of Environmental Sciences, University of Camerino, via Gentile III da Varano, Camerino, Italy b
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 2 May 2013 Accepted 2 July 2013 Available online 10 July 2013 Keywords: DGSD Differential SAR interferometry Remote sensing Central Apennine
a b s t r a c t A quantitative and innovative DGSD (deep gravitational slope deformation) assessment method that used integrated remote sensing was tested in the central Apennine mountain range (Italy). The movement rate was calculated for selected test areas using the differential SAR interferometry small baseline subset (SBAS) technique. The selected test areas were previously identified by interpreting both aerial photos and using 32 ERS radar images taken between 1993 and 2000. More than 15 cm of cumulative surface displacement occurred across the Podalla DGSD along the satellite line of sight (LoS). Moreover, the displacement time series showed non-linear deformation rates, which included periods of accelerated movement correlated with strong rainfall. The high estimated Podalla DGSD activity indicates that this type of study should be conducted to monitor the evolution of this phenomenon. In addition, the DInSAR movement rate can be used to improve mapping and identify DGSDs in specific landscapes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since the 1940s, various authors have observed morphologic evidence of mass movement of entire slopes. These movements occurred over areas that were larger than landslides and were caused by gravity (Dal Piaz, 1936; Ampferer, 1939; Stini, 1941). In the 1950s and 1960s, the concept of “depth creep” was developed to explain this surface evidence. In addition, the first definition of this phenomenon was supplied (Terzaghi, 1950; Jhan, 1964; Ter-Stepanian, 1966; Beck, 1968; Zischinsky, 1969; Nemcock, 1972; Radbruch-Hall, 1978). Over the last 20 years, several studies regarding DGSD phenomena have highlighted the differences between these phenomena and typical large landslides, which are characterized by a slide surface that is not well defined following DGSD (Savage and Varnes, 1987; Chigira, 1992). Recently, the term DGSD has been used to indicate slope movements on high relief-energy hillslopes with an upper portion that is characterized by tensional structures. These tensional structures are comparable to the entire slope regarding size, and have little displacement relative to the slope itself (Cendrero and Dramis, 1996; Agliardi et al., 2001; Goudie, 2004; Baron et al., 2005; Kellerer-Pirklbauer et al., 2010). Trenches, double ridges and counter slopes provide superficial evidence for the mass movement of scarp edges. Sags, cambers and widespread landslides provide ⁎ Corresponding author. E-mail address: [email protected] (C. Tolomei). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.07.002
evidence for compressional stress deformation in the lower portion of slopes (Hutchinson, 1988; Saroli et al., 2005; Galadini, 2006; Moro et al., 2007, 2009, 2011). Deep-seated gravitational slope deformations are generally subdivided into the following two types (Goudie, 2004): 1) sackung (or rock flow) and 2) lateral spreading (or rock spreading): 1) The sackung DGSD is a rock flow that bulges from depths and creeps along very high and steep slopes (Zischinsky, 1969; Hermann et al., 2000). In this case, a constant uplift (with a high catchment discharge at the bottom of the slope) and an isotropic and continuous faulting or jointing of a coherent lithotype are required. Surface evidence for a brittle ruptured middle portion of the slope is characterized by plastic behavior (Radbruch-Hall, 1978) that increases the volume of the bulging landform at the bottom of the slope. 2) When a coherent and thick layer of rock overlies a weaker lithotype with a very gentle dip angle and when the rock masses are affected by tectonic residual stress, lateral mass movement may occur towards the outside and bottom of the slope. These phenomena are clearly linked to active faults and tectonic settings (Dramis and Sorriso-Valvo, 1994; Galadini, 2006; Moro et al., 2007, 2009, 2011, 2012). First, uplift and subsidence may produce enough energy to cause gravitationally driven slope failures. Furthermore, tectonics also produce aligned fault surfaces or joints along the slope and can act as triggering planes for deep mass creep.
