Geomorphology xxx (2013) xxx–xxx

GEOMOR-04410; No of Pages 12

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Geomorphology

j ourna l homepage: www.e lsev ie r .com/ locate /geomorph

Analysis of the deep-seated gravitational slope deformations overMt. Frascare (Central Italy) with geomorphological assessmentand 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, Italyb ISPRA Institute for Environmental Protection and Research, via Vitaliano Brancati, 60, Rome, Italyc DICeM-Civil and Mechanical Engineering Departement, University of Cassino and Southern Lazio, 03043 Cassino, Italyd School of Environmental Sciences, University of Camerino, via Gentile III da Varano, Camerino, Italy

⁎ Corresponding author.E-mail address: cristiano.tolomei@ingv.it (C. Tolome

0169-555X/$ – see front matter © 2013 Elsevier B.V. Alhttp://dx.doi.org/10.1016/j.geomorph.2013.07.002

Please cite this article as: Tolomei, C., et al.,geomorphological assessment..., Geomorpho

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 August 2012Received in revised form 2 May 2013Accepted 2 July 2013Available online xxxx

Keywords:DGSDDifferential SAR interferometryRemote sensingCentral Apennine

A quantitative and innovative DGSD (deep gravitational slope deformation) assessment method that used in-tegrated remote sensing was tested in the central Apennine mountain range (Italy). The movement rate wascalculated for selected test areas using the differential SAR interferometry small baseline subset (SBAS) tech-nique. The selected test areas were previously identified by interpreting both aerial photos and using 32 ERSradar images taken between 1993 and 2000. More than 15 cm of cumulative surface displacement occurredacross the Podalla DGSD along the satellite line of sight (LoS). Moreover, the displacement time seriesshowed non-linear deformation rates, which included periods of accelerated movement correlated withstrong rainfall. The high estimated Podalla DGSD activity indicates that this type of study should beconducted to monitor the evolution of this phenomenon. In addition, the DInSAR movement rate can beused 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 evi-dence of mass movement of entire slopes. These movements occurredover 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 surfaceevidence. In addition, the first definition of this phenomenon wassupplied (Terzaghi, 1950; Jhan, 1964; Ter-Stepanian, 1966; Beck, 1968;Zischinsky, 1969; Nemcock, 1972; Radbruch-Hall, 1978). Over the last20 years, several studies regarding DGSD phenomena have highlightedthe differences between these phenomena and typical large landslides,which are characterized by a slide surface that is not well defined follow-ing DGSD (Savage and Varnes, 1987; Chigira, 1992). Recently, the termDGSD has been used to indicate slope movements on high relief-energyhillslopes with an upper portion that is characterized by tensional struc-tures. These tensional structures are comparable to the entire slope re-garding size, and have little displacement relative to the slope itself(Cendrero and Dramis, 1996; Agliardi et al., 2001; Goudie, 2004; Baronet al., 2005; Kellerer-Pirklbauer et al., 2010). Trenches, double ridgesand counter slopes provide superficial evidence for the mass movementof scarp edges. Sags, cambers and widespread landslides provide

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Analysis of the deep-seated glogy (2013), http://dx.doi.org

evidence for compressional stress deformation in the lower portion ofslopes (Hutchinson, 1988; Saroli et al., 2005; Galadini, 2006; Moro etal., 2007, 2009, 2011). Deep-seated gravitational slope deformationsare 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 creepsalong very high and steep slopes (Zischinsky, 1969; Hermann et al.,2000). In this case, a constant uplift (with a high catchment dis-charge at the bottom of the slope) and an isotropic and continuousfaulting or jointing of a coherent lithotype are required. Surface ev-idence for a brittle rupturedmiddle portion of the slope is character-ized by plastic behavior (Radbruch-Hall, 1978) that increases thevolume of the bulging landform at the bottom of the slope.

2) When a coherent and thick layer of rock overlies a weakerlithotype with a very gentle dip angle and when the rock massesare affected by tectonic residual stress, lateral mass movementmay occur towards the outside and bottom of the slope.

