Geomorphology xxx (2012) xxx–xxx

GEOMOR-04137; No of Pages 13

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Geomorphology

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Deformations and slope instability on Stromboli volcano: Integration of GBInSAR dataand analog modeling

Teresa Nolesini, Federico Di Traglia ⁎, Chiara Del Ventisette, Sandro Moretti, Nicola CasagliDep. Earth Sciences, University of Firenze, Italy

⁎ Corresponding author.E-mail addresses: ditraglia@unifi.it, ditragliafederico

0169-555X/$ – see front matter © 2012 Elsevier B.V. Alhttp://dx.doi.org/10.1016/j.geomorph.2012.10.014

Please cite this article as: Nolesini, T., et al.,modeling, Geomorphology (2012), http://d

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 July 2012Received in revised form 15 October 2012Accepted 16 October 2012Available online xxxx

Keywords:Ground-based InSARRemote sensingVolcano deformationsVolcano instabilityStromboliMafic volcano

To understand the relationship between volcano deformations, magma overpressure and flank instability, the re-sults of analog experiments of slope instability and ground deformations, recorded by the GBInSAR system on thewestern flank of the Stromboli volcano during the period 2009–2011 have been analyzed. Analog experimentsthat consider both the external (accumulation on the slope) and the endogenous (intrusion-related bulging) triggerphenomena. The effect of accumulation on the sub-aqueous slope on the initiation of sub-marinemass movementshas been analyzed. By combining the monitoring data with analog modeling, the observed deformations from thecombined action of overpressure in the volcanic system and gravity have been related. The results suggest thatthe superficial movements observed by the GBInSAR system represent the response of the Stromboli volcano tooverpressure changes in the conduit. The movements observed on the Sciara del Fuoco were slope instability phe-nomena where the gravitational component produced a constant creep, while changes in the magma overpressureexplainwhy certain periods are characterized by accelerations, which induce instability on the external flanks of thecrater area and in the Sciara del Fuoco and eventually promote failure of the volcanic slopes.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Understanding the deformations associated with magma ascent indikes and/or sills is a crucial issue in volcano monitoring. Magmaticdriving forces are responsible for the failure of volcanic slopes by in-creasing the shear stress and reducing the shear strength (Voightand Elsworth, 1997; Elsworth and Day, 1999). The pathway of intru-sive sheets at shallow levels depends on the geological, tectonic andgeomorphological evolution of the volcanic area (Nakamura, 1977;Tibaldi, 2001; Tibaldi et al., 2009). However, propagation of magma-filled cracks depends on internal over-pressure and magma rheology(Rubin, 1995; Scaillet et al., 1998; Takeuchi, 2004); these factors areintimately connected, and the best chance of identifying the regionsaffected by instability and their causes comes from understandingthe deformation of the volcanic edifice. High-frequency monitoringsystems may provide an advanced warning of volcano collapse, un-rest or changes in the level of activity (Casagli et al., 2009).

In this work, the ground deformations, recorded by the Ground Based(GB) InSAR system on thewestern flank of the Stromboli volcano (Fig. 1),have been analyzed with the aim of understanding the relationship be-tween volcano deformations and magma overpressure and, hence, theircorrelations with flank instability. In particular, the temporal and spatialdeformations during the period between 2009 and 2011 have been

@gmail.com (F. Di Traglia).

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Deformations and slope instax.doi.org/10.1016/j.geomorp

considered. This period was chosen because a large set of geochemical(Aiuppa et al., 2010), geophysical (Coppola et al., 2012; Di Traglia et al.,2012), volcanological (Andronico and Pistolesi, 2010) and petrologicaldata (La Felice and Landi, 2011) are available in the literature.

2. Geological background

The 916 m-high Stromboli Island is the emerged portion of a~3000 m-high collapsing stratovolcano located in the north-easterntip of the Aeolian Archipelago in the southern Tyrrhenian sea(Fig. 1). The rock composition varies between basaltic andesite,shoshonite and latite-trachyte (e.g. Hornig-Kjarsgaard et al., 1993),with the oldest exposed products dated approximately 100 ka (Gillotand Keller, 1993).