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In the last 20 years, many studies have identified the presence of DGSD phenomena in the central Apennine mountains, particularly along the calcareous mountain ranges (Farabollini et al., 1995; Aringoli et al., 1996). However, most of these studies found strong evidence of DGSDs in terrigenous rocks (Genevois and Tecca, 1984; Crescenti et al., 1994; Calabresi et al., 1995; Dramis et al., 1995; Baron et al., 2005). Most results contained field evidence (geomorphologic and stratigraphic) for the activity between the Pliocene and the Pleistocene and the maximum relief along slopes (Ambrosetti et al., 1982). All of the presented studies identified some morphologic evidence from the tectonic and lithologic structures of the Apennines (Galadini, 2006; Moro et al., 2007, 2009). Thus, we chose to study the central Apennines in the Umbria and Marche regions of central Italy (Fig. 1). The use of different satellite remote sensing approaches is a new and potentially useful way to identify the presence of DGSD phenomena (Hilley et al., 2004; Saroli et al., 2005; Ambrosi and Crosta, 2006; Taramelli and Melelli, 2009). Thus, the development of DGSD detection methods that use remote sensing data has become an important
topic of research in the last decade (Allievi et al., 2003; Catani et al., 2005; Moro et al., 2007, 2009; Melelli and Taramelli, 2010; Moro et al., 2011). Unfortunately, when treated separately, the characteristics of this slope instability phenomenon were not comparable to the acquisition parameters of different space-borne missions, such as spatial resolution, revisiting time and view geometry (Stramondo et al., 2005; Moro et al., 2007). Therefore, these results were not as good as the results obtained using other geophysical applications (Antonello et al., 2004; Stramondo et al., 2007). However, the use of integrated remote sensing methods could provide more flexibility regarding its operation, the acquisition of morphometric parameters for spatially distributed information and its independence relative to different acquisition conditions. In addition, remote sensing methods could eliminate the drawbacks that are related to different satellite platforms (Antonello et al., 2004). Recently the potential advantages of combining synthetic aperture radar (SAR) and aerial photography to analyze surface changes and classify DGSDs were investigated (Melelli et al., 2007).
Fig. 1. Study area. The rectangle shows the area of Figs. 5 and 9 (Frascare Mountain).
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SAR collects high-resolution radar echo maps of the Earth's surface from satellites (Bürgmann et al., 2006). In addition, SAR measures the backscattering coefficient of the surface with a pixel size of 10 m or less. Specifically, SAR measures the phase of the signal echoed by each ground pixel as an incoherent sum of the return echoes from the pixel's targets. Thus, SAR is strongly sensitive to surface changes. DGSDs modify observed scenarios and their electromagnetic behavior, which enables the detection of these changes by comparing the SAR images in terms of both amplitude and phase. Improving classical differential SAR interferometry (DInSAR) has expanded the application of this technique. For example, this technique can be used to detect very slow deformations and provide ground velocity and displacement measurements across a time series with sub-centimeter accuracy (Ferretti et al., 2000, 2001; Berardino et al., 2002; Lanari et al., 2004; Salvi et al., 2004). Here, an innovative approach for improving DGSD detection and analysis was tested. This approach was based on the multiscale application of DInSAR and aerial photography. We estimated the DGSD signature of the central Apennine ridge (terrigenous fractions) by using integrated analysis. We selected the NE slope of Mt. Frascare as a test area to study classical geological, geomorphological and advanced DlnSAR techniques for deformation measurements. This area contained the most impressive geomorphological evidence and the greatest velocities, which were measured across the study area with the interferometry technique. The paper is organized as follows. The interpretations of the aerial photographs were outlined to define the topographic analyses that were used to characterize the DGSD evidence in the central Apennine ridge (Italy). Next, the large-scale gravitational phenomena were identified by their morphological and geological descriptions at the individual study sites (detailed in the next section). The succeeding section describes the application of the DInSAR method. Finally, detailed analysis of the results and the displacement time series of the investigated area were used to explain the main DGSD characteristics