These phenomena are clearly linked to active faults and tectonicsettings (Dramis and Sorriso-Valvo, 1994; Galadini, 2006; Moro etal., 2007, 2009, 2011, 2012). First, uplift and subsidence may produceenough energy to cause gravitationally driven slope failures. Further-more, tectonics also produce aligned fault surfaces or joints along theslope 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 ofDGSD phenomena in the central Apennine mountains, particularlyalong the calcareous mountain ranges (Farabollini et al., 1995;Aringoli et al., 1996). However, most of these studies found strong evi-dence of DGSDs in terrigenous rocks (Genevois and Tecca, 1984;Crescenti et al., 1994; Calabresi et al., 1995; Dramis et al., 1995; Baronet al., 2005). Most results contained field evidence (geomorphologicand stratigraphic) for the activity between the Pliocene and the Pleisto-cene and the maximum relief along slopes (Ambrosetti et al., 1982). Allof the presented studies identified some morphologic evidence fromthe tectonic and lithologic structures of the Apennines (Galadini,2006; Moro et al., 2007, 2009). Thus, we chose to study the centralApennines in the Umbria and Marche regions of central Italy (Fig. 1).

The use of different satellite remote sensing approaches is a newand potentially useful way to identify the presence of DGSD phenom-ena (Hilley et al., 2004; Saroli et al., 2005; Ambrosi and Crosta, 2006;Taramelli and Melelli, 2009). Thus, the development of DGSD detec-tion methods that use remote sensing data has become an important

Fig. 1. Study area. The rectangle shows the a

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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 etal., 2011). Unfortunately, when treated separately, the characteristicsof this slope instability phenomenon were not comparable to theacquisition parameters of different space-borne missions, such asspatial resolution, revisiting time and view geometry (Stramondo etal., 2005; Moro et al., 2007). Therefore, these results were not asgood as the results obtained using other geophysical applications(Antonello et al., 2004; Stramondo et al., 2007). However, the use ofintegrated remote sensing methods could provide more flexibility re-garding its operation, the acquisition of morphometric parameters forspatially distributed information and its independence relative to dif-ferent acquisition conditions. In addition, remote sensing methodscould eliminate the drawbacks that are related to different satelliteplatforms (Antonello et al., 2004). Recently the potential advantagesof combining synthetic aperture radar (SAR) and aerial photographyto analyze surface changes and classify DGSDs were investigated(Melelli et al., 2007).

rea of Figs. 5 and 9 (Frascare Mountain).

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SAR collects high-resolution radar echo maps of the Earth's surfacefrom satellites (Bürgmann et al., 2006). In addition, SAR measures thebackscattering coefficient of the surface with a pixel size of 10 m orless. Specifically, SAR measures the phase of the signal echoed by eachground pixel as an incoherent sum of the return echoes from the pixel'stargets. Thus, SAR is strongly sensitive to surface changes. DGSDsmodifyobserved scenarios and their electromagnetic behavior, which enablesthe detection of these changes by comparing the SAR images in termsof both amplitude and phase. Improving classical differential SAR inter-ferometry (DInSAR) has expanded the application of this technique. Forexample, this technique can be used to detect very slow deformationsand provide ground velocity and displacement measurements across atime 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 andanalysis was tested. This approachwas based on themultiscale applica-tion of DInSAR and aerial photography. We estimated the DGSD signa-ture of the central Apennine ridge (terrigenous fractions) by usingintegrated analysis. We selected the NE slope of Mt. Frascare as a testarea to study classical geological, geomorphological and advancedDlnSAR techniques for deformationmeasurements. This area containedthemost impressive geomorphological evidence and the greatest veloc-ities, whichweremeasured across the study area with the interferome-try technique.