2.1. Instability phenomena on Stromboli volcano

Stromboli build-up was repeatedly interrupted by three caldera col-lapses and five lateral collapse events, which were followed by reorgani-zations of eruptive centers (Tibaldi, 2001). The older recognized flankcollapse affected the SE flank of the edifice and was dated to between35 ka and 26 ka (Tibaldi et al., 2008; Romagnoli et al., 2009). From13 ka, lateral collapses only developed on the NW side of the volcano,which formed a nested horseshoe-shaped scar (Fig. 1) opening to thenorthwest called the Sciara del Fuoco depression (Tibaldi, 2001). Themost recent collapse event, which occurred 5.6±3.3 ka (Tibaldi, 2001),has been related to a large phreatomagmatic eruption (Secche di Lazzaropyroclastic succession; Bertagnini and Landi, 1996) triggeredby amassive

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Fig. 1. a) Stromboli volcano. The Sciara del Fuoco is outlined (dotted line) along with thelocation of the GBInSAR instrument (white diamond). The studied sectors of the NE craterarea are numbered (1 to 4). Themain effusive vents from the 2002–2003 and 2007 parox-ysmal eruptions are shown (Neri et al., 2008; Casagli et al., 2009). b) 3D model of theStromboli Island (Sciara del Fuoco slope and NE crater) with an interferogram obtainedfrom the GBInSAR showing a velocity greater than 300 mm/h (from Casagli et al., 2009).The eruptive vents of the 2002–2003 and 2007paroxysmal eruptions are also shownhere.

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landslide (0.73±0.22 km3, Tibaldi, 2001; Di Roberto et al., 2010). Asmaller subaerial landslide that has cut a lava overflow in the NW sectorof the Sciara del Fuoco, most likely occurred ~700 years ago (AD1350±60; Arrighi et al., 2004; Speranza et al., 2008), producing a subma-rine turbidite deposit (Di Roberto et al., 2010). Flank instability and relat-ed tsunamis occurred during the recentflank effusions, which occurred in1879, 1916, 1919, 1930, 1944 and 1954 (Barberi et al., 1993). Themost re-cent instability event of notable importance is related to the 30th Decem-ber 2002 landslides (Baldi et al., 2008), which caused two tsunamisequences (max run-up 6–7 m at Stromboli village; Tinti et al., 2006).The landslides were caused by the injection of lateral dikes through theSciaradel Fuoco (Neri et al., 2008). Observations indicated that a relativelydeep-seated landslide (Baldi et al., 2008) affected the northern side of theSciara del Fuoco. Although significant deformations developed during the30th December 2002 event, the displaced mass did not slide down intothe sea, but the slope relentlessly deformed and fragmented into several“blocks” (Boldini et al., 2009). The largest block collapsed producing thesecond tsunamiwave, while the first tsunamiwas generated by a subma-rine slide (Chiocci et al., 2008).

During the last paroxysmal eruption (February–April 2007), aflank ef-fusion occurred, and the opening of the effusive vents at the base of theNW craters and in the Sciara del Fuoco area produced small landslides,but tsunamis were not detected (Barberi et al., 2009). Despite the similarfeeding system, no catastrophic landslide occurred in 2007. This lack ofactivity appears to be related to the lowermagmatic pressures that devel-oped at the tip of the NW–SE-striking dike (Neri et al., 2008).

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

The northernmost part of the Sciara del Fuoco has been continu-ously monitored since the 2002 collapse event by a GBInSAR system(Casagli et al., 2010), which is able to detect Sciara del Fuoco instabil-ity phenomena, summit area collapse, pit crater formation, lateralvent opening and explosion-related crater deformations (Tarchi etal., 2008; Casagli et al., 2009).