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and how the approach can be directly used to quantitatively detect these phenomena. 2. Geology and geomorphology of the study area Most mass wasting studies on the central Apennine ridge have focused on quantifying shallow landslides, flow erosion debris (Melelli and Taramelli, 2004) and their effects on landscape functions on the human and geologic time scales (Galadini, 2006). The presence of a shallow landslide near a lake dam in the Mt. Frascare area suggests that DGSDs potentially played an important role in shaping the region (Dramis and Sorriso-Valvo, 1994; Aringoli et al., 1996). The study area is located at the top of the Sibillini Mountain thrust next to the Lower Lias–Upper Eocene calcareous formations, which contain Oligocene marls that were formed during the Tortonian–Lower Pliocene compressive tectonic phase (Calamita, 1990). The calcareous units form a complex folded setting with a northward axial depression (direction of the axes N170°). These units are dissected by faults in the following directions: Apenninic (N150°–170°), anti-Apenninic (N30°–50°) and N–S (Fig. 2). The Apenninic faults are related to the compressive tectonic phase (Lower Pliocene) and were reused as normal faults during the Pleistocene, which lowered the entire structure to the west. The anti-Apenninic faults are related to the same compressive phase, but they have more complex kinematics. For example, the main movement was mainly inverse (left-transpressive) and resulted from a more recent reactivation of the normal faults (right-transtensive). The N–S fault system is characterized by right-transtensive kinematics and an en-echelon geometry that was generated during the reactivation (Upper Pliocene) of the Sibillini Mountain thrust by a gravitational tectonic mechanism. The area is crossed by an important shallow (approximately 200 m deep) thrust plane that strikes NE and dips approximately 10° to the NW (Fig. 2). The thrust is superimposed on calcareous formations that occur on the more plastic marly formations. The complex litho-
Fig. 2. Geological and geomorphological sketch. 1) calcareous formations; 2) marly formations; 3) slope waste deposits; 4) slides; 5) flows; 6) rock falls; 7) strata attitudes; 8) normal faults (a) and thrusts (b); 9) tectonized zones; 10) main edges of structural scarps; 11) trenches; 12) structurally conditioned downcutting streams; and 13) the DSGSD upper limit.
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structural setting was affected during the Quaternary period by extensional tectonics and isostatic uplift, which generated new discontinuities (Ambrosetti et al., 1982; Dramis et al., 1995) and renewed previous discontinuities. In this area, photogeological analysis allowed us to identify different morphological features that indicated DGSDs, such as double crest lines, scarps, counterslope scarps, slope-parallel trenches, fractures, open fissures and small depressions that were linearly aligned to the NE (Fig. 3). We determined that the Podalla DGSD (hereinafter PD) is a sackung, which moves towards the NW, is controlled by NE-striking, and has an NW-dipping sliding plane that reaches a
depth of approximately 500–600 m and evolves in a shear zone at these depths. The top of the sackung is limited by the presence of arcuated trenches (Figs. 3 and 4). From a geomorphologic point of view, the folded structures in the study area, which were mainly composed of calcareous rocks, were eroded during the Pliocene period (almost exclusively during continental conditions) to produce a low relief erosional surface. This ancient landscape was deeply deformed and incised, which began during the Lower Pleistocene (Ciccacci et al., 1985; Coltorti and Dramis, 1995). This process and an important anti-Apenninic tectonic line formed the Fiastrone River Gorge, which borders the study area
Fig. 3. Photogeological interpretation. 1) slope waste deposits; 2) slides; 3) flows; 4) rock falls; 5) DSGSD upper limit; 6) strata attitudes; 7) normal faults (a) and thrusts (b); 8) tectonized zones; 9) main edges of structural scarps; 10) trenches; and 11) structurally conditioned downcutting streams.