The paper is organized as follows. The interpretations of the aerialphotographs were outlined to define the topographic analyses thatwere used to characterize the DGSD evidence in the central Apennineridge (Italy). Next, the large-scale gravitational phenomena wereidentified by their morphological and geological descriptions at theindividual study sites (detailed in the next section). The succeedingsection describes the application of the DInSAR method. Finally, de-tailed analysis of the results and the displacement time series of theinvestigated area were used to explain the main DGSD characteristics

Fig. 2. Geological and geomorphological sketch. 1) calcareous formations; 2) marly formationfaults (a) and thrusts (b); 9) tectonized zones; 10) main edges of structural scarps; 11) trench

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and how the approach can be directly used to quantitatively detectthese phenomena.

2. Geology and geomorphology of the study area

Most mass wasting studies on the central Apennine ridge have fo-cused on quantifying shallow landslides, flow erosion debris (Melelliand Taramelli, 2004) and their effects on landscape functions on thehuman and geologic time scales (Galadini, 2006). The presence of ashallow landslide near a lake dam in the Mt. Frascare area suggeststhat 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 thrustnext to the Lower Lias–Upper Eocene calcareous formations, which con-tain Oligocene marls that were formed during the Tortonian–Lower Pli-ocene compressive tectonic phase (Calamita, 1990). The calcareous unitsform a complex folded setting with a northward axial depression (direc-tion of the axes N170°). These units are dissected by faults in the follow-ing directions: Apenninic (N150°–170°), anti-Apenninic (N30°–50°) andN–S (Fig. 2). The Apenninic faults are related to the compressive tectonicphase (Lower Pliocene) and were reused as normal faults during thePleistocene, which lowered the entire structure to the west. Theanti-Apenninic faults are related to the same compressive phase, butthey have more complex kinematics. For example, the main movementwasmainly inverse (left-transpressive) and resulted from amore recentreactivation of the normal faults (right-transtensive). The N–S fault sys-tem is characterized by right-transtensive kinematics and an en-echelongeometry 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 mdeep) thrust plane that strikes NE and dips approximately 10° to theNW (Fig. 2). The thrust is superimposed on calcareous formations thatoccur on the more plastic marly formations. The complex litho-

s; 3) slope waste deposits; 4) slides; 5) flows; 6) rock falls; 7) strata attitudes; 8) normales; 12) structurally conditioned downcutting streams; and 13) the DSGSD upper limit.

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structural setting was affected during the Quaternary period by exten-sional tectonics and isostatic uplift, which generated new discontinuities(Ambrosetti et al., 1982; Dramis et al., 1995) and renewed previousdiscontinuities.

In this area, photogeological analysis allowed us to identify differ-ent morphological features that indicated DGSDs, such as double crestlines, scarps, counterslope scarps, slope-parallel trenches, fractures,open fissures and small depressions that were linearly aligned tothe NE (Fig. 3). We determined that the Podalla DGSD (hereinafterPD) is a sackung, which moves towards the NW, is controlled byNE-striking, and has an NW-dipping sliding plane that reaches a

Fig. 3. Photogeological interpretation. 1) slopewaste deposits; 2) slides; 3)flows; 4) rock falls; 5zones; 9) main edges of structural scarps; 10) trenches; and 11) structurally conditioned dow

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depth of approximately 500–600 m and evolves in a shear zone atthese depths. The top of the sackung is limited by the presence ofarcuated trenches (Figs. 3 and 4).

From a geomorphologic point of view, the folded structures in thestudy area, which were mainly composed of calcareous rocks, wereeroded during the Pliocene period (almost exclusively during conti-nental conditions) to produce a low relief erosional surface. This an-cient landscape was deeply deformed and incised, which beganduring the Lower Pleistocene (Ciccacci et al., 1985; Coltorti andDramis, 1995). This process and an important anti-Apenninic tectonicline formed the Fiastrone River Gorge, which borders the study area

)DSGSDupper limit; 6) strata attitudes; 7) normal faults (a) and thrusts (b); 8) tectonizedncutting 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.