2.2. Volcanic activity on Stromboli volcano

The “ordinary activity” on Stromboli volcano consists of quiescentmagma degassing through crater vents that are grouped in threeareas (NE, central and SW) and located 750 m above sea level, aswell as brief (10 to 15 s) 100 to 200 m-high scoria-rich jets that areproduced by explosions of variable energy every 10 to 20 min(Patrick et al., 2007; Taddeucci et al., 2012). These explosions are re-lated to the uprising and break-up of gas slugs (Parfitt, 2004). Thistype of activity (termed Strombolian; Blackburn et al., 1976) is peri-odically interrupted by more energetic blasts, which are character-ized by larger emitted volumes and column heights (i.e., greatermass discharge rates) and defined as “paroxysms” (higher intensity)and “major” (moderate intensity) explosions (Rosi et al., 2000,2006; Pistolesi et al., 2011). In terms of conduit dynamics,“paroxysmal” and “major” explosions are considered the twohigh-intensity “variations on the Strombolian theme” (Hougthonand Gonnermann, 2008) and are related to the uprising of largegas-slugs derived from deep-seated, volatile-rich parental magmas(Burton et al., 2007a; Allard, 2010; Métrich et al., 2010; Del Bello et al.,2012). Ordinary activity is fed by high-porphyritic (HP), volatile-poormagma, while low-porphyritic (LP), volatile-rich magma is alwaysejected during paroxysmal and major activity (Bertagnini et al., 2003;Pistolesi et al., 2008; La Felice and Landi, 2011). HP magma is also char-acterized by density variations, which demonstrates the complex con-duit dynamics within the shallow conduit-dike system (Lautze andHoughton, 2005). Major and paroxysmal explosions are characterizedby similar seismic waveforms, while “ordinary” explosions are quite dif-ferent, regardless of their energy (D'Auria et al., 2006).

Paroxysmal explosions are generally anticipated by major changes involcanic activity (Casagli et al., 2009; Aiuppa et al., 2010; Inguaggiato etal., 2011), and hence their occurrence can theoretically be forecasted.Recently, precursors such as an increase in the CO2 plume flux(>1000 tonnes per day) and in the deformation rate at the base of thesummit craters area have marked the onset of periods characterized bygreater explosion frequency (>5 events per hour; Andronico et al.,2008) and the occurrence of major explosions (Aiuppa et al., 2010; DiTraglia et al., 2012), which is defined here for simplicity as “majorexplosion-dominated periods”. In these “anomalous” periods, whichgenerally last 1–2 months, clustered major explosions and frequent“ordinary” Strombolian explosions can be associatedwith lava emissionfrom the summit craters (Andronico and Pistolesi, 2010; Coppola et al.,2012).

3. Ground-based radar interferometry

TheGround-based Interferometric Syntetic Aperture Radar (GBInSAR)is a remote sensing technique based onmicrowaves that permits the pro-duction of 2D displacement maps (interferograms) of an area with highaccuracy; the apparatus installed at Stromboli exploits ametric-scale spa-tial resolution and an acquisition frequency of approximately 11 min (fortechnical details, refer to Antonello et al., 2004). Only the component ofthe displacement vector parallel to the line of sight can be assessed.

The study of explosions and rapid deformations is usually carriedout using interferograms calculated on relatively brief time intervalsthat span from a fewminutes to a few days because longer time inter-vals produce noise and decorrelation phenomena on the crater area.Conversely, the best way to characterize the behavior of the craterarea during periods of normal activity is to analyze the interferograms

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that are calculated from long time intervals spanning from weeks tomonths. This approach permits the distinction of areas affected by differ-ent types ofmovement to assess amean velocity for each area and to ob-tain a spatial sub-division of the Sciara del Fuoco based on the recordeddisplacements. Most importantly, the study of the deformations thatare not detectable on daily basis allows us to identifymore or less intenseperiods, to understand the mechanisms inducing the movements and tocorrelate the movements with the volcanic activity.

3.1. Geometry of the deformed zones

The analysis of ground motion allowed us to identify three mainareas affected by significant variations in the deformation rate(Figs. 2, 3). One of these areas (CR; Fig. 2) is located at the base ofthe NE craters and corresponds to the intersection of the summitarea with the lateral propagation of the shallow-conduit(Bonaccorso, 1998; Ripepe et al., 2005; Chouet et al., 2008; Neri etal., 2008). The other two zones (SdF1 and SdF2; Fig. 3) are locatedin the Sciara del Fuoco area and correspond to the vents of the2002–03 and 2007 flank effusions (Marsella et al., 2012).