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Fig. 4. Three dimensional view and simplified cross-section for Mt. Frascare. Vertical exaggeration = 3×. Highlighted typical DGSD features are trenches, scarps, and surficial slides.
to the north (Coltorti et al., 1996). Along the base of this small river in the northwest area, valley incision created steep slopes at the channel margin. Similarly, steep sections were observed during our field investigations along the northern slope of Mt. Frascare (Fig. 5). Different patterns were observed between the NW and NE river profiles. Profiles 1 and 2 show obvious knickpoints while profiles 3 and 4 show more regular patterns with only minor deviations (Fig. 5). A steep composite slope in the lower part characterizes the area that contains rivers 1 and 2 with a convex shape to the west and concave shape to the east. The central portion of the slope has a low-gradient topography (Fig. 6) that is affected by a set of N50°-trending scarps and small incisions that isolate elongated blocks with nearly flat surfaces (Fig. 5). The tip of the triangle connects the headwaters of rivers 1 and 2 with the flat summit of Mt. Frascare by a steep scarp. Upslope of this scarp and inside a belt that has a width of approximately 600 m, several near-parallel trenches outcrop at the mesoscale (Fig. 7). These trenches vary in length from a few to 800 m, width from 50 cm to N20 m, and depth up to 10 m. In addition, repeated field surveys showed that these fractures are subject to constant evolution. Very fresh-looking scarps, up to 50 m long and 50 cm wide, were first surveyed in 1995 (Aringoli et al., 1996). In the following years, the increasing length and width, opening up to few cm per year, were observed (Fig. 8). In fact, the wider area around the triangular block is subject to structural control by (mainly) the N60° tectonic trend, as shown by the alignment of the main incisions and scarps (Figs. 3 and 5). The main boundary condition that constrains the re-use of these fractures and joints by gravitational forces is the deep incision of the Fiastrone gorge. Other important factors that may control the deformation rates include the direction of the slope with respect to structural trends and the presence of marl layers that were locally isolated by previous tectonics (Figs. 3 and 5). The central triangular block that is described above provides evidence for the largest deformation. This block has been identified as PD. 3. DInSAR data analysis and active ground deformation measurements The differential interferometric SAR technique was used to investigate the current deformation rates. In fact, the classical differential SAR Interferometry is a technique that uses synthetic aperture radar image data (usually collected by space sensors) to calculate ground surface movements that occur between two satellite passes over the
same area. This technique is based on the radar concept. The phase of the radar signal that is returned to the satellite conveys quantitative information regarding changes in the sensor-to-ground distance (range) that are caused by surface deformation (Bϋrgmann et al., 2000). By subtracting the phase of the two images and then simulating and subtracting the phase contributions due to topographic relief (a DEM is needed for this), a differential interferogram is formed. This interferogram contains the ground deformation signal that occurred between the two passes (Massonnet and Feigl, 1998). The accuracy of the surface movements that are measured by the DInSAR technique depends on various factors such as atmospheric effects, orbital effects, stability of ground scatterers, and unwrapping errors. However, in favorable cases, the accuracy of the displacement is less than 1 cm (Hanssen, 2001). In the last decade, this technique was improved by the development of a multi-temporal DInSAR approach, which allows users to obtain displacement time series and surface mean velocities data at accuracies of ~ 1 mm yr−1 (Casu et al., 2006). Here, we used the small baseline subset (SBAS) algorithm (Berardino et al., 2002). With this algorithm, we processed SAR data acquired by the ERS-1/2 satellites of the European Space Agency (ESA) to estimate the displacement time-series and the mean velocities of the coherent ground areas. This method has been used to accurately measure ground deformation for a variety of applications, including urban subsidence (Stramondo et al., 2007), volcanic eruptions (Pritchard and Simons, 2004), post-seismic and inter-seismic monitoring (Hunstad et al., 2009), and analyses of gravitational phenomena (Catani et al., 2005; Saroli et al., 2005; Ambrosi and Crosta, 2006). The SBAS algorithm uses a large number (several tens) of radar images to reduce various noise components of the DInSAR interferograms, which increases the accuracy of the displacement measurements (Casu et al., 2006). Initially, the operator defines the acceptable temporal and orbital separations that occur between each of the DInSAR interferogram images to be generated. Next, a consistent number of differential interferograms are generated with a DEM of comparable resolution and are unwrapped to generate actual displacement maps. Then, the reference pixel, for which all calculated displacements and velocities will be referenced, is chosen. The unwrapped phases are inverted with the singular value decomposition (SVD) technique, and the displacement time series for each image (at a given date) are retrieved for each pixel that has a coherence value greater than the fixed threshold. The SVD method is used to compute the matrix pseudoinverse to solve the over-determined linear
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Fig. 5. Rivers and their profiles along the northern slope of Mt. Frascare. A) Location of the rivers. The areas marked as UT, PD and AB are discussed in the text. The locations of knickpoints (np) are marked in green. B) River profiles. Streams 1 and 2 have prominent knickpoints.