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to the north (Coltorti et al., 1996). Along the base of this small river inthe northwest area, valley incision created steep slopes at the channelmargin. Similarly, steep sections were observed during our field in-vestigations along the northern slope of Mt. Frascare (Fig. 5). Differ-ent patterns were observed between the NW and NE river profiles.Profiles 1 and 2 show obvious knickpoints while profiles 3 and 4show more regular patterns with only minor deviations (Fig. 5).

A steep composite slope in the lower part characterizes the area thatcontains rivers 1 and 2 with a convex shape to the west and concaveshape to the east. The central portion of the slope has a low-gradient to-pography (Fig. 6) that is affected by a set of N50°-trending scarps andsmall incisions that isolate elongated blocks with nearly flat surfaces(Fig. 5). The tip of the triangle connects the headwaters of rivers 1 and2 with the flat summit of Mt. Frascare by a steep scarp. Upslope of thisscarp and inside a belt that has awidth of approximately 600 m, severalnear-parallel trenches outcrop at themesoscale (Fig. 7). These trenchesvary in length from a few to 800 m, width from 50 cm to N20 m, anddepth up to 10 m. In addition, repeated field surveys showed thatthese fractures are subject to constant evolution. Very fresh-lookingscarps, up to 50 m long and 50 cm wide, were first surveyed in 1995(Aringoli et al., 1996). In the following years, the increasing lengthand width, opening up to few cm per year, were observed (Fig. 8).

In fact, the wider area around the triangular block is subject tostructural control by (mainly) the N60° tectonic trend, as shown bythe alignment of the main incisions and scarps (Figs. 3 and 5). Themain boundary condition that constrains the re-use of these fracturesand joints by gravitational forces is the deep incision of the Fiastronegorge. Other important factors that may control the deformation ratesinclude the direction of the slope with respect to structural trendsand the presence of marl layers that were locally isolated by previoustectonics (Figs. 3 and 5). The central triangular block that is describedabove provides evidence for the largest deformation. This block hasbeen identified as PD.

3. DInSAR data analysis and active grounddeformation measurements

The differential interferometric SAR technique was used to inves-tigate the current deformation rates. In fact, the classical differentialSAR Interferometry is a technique that uses synthetic aperture radarimage data (usually collected by space sensors) to calculate groundsurface movements that occur between two satellite passes over the

Please cite this article as: Tolomei, C., et al., Analysis of the deep-seated ggeomorphological assessment..., Geomorphology (2013), http://dx.doi.org

same area. This technique is based on the radar concept. The phaseof the radar signal that is returned to the satellite conveys quantita-tive 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 simulat-ing and subtracting the phase contributions due to topographic relief(a DEM is needed for this), a differential interferogram is formed. Thisinterferogram contains the ground deformation signal that occurredbetween the two passes (Massonnet and Feigl, 1998).

The accuracy of the surface movements that are measured by theDInSAR technique depends on various factors such as atmospheric ef-fects, orbital effects, stability of ground scatterers, and unwrappingerrors. However, in favorable cases, the accuracy of the displacementis less than 1 cm (Hanssen, 2001). In the last decade, this techniquewas improved by the development of a multi-temporal DInSAR ap-proach, which allows users to obtain displacement time series andsurface mean velocities data at accuracies of ~1 mm yr−1 (Casu etal., 2006).