In the CR area, increases in the deformation rate have been ob-served using 4–8 hour interferograms, while the CR area is generallyde-correlated in interferograms that span a longer time interval (12 hto 7 days) due to the fast accumulation and remobilization of theejecta, forming a debris fan. Conversely, to constrain the differencesbetween the erosion and the deformations in SdF1 and SdF2,long-term analysis (365-day interferograms) has been performed(Fig. 3). August-December 2011 interval has been analyzed moreclosely because it was characterized by an overflow from the NE cra-ter, which produced lava that flowed toward the SdF (1st–2nd August2011; Coppola et al., 2012) and was subsequently eroded. TheAugust-December 2011 reveals three deformation patterns charac-terized by different geometries and velocities. One pattern is faster,produces de-correlation on the interferograms and is related to ero-sion of the 1st–2nd August lava flow (Fig. 3). The other pattern char-acterizes the upper (SdF1) and lower sector (SdF2) and was identifiedin the shorter term (8 h–7 days) interferograms. SdF1 and SdF2 pat-terns are similar to the deformations observed in the Sciara del

Fig. 2. Interferogram (07:11–11:20, 09th May 2011) showing the deformations in thecrater area. Black color corresponds to “no-data” (“shadows zones” with respect to theradar or very low-coherence areas).

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

Fuoco by Casagli et al. (2009) before the opening of the effusivevent during the 2007 paroxysmal eruption.

3.2. Deformation rate trend

3.2.1. Summit crater areaAn increase in the deformations has been observed since 20th

March 2009, when the rate of deformation increased to 1.1 mm/h(Fig. 4). The maximum deformation rate was achieved on 27thMarch 2009 (0.35 mm/h), but the deformations at the summit arearemained high for the following two months, with other peaks on3rd April 2009 (0.27 mm/h), 14th April 2009 (0.3 mm/h), 3rd May2009 (0.25 mm/h) and 22ndMay 2009 (0.23 mm/h). During the periodof November 2009 – January 2010, an increase in the deformation ratewas observed, starting on 2nd November 2009 (0.56 mm/h) and end-ingwith the explosion on 8th November 2009 (0.6 mm/h). Comparabletrends were observed during the periods between (i) 19th and 23rdNovember 2009 (0.25–0.3 mm/h), (ii) 20th and 28th December(0.25–0.29 mm/h), (iii) 6th and 9th January 2010 (0.24–0.29 mm/h)and (iv) 16th and 20th January 2010 (0.2–0.3 mm/h). The GBInSAR re-vealed that an increase in the deformation rate was observed in the CRsector during the period of 9th-18th May 2011 (Figs. 3, 4). Clear evi-dence of deformation was observed during the night between 8th and9th May 2011, with an increase in the deformation at the base of theNE crater area with rates up to 0.38 mm/h. The deformation ratereached its peak (0.55 mm/h) during the early morning of 10th May2011 and then gradually decreased (Fig. 4).

3.2.2. Sciara del FuocoThe detected deformation in the Sciara del Fuoco area (Fig. 4) oc-

curred more slowly than the deformations measured in the CR area.The Sciara del Fuoco area was subjected to an increase in the defor-mation rate starting on 8th November 2009 (0.43 mm/day) and end-ing on 13th November. Similar trends have been observed during theperiods between (i) 19th and 23rd February 2010 (0.5 mm/day). In2011, the Sciara del Fuoco demonstrated four different patterns of de-formation. During the first month (1st January–21st April), the defor-mation rate had strong variations over the range 0.03–0.34 mm/day.After these periods, the deformations were very low (0.01–0.14 mm/day) for a long period of time (22nd April to 24th Septem-ber). Subsequently, there was a sudden increase in the deformationrate, which reached a maximum (0.5 mm/day) on 23rd–24th Octo-ber. Then, the deformation rate decreased again, reaching the samevalue observed in early 2011.