equation system. At this step, the orbital residual and topographic errors are estimated before subtracting. Finally, the short-term atmospheric contribution is removed by double filtering in time and space. From our SAR data processing, only the pixels with an interferometric coherence of more than 0.7 (over 30% of the total number of interferograms) were considered to be reliable (Burgmann et al., 2000). Thus, for each retained pixel of 80 × 80 m, a time series of the ground displacement was calculated for each image. All displacements are relative to the reference pixel (or area) and are assumed to be stable in the image (Burgmann et al., 2000) and along the sensor line of sight (LoS). We applied the SBAS technique to a data set of 32 ERS1–2 images that were acquired from the ascending pass, track 401, and frame 858
between 1993 and 2000 (Tables 1 and 2). To generate the DInSAR couples, we imposed a maximum orbital separation of 300 m to reduce spatial decorrelation, and a maximum temporal distance of 1200 days between two passes to limit the temporal decorrelation effects. By using these constraints, a maximum of 73 interferograms were generated. Of these interferograms, 16 were excluded due to their scarce coherence (leaving 57 usable interferograms). We used the SRTM DEM for topography subtraction (http://www2.jpl.nasa. gov/srtm — Farr et al., 2007) and processed a spatial subset of the images in the Mt. Frascare area that contained pixels of 80 × 80 m. The obtained ascending mean velocity map is shown in Fig. 9, where the image pixels are symbolized as points. Due to high relief, diffuse
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Fig. 6. Counterslope between the Fiastrone River and Mt. Frascare with different trench types (at the top and in the mid part of the slope) in the SE–NW direction.
vegetation cover and scarce rock outcrops, the temporal coherence is relatively low in this area. Only a few pixels reached the minimum coherence level of 0.7. As previously mentioned, the displacement time series values were calculated relative to a given pixel that was located in a stable area. This reference area was selected near the summit of Mt. Frascare, where no geomorphological evidence of long-term gravitational processes was found (Fig. 9). To reduce the uncertainty, we averaged the time series that were available for the reference area. Thus, all of the displacement time series and ground velocities discussed below are relative to that average. The mean ground velocities shown in Fig. 9 were calculated from the time series by assuming a linear deformation between 1993 and 2000. They represent the general ground deformation pattern. However, the time series also contains some non-linear components, as demonstrated by the change of slope in the time series in Fig. 10. Surface movements measured by the DInSAR techniques always include scalar measurements along the satellite's LoS (Burgmann et al., 2000). In our case (ERS imagery), the LoS is vertically inclined by approximately 23° and looks eastward from the ascending orbit (i.e., a line running N13°W) (Fig. 9). Fig. 9 indicates that the ground in the central and upper portions of the PD moved away from the satellite at rates of up to 3–4 cm yr−1
between 1993 and 2000. However, areas located outside the mapped DGSD limits were relatively stable during this period. Based on the field observations and the general geological setting, we hypothesize that the main horizontal component of DGSD movement was perpendicular to the main slope (Fig. 3). In addition, we hypothesize that negligible deformation occurred along the slope. These horizontal movements, which occurred nearly parallel to the ascending orbit (Fig. 9), cannot be resolved in the DInSAR interferograms because they only caused very small changes in distance between the SAR antenna and the ground (Burgmann et al., 2000). Therefore, we assume that the displacement that resulted from our SBAS analysis represents the projection in the vertical component LoS of ground deformation. In addition, we can calculate the actual vertical displacement or velocity by dividing the LoS value by 0.9, i.e., the cosine of the local incidence angle. 4. Discussion We investigated the geomorphological evidence of a DGSD (PD) in the study area at increasingly larger scales. The lower triangular block that corresponded to the PD showed well-developed gravitational landforms. The parallel NE–SW scarps in the central part of the block are up to 25 m tall and consist of several dislocated and