Here,we used the small baseline subset (SBAS) algorithm(Berardinoet al., 2002).With this algorithm,weprocessed SARdata acquired by theERS-1/2 satellites of the European Space Agency (ESA) to estimate thedisplacement time-series and the mean velocities of the coherentground areas. This method has been used to accurately measure grounddeformation 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 algorithmuses a large number (several tens) of radar images to reduce variousnoise components of the DInSAR interferograms, which increases theaccuracy of the displacementmeasurements (Casu et al., 2006). Initially,the operator defines the acceptable temporal and orbital separationsthat occur between each of the DInSAR interferogram images to be gen-erated. Next, a consistent number of differential interferograms are gen-erated with a DEM of comparable resolution and are unwrapped togenerate actual displacement maps. Then, the reference pixel, forwhich all calculated displacements and velocities will be referenced, ischosen. The unwrapped phases are inverted with the singular value de-composition (SVD) technique, and the displacement time series for eachimage (at a given date) are retrieved for each pixel that has a coherencevalue greater than the fixed threshold. The SVDmethod is used to com-pute 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 ofknickpoints (np) are marked in green. B) River profiles. Streams 1 and 2 have prominent knickpoints. (For interpretation of the references to color in this figure, the reader isreferred to the web version of this article.)

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equation system. At this step, the orbital residual and topographic errorsare estimated before subtracting. Finally, the short-term atmosphericcontribution is removed by double filtering in time and space. Fromour SAR data processing, only the pixels with an interferometric coher-ence of more than 0.7 (over 30% of the total number of interferograms)were considered to be reliable (Burgmann et al., 2000). Thus, for eachretained pixel of 80 × 80 m, a time series of the ground displacementwas calculated for each image. All displacements are relative to the ref-erence 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 imagesthat were acquired from the ascending pass, track 401, and frame 858

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between 1993 and 2000 (Tables 1 and 2). To generate the DInSARcouples, we imposed a maximum orbital separation of 300 m to re-duce spatial decorrelation, and a maximum temporal distance of1200 days between two passes to limit the temporal decorrelation ef-fects. By using these constraints, a maximum of 73 interferogramswere generated. Of these interferograms, 16 were excluded due totheir scarce coherence (leaving 57 usable interferograms). We usedthe SRTM DEM for topography subtraction (http://www2.jpl.nasa.gov/srtm— Farr et al., 2007) and processed a spatial subset of the im-ages in the Mt. Frascare area that contained pixels of 80 × 80 m.

The obtained ascendingmean velocitymap is shown in Fig. 9, wherethe 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.

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vegetation cover and scarce rock outcrops, the temporal coherence isrelatively low in this area. Only a few pixels reached the minimum co-herence level of 0.7.

As previously mentioned, the displacement time series valueswere calculated relative to a given pixel that was located in a stablearea. This reference areawas selected near the summit of Mt. Frascare,where no geomorphological evidence of long-term gravitational pro-cesses was found (Fig. 9). To reduce the uncertainty, we averagedthe time series that were available for the reference area. Thus, all ofthe displacement time series and ground velocities discussed beloware relative to that average.

The mean ground velocities shown in Fig. 9 were calculated fromthe time series by assuming a linear deformation between 1993 and2000. They represent the general ground deformation pattern. How-ever, the time series also contains some non-linear components, asdemonstrated by the change of slope in the time series in Fig. 10.

Surface movements measured by the DInSAR techniques alwaysinclude scalar measurements along the satellite's LoS (Burgmann etal., 2000). In our case (ERS imagery), the LoS is vertically inclined byapproximately 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 ofthe PD moved away from the satellite at rates of up to 3–4 cm yr−1

Fig. 7. Trenches near the top of Mt. Frascare. Greater trenches are mainly occupied by debrisby debris, and do not contain vegetation.

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between 1993 and 2000. However, areas located outside the mappedDGSD limits were relatively stable during this period. Based on thefield observations and the general geological setting, we hypothesizethat the main horizontal component of DGSD movement was perpen-dicular to the main slope (Fig. 3). In addition, we hypothesize that neg-ligible deformation occurred along the slope. These horizontalmovements, which occurred nearly parallel to the ascending orbit(Fig. 9), cannot be resolved in the DInSAR interferograms becausethey only caused very small changes in distance between the SAR an-tenna and the ground (Burgmann et al., 2000). Therefore, we assumethat the displacement that resulted from our SBAS analysis representsthe projection in the vertical component LoS of ground deformation.In addition, we can calculate the actual vertical displacement or velocityby dividing the LoS value by 0.9, i.e., the cosine of the local incidenceangle.