4. Analog models

The analog modeling technique represents a useful tool for under-standing many geological processes because it allows for the study ofprogressive deformation, which provides useful indications of the roleof several distinct factors that control the final deformation pattern. An-alog experiments have been performed with the aim of understandingthe effects of different trigger mechanisms of slope instability andconstraining the geometry of the induced deformations. Experimentshave been conducted to consider both the external (accumulation onthe slope) and the endogenous (intrusion-related bulging) trigger phe-nomena. The effects of accumulation on the sub-aqueous slope on theinitiation of sub-marine mass movements have also been simulated.

For this study 30 different models have been performed using dif-ferent materials and simulating different landslide-triggering mecha-nism (5 different series; Fig. 5). In this paper, the results ofrepresentative experiment are described. Three models with differentset-ups were chosen (Fig. 5): i) accumulation of material — subaerialcondition (AA series), ii) bulging — subaerial condition (BA), and iii)accumulation of material — submerged condition (AS).

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Fig. 3. Interferograms (~1 year differences between each SAR image) showing the different deformation patterns in the Sciara del Fuoco area. These differential movements arerelated to different processes: i) dike injection from the central conduit related to over-pressure increase and ii) erosion of the 1st–2nd August 2011 lava flow. The long-lastingerosion in the Sciara del Fuoco is related to the presence of lava remnants in the upper part of the Sciara del Fuoco that produced the slope instability.

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One of the main limitations of the analog modeling technique con-cerns the submarine environment. The subaqueous landslide sedi-ments are completely saturated by the same fluid, which in theliterature concerning the use of the analog modeling technique insubaqueous environment is approximate to air, and thus the modelis carried out in a subaerial environment (Hampton et al., 1996;Caplan-Auerbach et al., 2001; Imran et al., 2001; Stake, 2001;Lobkovskya et al., 2002; Ward and Day, 2002; Bonnet et al., 2004;Ellis et al., 2004; Rosemberg et al., 2004; McAdoo and Simpson,2005; Yamada et al. 2010). Recently, the analog modeling techniquewas applied to coastal landslides in a subaerial, submerged environment

Fig. 4. GBInSAR data during the January 2009–December 2011 period. To follow the different vhere been plotted against time. The recorded movements are directed away from the GBInSARabsolute value. The main events that occurred in the analyzed periods are reported. Syn-explodeformations in the crater flanks coincide with major explosions and high-energy “ordinary” S

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

(Mazzanti and De Blasio, 2011), but no one has used this model up tothe air–water interface.

4.1. Materials

Much of the Sciara del Fuoco is inaccessible due to continuousejection of products from the frequent explosions and frequent fallingrocks. The samples taken in the marginal position of Sciara del Fuocoare considered representative of the deep position. In fact, observa-tions of the deposition process on the Sciara slope before and afterthe 2002 landslides indicate that the lithology, the grain size and

elocities of movement, the velocities in the three areas (craters flanks, CR and SdF2) haveposition as they represent inflation (i.e., negative values), and values are represented assive deformation rates of the crater flanks are separated with respect to the CR data. Thetrombolian explosions, but some bursts do not produce “spikes”.

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Fig. 5. Grain-size distribution curves of models materials: A) Fontainebleau's sand; B) uniform sand; C) Sciara del Fuoco material; D) silty-sand.

Fig. 6. Initial model set-up. Each series is built into a plexilglass box on an inclinedplane. The inclination of the plane change between 25° and 45°. AA series: models sim-ulated material accumulation in sub aerial condition; AB series: models simulatedbulging before vent opening in sub aerial condition; TA series: models simulated ma-terial accumulation in transitional condition; TB series: models simulated bulging be-fore vent opening in transitional condition; SA series: models simulated materialaccumulation in submerged condition.