Fig. 7. Trenches near the top of Mt. Frascare. Greater trenches are mainly occupied by debris and vegetation. However, small recent trenches (on the right) are less open, not filled by debris, and do not contain vegetation.
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Fig. 8. Fresh crack widening up to a few cm per year. The crack has opened more than 0.5 m.
elongated flat-tops and tilted blocks of up to 1 km long and 500 m wide (Fig. 9). A less pronounced deformation can be observed in the upper trench area (hereinafter UT), where the fractures are smaller (up to a few meters) and are closer together. The strong extension that affects the UT acts approximately perpendicularly to the steep slope, which represents the scarp of the upper PD. This result suggests that the recent trench development corresponds with a continuously decreasing DGSD. at least in its upper portion. To obtain a better representation of ongoing deformation, we averaged the DInSAR displacement time series over five areas (Fig. 9). These areas correspond to the following homogeneous blocks, which were defined by geomorphological analysis (Figs. 3, 4, 5, and 7): 1) the flat area upslope of the UT zone, where no fresh scarps are observed; 2) the proper UT zone where the recent fracture field is present; 3) flat and 4) tilted terraces in the central portion of the PD; and 5) a stable area west of the UT zone which shows no sign of recent deformation.
Table 1 Specifications of ERS1 and 2 satellites. Satellite Launch date Altitude Revisiting cycle Acquisition time
ERS-1
17 July 1991 800 km 35 days 21.16 in ascending orbit 9.40 in descending orbit Orbit inclination 98.5° inclination orbit Look angle 23 deg look angle towards right Band/wavelength C/5.8 cm Frame dimension 100 × 100 km SAR pixel size 20 × 4 m
ERS-2 21 April 1995 800 km 35 days 21.16 in ascending orbit 9.40 in descending orbit 98.5° inclination orbit 23 deg look angle towards right C/5.8 cm 100 × 100 km 20 × 4 m
Acquisition date
Sensor
23/07/1993 02/04/1995 07/05/1995 17/07/1995 25/09/1995 07/01/1996 08/01/1996 17/03/1996 18/03/1996 21/04/1996 22/04/1996 01/07/1996 18/11/1996 07/04/1997 12/05/1997 16/06/1997 21/07/1997 29/09/1997 08/12/1997 16/02/1998 06/07/1998 10/08/1998 14/09/1998 23/11/1998 21/06/1999 29/08/1999 08/11/1999 16/01/2000 17/01/2000 01/05/2000 23/10/2000
ERS1 ERS1 ERS1 ERS2 ERS2 ERS1 ERS2 ERS1 ERS2 ERS1 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS2 ERS1 ERS2 ERS1 ERS2 ERS2 ERS2
By averaging the displacements, we reduced the data uncertainty and estimated the local variability of the deformation. The large displacement error bars in Fig. 10 indicate the presence of internal deformation. In all areas, the average time series exhibited a non-linear temporal trend with periods of more stable deformation and of increased ground velocity (Fig. 10). The same variations, albeit with variable intensities, are present in all of the time series, suggesting that the variations originated from a common cause but were different due to local responses. When we calculated the mean velocities before and after the sharp change that occurred in November 1999 (Fig. 10), we observed strong acceleration in the unstable areas (Table 3). Moreover, while the velocities prior to August 1999 were similar, a greater acceleration was observed during the second period in the upper zones (1 and 2) than in the central blocks of the PD (zones 3 and 4). These results indicate larger gravitational deformation rates on the UT relative to the central PD. In addition, a rather sharp transition occurs between the stable ground velocities in the SW area and the rapid deformations in the UT and PD. This finding suggests the presence of a structural discontinuity that is oriented in the NW and SE direction along incision no. 1 (Fig. 5). Based on the geomorphological analysis and the current patterns of ground