4. Discussion

We investigated the geomorphological evidence of a DGSD (PD) inthe study area at increasingly larger scales. The lower triangular blockthat corresponded to the PD showed well-developed gravitationallandforms. The parallel NE–SW scarps in the central part of theblock are up to 25 m tall and consist of several dislocated and

and vegetation. However, small recent trenches (on the right) are less open, not filled

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Fig. 8. Fresh crack widening up to a few cm per year. The crack has opened more than0.5 m.

Table 2List of ascending images used.

Acquisition date Sensor

23/07/1993 ERS102/04/1995 ERS107/05/1995 ERS117/07/1995 ERS225/09/1995 ERS207/01/1996 ERS108/01/1996 ERS217/03/1996 ERS118/03/1996 ERS221/04/1996 ERS122/04/1996 ERS201/07/1996 ERS218/11/1996 ERS207/04/1997 ERS212/05/1997 ERS216/06/1997 ERS221/07/1997 ERS229/09/1997 ERS208/12/1997 ERS216/02/1998 ERS206/07/1998 ERS210/08/1998 ERS214/09/1998 ERS223/11/1998 ERS221/06/1999 ERS229/08/1999 ERS108/11/1999 ERS216/01/2000 ERS117/01/2000 ERS201/05/2000 ERS223/10/2000 ERS2

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elongated flat-tops and tilted blocks of up to 1 km long and 500 mwide (Fig. 9). A less pronounced deformation can be observed in theupper trench area (hereinafter UT), where the fractures are smaller(up to a few meters) and are closer together. The strong extensionthat affects the UT acts approximately perpendicularly to the steepslope, which represents the scarp of the upper PD. This result suggeststhat the recent trench development corresponds with a continuouslydecreasing DGSD. at least in its upper portion.

To obtain a better representation of ongoing deformation, we aver-aged the DInSAR displacement time series over five areas (Fig. 9). Theseareas correspond to the following homogeneous blocks, which were de-fined by geomorphological analysis (Figs. 3, 4, 5, and 7): 1) the flat areaupslope of theUT zone,whereno fresh scarps are observed; 2) the properUT zonewhere the recent fracturefield is present; 3)flat and4) tilted ter-races in the central portion of the PD; and 5) a stable area west of the UTzone which shows no sign of recent deformation.

Table 1Specifications of ERS1 and 2 satellites.

Satellite ERS-1 ERS-2

Launch date 17 July 1991 21 April 1995Altitude 800 km 800 kmRevisiting cycle 35 days 35 daysAcquisition time 21.16 in ascending orbit

9.40 in descending orbit21.16 in ascending orbit9.40 in descending orbit

Orbit inclination 98.5° inclination orbit 98.5° inclination orbitLook angle 23 deg look angle towards right 23 deg look angle towards rightBand/wavelength C/5.8 cm C/5.8 cmFrame dimension 100 × 100 km 100 × 100 kmSAR pixel size 20 × 4 m 20 × 4 m

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By averaging the displacements, we reduced the data uncertaintyand estimated the local variability of the deformation. The large dis-placement error bars in Fig. 10 indicate the presence of internal defor-mation. In all areas, the average time series exhibited a non-lineartemporal trend with periods of more stable deformation and of in-creased ground velocity (Fig. 10). The same variations, albeit withvariable intensities, are present in all of the time series, suggestingthat the variations originated from a common cause but were differ-ent due to local responses.

When we calculated the mean velocities before and after the sharpchange that occurred in November 1999 (Fig. 10), we observed strongacceleration in the unstable areas (Table 3). Moreover, while the veloc-ities prior to August 1999 were similar, a greater acceleration was ob-served during the second period in the upper zones (1 and 2) than inthe central blocks of the PD (zones 3 and 4). These results indicate largergravitational deformation rates on the UT relative to the central PD. Inaddition, a rather sharp transition occurs between the stable ground ve-locities in the SWarea and the rapid deformations in theUTand PD. Thisfinding suggests the presence of a structural discontinuity that is orient-ed in the NW and SE direction along incision no. 1 (Fig. 5).