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the structure of volcaniclastic materials remain unaltered in time(Boldini et al., 2009). Volcanic edifices subjected to hydrothermal al-teration develop weak cores and they respond to internal weakeningby deforming slowly, but may then collapse catastrophically (Cecchiet al., 2004). Hydrothermal alteration characterizes only the innerpart of Stromboli volcano (Del Moro et al., 2011; Revil et al., 2011)and hence its effect on the stability of the shallower part of the Sciaradel Fuoco (b100 m) can be neglected (Apuani et al., 2005). Addition-ally, the portion of the Sciara del Fuoco below sea level has the samelithostatic and geological characteristic as the subaerial portion (Baldiet al., 2008; Boldini et al., 2009).

To simulate the brittle behavior of Sciara del Fuoco material we testdifferent granular materials. Four different samples were used. SamplesA, B and D were material commonly used in analog models; they werequartz Fontainebleau's sand, uniform sand and silty-sand, respectively(Fig. 6). Sample C was made from Sciara del Fuoco material taken atan altitude of 400 m. For Fontainebleau's sand previous laboratorymea-surements on grain dimension lower than 250 mm indicate this sandhas a density of ρ=1300 g m−3, a coefficient of internal frictionμ≈0.83 (with an angle of internal friction ϕ≈40°) and a cohesivestrength (c) of about 66 Pa (Del Ventisette et al., 2005).

Grain-size characterization and shear tests were carried out on allmaterials. The results are shown in Table 1 and Fig. 6. All parameterswere calculated using the ASTM (American Society for Testing andMaterials) standard procedure. The shear behavior of the dry, 10%saturated and completely saturated material was investigated usingthe direct shear test (DS) given by Casagrande.

The laboratory tests highlighted that the data obtained from sampleC are in accord with the literature data (Apuani et al., 2005; Boldini etal., 2009). We can also observe that the Stromboli Sciara del Fuoco ma-terial is quite different from the other type of granular materials ana-lyzed due to the large heterogeneity and, specifically, the presence offine material. The fine material results in a high internal friction anglein the Stromboli material, which is a limit in the construction of analogmodel. For this reason, four different materials for use in the analogmodels were tested.

4.2. Scaling and model construction

An analog model can represent a real situation, but the models mustbe scaled comparably to the natural prototype, specifically with respectto the geometric similarities and the rheological, kinematic anddynamicalconditions (e.g. Hubbert, 1937; Ramberg, 1955, analogmodel parameters

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

are given in Table 2). Following the scaling methodology generally usedfor simulating different geological processes through analogmodeling de-formed both in the natural gravitational field (i.e. Tibaldi, 1995; DelVentisette et al., 2006, 2007; Tibaldi et al., 2006; Montanari et al., 2010a,b; Bialas et al., 2011; Ferré et al., 2012) and in an artificial gravity field(i.e. Bonini et al., 2007; Dietl and Koyi, 2011; Corti, 2012). The brittle be-havior of rocks can be expressed by theMohr-Coulomb criterion of failure(τ=μσ(1−λ)+c; where τ and σ are the shear and normal stress on thesliding surface, μ is the internal friction coefficient, λ is the Hubbert–Rubey coefficient of fluid pressure and c is the cohesion). Since cohesionhas stress dimension, it must share a similar scaling ratio. In the sameway the internal friction coefficient must have similar values both inmodels and in nature.

The models were suitably scaled such that 1 cm in the model rep-resents 100 m in nature, involving a geometrical length ratio l*=lmod= lnat=10−4.

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Table 1Results of the Casagrande shear test on samples A, B, C, D and E in dry conditions, with10% humidity and saturated conditions.

Sand A Sand B Sand C Silt Rice

DryΦ 31° 33° 42° 26° 50°c′ 14,91 16,14 19,35 18,54 18,47

Wet (10%)Φ 28° 31° 36° 25° 19°c′ 15,54 17,44 12,14 19,55 41,11

Wet (100%)Φ 27° 27° 26° 24° 17°c′ 32,59 43,79 37,89 22,01 39,33

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Considering the length ratio, the gravity ratio (g*=1; the modelswhere performed in the natural gravity field) and the density ratioρ*≈0.5, the stress σ* acting on the model is 5×10−5 Pa (Table 2).