deformation, the entire Mt Frascare NW slope can be partitioned into three different sub-regions that are characterized by different activity rates. These sub-regions include the central triangular block including its upslope extension in the UT, the southwestern area and the northeastern area. The central triangular block corresponds to the PD as defined in Section 2. Here, the surfaces of the tilted blocks show considerable ongoing vertical deformation (~− 2 cm yr−1, Fig. 9). The flat and tilted surfaces in this area are likely relics of the same erosional surface that formed the upper portion of Mt. Frascare. Because the two surfaces now differ in elevation by 80 to 100 m, gravitational deformation occurred over a time frame of between 4000 and 5000 years (this is only an estimation because deformation rates are not constant).
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Fig. 9. Ascending velocity map of the Fiastra area. Five areas shown by yellow outlines are where the average time series were obtained (Fig. 10).
Geomorphological investigations indicate that in the UT zone of the PD, the gravitational deformations began more recently. This may be due to the fragile response of the pre-existing NE–SW fractures to slope disequilibrium from the movement of the PD block. This zone has the highest vertical velocity (up to 10 cm yr−1) and different deformation mechanisms from that of the PD (toppling). Additional evidence regarding the strong influence of the NE–SW structural discontinuities that control landform development in this area is provided by the analysis of river profiles. As shown in Fig. 5A, streams 1 and 2 cross the structural trend at a high angle and streams 3 and 4 follow nearly the same trend. Only streams 1 and 2 have well developed knickpoints (Fig. 5B) that are aligned with one of the main scarps in the PD block (Fig. 5A), which may represent the same deformation surface. Finally, the western portion of the lower PD slope and the presence of a more advanced block (AB in Fig. 5A) with a prominently convex slope, which is separated from the PD main body by a large and well developed trench (Figs. 4 and 5A), suggests strong ongoing deformation. Unfortunately, no coherent DInSAR pixel is available to measure the ground velocity at this site. In the second area, located SW of the PD, the drainage network indicates the presence of a structural control based on the NE–SW fracture pattern. However, no marked gravitational landforms exist and the current ground velocities indicate relative stability (Fig. 5A). In the third area east of the PD, the dominant landforms include two nearly parallel streams (3 and 4 in Fig. 5A) that cause strong localized erosion. The development of these landforms is controlled by the same NE–SW structural trend. The slopes in this area do not indicate recent or historical large mass movements. In addition, the main stream profiles do not contain evident knickpoints. We interpreted the different behaviors of the three areas based on the complex interactions of various factors. The rapid deepening of
the Fiastra River was the first factor that we considered, which caused the valley slopes to move laterally and progressively, resulting in toe unloading. Given the curvature of the Fiastra valley, the main-slope stress direction in the central block (the PD and UT) was favorably oriented perpendicular to the extension of the NE–SW joint set. In the other two sectors, the angle was reduced to 30° (Fig. 5A). In addition, the slope of the SW sector has lower relief and inclination than the other two sectors. Although the NE sector has the highest relief and steepest slope inclination, the joint set was rapidly exploited by erosion, which effectively reduced the gravitational stress. Another potentially important control factor regarding the development of the PD is the shallow thrust (see Section 2) that outcrops to the east, which likely reaches a depth of 200 m beneath the PD block. This outcrop potentially served as a detachment surface that was favorably oriented with the PD block movement (NNW). However, we hypothesize that this outcrop was related to the different behavior between the more rigid limestone formation and