Based on the geomorphological analysis and the current patternsof ground deformation, the entire Mt Frascare NW slope can bepartitioned into three different sub-regions that are characterizedby different activity rates. These sub-regions include the central trian-gular block including its upslope extension in the UT, the southwest-ern area and the northeastern area.

The central triangular block corresponds to the PD as defined inSection 2. Here, the surfaces of the tilted blocks show considerableongoing vertical deformation (~−2 cm yr−1, Fig. 9). The flat andtilted surfaces in this area are likely relics of the same erosional sur-face that formed the upper portion of Mt. Frascare. Because the twosurfaces now differ in elevation by 80 to 100 m, gravitational defor-mation occurred over a time frame of between 4000 and 5000 years(this is only an estimation because deformation rates are notconstant).

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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). (For interpretation of the refer-ences to color in this figure, the reader is referred to the web version of this article.)

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Geomorphological investigations indicate that in the UT zone ofthe PD, the gravitational deformations began more recently. Thismay be due to the fragile response of the pre-existing NE–SW frac-tures to slope disequilibrium from the movement of the PD block.This zone has the highest vertical velocity (up to 10 cm yr−1) and dif-ferent deformation mechanisms from that of the PD (toppling).

Additional evidence regarding the strong influence of the NE–SWstructural discontinuities that control landform development in thisarea 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 streams3 and 4 follow nearly the same trend. Only streams 1 and 2 have welldeveloped knickpoints (Fig. 5B) that are aligned with one of the mainscarps in the PD block (Fig. 5A), which may represent the same defor-mation surface.

Finally, the western portion of the lower PD slope and thepresence of a more advanced block (AB in Fig. 5A) with a prominentlyconvex slope, which is separated from the PD main body by a largeand well developed trench (Figs. 4 and 5A), suggests strong ongoingdeformation. Unfortunately, no coherent DInSAR pixel is available tomeasure the ground velocity at this site.

In the second area, located SW of the PD, the drainage network in-dicates the presence of a structural control based on the NE–SW frac-ture pattern. However, no marked gravitational landforms exist andthe current ground velocities indicate relative stability (Fig. 5A).

In the third area east of the PD, the dominant landforms includetwo nearly parallel streams (3 and 4 in Fig. 5A) that cause strong lo-calized erosion. The development of these landforms is controlledby the same NE–SW structural trend. The slopes in this area do not in-dicate recent or historical large mass movements. In addition, themain stream profiles do not contain evident knickpoints.

We interpreted the different behaviors of the three areas based onthe complex interactions of various factors. The rapid deepening of

Please cite this article as: Tolomei, C., et al., Analysis of the deep-seated ggeomorphological assessment..., Geomorphology (2013), http://dx.doi.org

the Fiastra River was the first factor that we considered, which causedthe valley slopes to move laterally and progressively, resulting in toeunloading. Given the curvature of the Fiastra valley, the main-slopestress direction in the central block (the PD and UT) was favorablyoriented perpendicular to the extension of the NE–SW joint set. Inthe other two sectors, the angle was reduced to 30° (Fig. 5A). In addi-tion, the slope of the SW sector has lower relief and inclination thanthe other two sectors. Although the NE sector has the highest reliefand steepest slope inclination, the joint set was rapidly exploited byerosion, which effectively reduced the gravitational stress.

Another potentially important control factor regarding the devel-opment of the PD is the shallow thrust (see Section 2) that outcropsto the east, which likely reaches a depth of 200 m beneath the PDblock. This outcrop potentially served as a detachment surface thatwas favorably oriented with the PD block movement (NNW). Howev-er, we hypothesize that this outcrop was related to the different be-havior between the more rigid limestone formation and the moreplastic underlying marly units.