The Plexiglas tank is 25×30×50 cm, and these dimensions limitthe boundary effects due to confinement. The modeling materialwas sieved on a slope with an inclination that varied between 45°and 25° depending on whether the simulated environment was sub-aerial or submarine, according to the average inclination of the Sciaradel Fuoco (Chiocci et al., 2008; Boldini et al., 2009). The selected fill-ing thickness of the Sciara del Fuoco is approximately 200 m and rep-resents the deepest discontinuity of the system (Tibaldi, 2001).During the deposition of the material, a colorful reference level wasused as a marker, and a final grid 5×5 cm was put on the last sandlayer to better observe the deformations. The tank was then situatedin a horizontal position, and the experiment was started. In the caseof the submerge models, the water was added to the tank after thesedimentation of the material on the slope to maintain a balance onthe plane. Water was added to the tank very slowly through a seriesof tubes to avoid creating landslides. All of the models were devel-oped in the Earth Science Department Laboratory of the Universityof Firenze.

4.3. Triggering mechanism simulations

The evolution of a volcano generally involves a large range of deg-radation processes, such as tephra and lava re-mobilization as well aserosion due to breaking waves on the coastline and erosion at the air–water interface (Romagnoli et al., 2009). In this case, two differenttriggering mechanisms were simulated in both a sub-aerial and a sub-merged environment.

− The accumulation of material in the summit part of the slopereproduced the effect of lava loading on the craters flanks and/oron the Sciara del Fuoco slope (accumulation experiments).

− The bulging was due to magma intrusion beneath the Sciara delFuoco and the possible opening of new eccentric vent(s) (bulgingexperiments).

Table 2Analog model parameters. The asterisk indicates the ratio between the model and thenatural prototype.

Model Nature Model/nature

ParameterGM density ρ (kg m−3) 1.3 ≈0.5GM friction coefficient ≈0.8

Scaling parametersGravity acceleration, g (m s−2) 9.81 9.81 1Length, l (m) 0.01 100 1×10−4

Stress, σ (Pa) σ*=ρ*g*l* 5×10−5

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

The first mechanism, simulated with the addition of material tothe slope, creates a greater tension at a single point, changes theslope balance and is the origin of the material sliding down theslope. Accumulation corresponds to the continuous deposition of ma-terial (mainly lava) and an increase in the thickness of the slope.

The bulging was achieved by positioning balloons under the depo-sitional plane of the material. The balloons were constantly inflatedwith air causing an instability in the upper material. The balloon po-sition on the depositional plane was chosen to reproduce bulging pre-ceding vent opening on the Stromboli volcano during the 2002–03and 2007 lateral effusions or the zones affected by an increase inthe deformation rate during the post-2007 period at 600 m (belowthe NE craters) and 400 m a.s.l. (Sciara del Fuoco).

4.4. Results

In total, 38 different experiments were conducted and were dividedinto the subaerial, submerged and transitional environment (Table 3).Although the models may have differed in small details, the geometry,the deformation and the landslide evolution were always comparable(similar volumes, cross sectional geometries and event sequences). Allmodels were constructed in a series of progressive steps, starting froma simplified model and moving toward a more realistic representation.Using sample A, the Fountainebleau sand, a subaerial condition (MAS1-MAS5)was established. FromMAS1 toMAS13, the rheological prop-erties of the sandwere tested at different slope angles over the range of35° to 40°, and the best angle to approximate the Sciara del Fuoco con-dition, according to the data, is 35°. Themodels achieved using sampleAhad a 35° inclination slope. For models in the submarine environment,the angle of the slope decreased to 25°. MAS 11 was achieved in a tran-sitional condition, and these type of experiments highlighted the mainproblems of this type of model. In fact, at the air–water interface,there is rapid capillary increase in the deposited material that isnon-existent in the real condition. Given this limitation, the experi-ments performed in transitional conditions are not considered furtherin the discussions.