the more plastic underlying marly units. Finally, we searched for a possible correlation between the patterns that were observed in the displacement time series, the patterns that were observed in the precipitation time series, the temporal variations of the lake level and seismic activity. Based on the 8-year SAR data, the displacement rate variations (Fig. 10) were not correlated with the main oscillations of the reservoir level (Fig. 11). We compared the total precipitation rates (rain + snow) with 3-month averages and a deformation time series (Fig. 11B). A strong temporal correlation between higher precipitation values and the beginning of acceleration events occurred (Fig. 11B). The largest discrepancy occurred for the acceleration episode that started at the end of 1997. This acceleration period was correlated with the Colfiorito seismic event (see below). On September 26, 1997, two earthquakes with magnitudes of 5.8 and 6.0 occurred on the Colfiorito plain, which is approximately
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Fig. 10. InSAR time series of the surface displacements of the five areas. Data for the reference area are also shown. Error bars represent the standard deviations around the average.
20 km from our study area. We observed a minor positive anomaly in the displacement time series in September 1997 (Fig. 10) for all of the areas that were strongly deformed (Fig. 9). Therefore, we cannot exclude the possible transient reactivation of the PD by local ground accelerations from the Colfiorito earthquake (Moro et al., 2007). 5. Conclusions This study led to the following conclusions. 1) Ground deformation rates were measured with the DInSAR SBAS technique and 32 ERS images that were acquired between 1993 and 2000. Over 15 cm of cumulated vertical surface displacement was detected across the PD. In addition, the highest deformation rates occurred where very fresh-looking extensional fractures occurred in the Upper Trench zone. Table 3 The mean ground velocity (mm y−1) for the areas shown in Fig. 9 and two different periods.
Area Area Area Area Area
no. no. no. no. no.
1 2 3 4 5
7/23/1993 to 8/29/1999
7/23/1993 to 8/29/1999
~0 −15.9 −14.3 −14.0 −15.6
~0 −67.7 −95.7 −59.4 −52.3
The 8-year displacement time series indicates the presence of non-linear deformation rates with 1–2 year periods of more stable behavior and accelerated movement. Additional investigations are needed to explain the causes of the acceleration events. However, we determined that the acceleration events were not caused by fluctuations in the Fiastra lake level. The observed correlation between these acceleration events and strong precipitation can be explained by the positive effect of pore water pressure on deformation rates (Terzaghi, 1950; Forlati et al., 2001; Kilburn and Petley, 2003). In addition, our results suggest that the short-term dynamic stresses imposed during the earthquakes (even at relatively long distances, such as 20 km) potentially triggered the release of gravitational strain that had accumulated in the deep-seated slope deformations (Moro et al., 2007, 2009, 2011, 2012). 2) The Mt. Frascare case study provided strong evidence for a DGSD in terrigenous rocks, as indicated by most of the reported evidence. The PD and UT areas should be closely monitored due to the rate of activity in these areas. Additional studies are needed to quantify the displacement rates across a current time series with SAR images. 3) This combined and multiscale approach between DInSAR and aerial photograph interpretations may have additional positive implications. The application of this method to different areas that are characterized by the presence of different DGSD typologies and an evident meteorological event could allow the separation of gravity from tectonic contributions.
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Fig. 11. DInSAR displacement, reservoir water level and average precipitation. (A) Time series of water level between 1993 and 2001 in response to the Fiastra dam storage. (B) Total precipitation (rain + snow) based on three-month averages (gray diamonds) and deformation (black diamonds).
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