Finally, we searched for a possible correlation between the patternsthat were observed in the displacement time series, the patterns thatwere observed in the precipitation time series, the temporal variationsof the lake level and seismic activity. Based on the 8-year SAR data,the displacement rate variations (Fig. 10) were not correlated withthe main oscillations of the reservoir level (Fig. 11). We compared thetotal precipitation rates (rain + snow) with 3-month averages and adeformation time series (Fig. 11B). A strong temporal correlation be-tween higher precipitation values and the beginning of accelerationevents occurred (Fig. 11B). The largest discrepancy occurred for theacceleration episode that started at the end of 1997. This accelerationperiod was correlated with the Colfiorito seismic event (see below).

On September 26, 1997, two earthquakes with magnitudes of 5.8and 6.0 occurred on the Colfiorito plain, which is approximately

ravitational slope deformations over Mt. Frascare (Central Italy) with/10.1016/j.geomorph.2013.07.002

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.

10 C. Tolomei et al. / Geomorphology xxx (2013) xxx–xxx

20 km from our study area. We observed a minor positive anomaly inthe displacement time series in September 1997 (Fig. 10) for all of theareas that were strongly deformed (Fig. 9). Therefore, we cannot ex-clude the possible transient reactivation of the PD by local ground accel-erations 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 SBAStechnique and 32 ERS images that were acquired between 1993and 2000. Over 15 cm of cumulated vertical surface displacementwas detected across the PD. In addition, the highest deformationrates occurred where very fresh-looking extensional fractures oc-curred in the Upper Trench zone.

Table 3The mean ground velocity (mm y−1) for the areas shown in Fig. 9 and two differentperiods.

7/23/1993 to 8/29/1999 7/23/1993 to 8/29/1999

Area no. 1 ~0 ~0Area no. 2 −15.9 −67.7Area no. 3 −14.3 −95.7Area no. 4 −14.0 −59.4Area no. 5 −15.6 −52.3

Please cite this article as: Tolomei, C., et al., Analysis of the deep-seated ggeomorphological assessment..., Geomorphology (2013), http://dx.doi.org

The 8-year displacement time series indicates the presence ofnon-linear deformation rates with 1–2 year periods of more stablebehavior and accelerated movement. Additional investigations areneeded to explain the causes of the acceleration events. However,we determined that the acceleration eventswere not caused byfluc-tuations in the Fiastra lake level. The observed correlation betweenthese acceleration events and strong precipitation can be explainedby the positive effect of pore water pressure on deformation rates(Terzaghi, 1950; Forlati et al., 2001; Kilburn and Petley, 2003). In ad-dition, our results suggest that the short-term dynamic stresses im-posed during the earthquakes (even at relatively long distances,such as 20 km) potentially triggered the release of gravitationalstrain 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 DGSDin terrigenous rocks, as indicated by most of the reported evi-dence. The PD and UT areas should be closely monitored due tothe rate of activity in these areas. Additional studies are neededto quantify the displacement rates across a current time serieswith SAR images.

3) This combined andmultiscale approach between DInSAR and aeri-al photograph interpretations may have additional positive impli-cations. The application of this method to different areas that arecharacterized by the presence of different DGSD typologies andan evident meteorological event could allow the separation ofgravity from tectonic contributions.

ravitational slope deformations over Mt. Frascare (Central Italy) with/10.1016/j.geomorph.2013.07.002

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).

11C. Tolomei et al. / Geomorphology xxx (2013) xxx–xxx

Acknowledgments

We thank S. Atzori and R. Lanari for their fruitful discussions and Ing.Silvi (ENEL) for providing the rainfall and dam water level data. A.Taramelli was affiliated with LDEO of Columbia University and PerugiaUniversity when this study began. The contribution of A. Taramelli tothis study was supported by Fondazione Cassa di Risparmio di FolignoAward n.1 Universita' degli Studi di Perugia - Corso di Laurea inProtezione Civile.

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