In the AA series (Figs. 5, 7), an increase in the material always leadsto more frequent landslides, which are localized in the lower part of theslope. Thematerial inserted in themodel breaks the slope stability, gen-erates a complex system of landslides and increases the volume of ma-terial involved in the sliding processes. The succession of phenomena isderived from an instability in the lower part of the slope onwhich land-slide material progressively accumulates. This lower part achieves astate of equilibrium each time until the arrival of the new materialfrom the upper portion, which determines a remobilization of the pre-viousmaterial in a larger landslide. The surface involved in the landslidemovement that is related to the initiation time is show in Fig. 7. The areainvolved in the landslide is 35 cm2 in the model, which would corre-spond in reality to 350,000 m2.

In the BA series, the landslides triggered by balloon pressure ex-pansion primarily forward, and when no more air is injected, suchthat the slope has no more disturbance elements, and retrogressivemovements begin. This suggests that landslides occurring on theslope generate a general instability system that continues even ifthe trigger mechanism is stopped. The surface involved in the land-slide movement is approximately 43 cm2, which corresponds in real-ity to 430000 m2. Fig. 8 shows how the latter landslide is faster (0.4–0.5 s) than the former (0.0–0.04 s) even if the surface area involvedin the landslide movement is smaller. Two landslides developed;the second landslide developed in a shorter time (0.1 s) than thefirst landslide (0.4 s), which involved a greater surface area.

The AS models represent an innovation as far as analog modelingis concerned. Subaqueous landslide sediments are completely satu-rated by the same fluid, which generally approximates air (Yamadaet al. 2010). In this paper, we propose a model completely achievedusing subaqueous conditions. The initiation mechanisms of the

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Table 3Summary of all the analog experiments achieved (MAS). The experiments differ in am-bient conditions (subaerial, subaqueous, and transitional), in the building material(Fontainebleau sand, sand), in the friction angle (25°–45°), in the triggering mecha-nism (accumulation of the material and bulging) and in the base friction (glass,Plexiglass and sandpaper).

Model Environment Material Slope Trigger Basefriction

MAS 1 Sub-aerial Fontainebleau sand 30° Accumulation GlassMAS 2 Sub-aerial Fontainebleau sand 33° Accumulation GlassMAS 3 Sub-aerial Fontainebleau sand 35° Accumulation GlassMAS 4 Sub-aerial Fontainebleau sand 40° Accumulation GlassMAS 5 Sub-aerial Fontainebleau sand 45° Accumulation GlassMAS 6 Sub-aerial Fontainebleau sand 30° Accumulation PlexiglassMAS 7 Sub-aerial Fontainebleau sand 33° Accumulation PlexiglassMAS 8 Sub-aerial Fontainebleau sand 35° Accumulation PlexiglassMAS 9 Sub-aerial Fontainebleau sand 40° Accumulation PlexiglassMAS 10 Sub-aerial Fontainebleau sand 45° Accumulation PlexiglassMAS 11 Transitional Fontainebleau sand 35° Accumulation PlexiglassMAS 12 Transitional Fontainebleau sand 35° Accumulation PlexiglassMAS 13 Subaqueous Fontainebleau sand 25° Accumulation PlexiglassMAS 14 Subaqueous Fontainebleau sand 35° Accumulation PlexiglassMAS 15 Sub-aerial Sand B 35° Accumulation PlexiglassMAS 16 Sub-aerial Sand B 33° Accumulation PlexiglassMAS 17 Sub-aerial Sand B 35° Accumulation PlexiglassMAS 18 Subaqueous Sand B 35° Accumulation PlexiglassMAS 19 Sub-aerial Sciara del Fuoco

material30° Accumulation Plexiglass

MAS 20 Sub-aerial Sciara del Fuocomaterial

33° Accumulation Plexiglass

MAS 21 Sub-aerial Sciara del Fuocomaterial

35° Accumulation Plexiglass

MAS 22 Sub-aerial Sciara del Fuocomaterial

45° Accumulation Plexiglass

MAS 23 Subaqueous Sciara del Fuocomaterial

35° Accumulation Plexiglass

MAS 24 Sub-aerial Silt 30° Accumulation PlexiglassMAS 25 Sub-aerial Silt 33° Accumulation PlexiglassMAS 26 Sub-aerial Silt 35° Accumulation PlexiglassMAS 34 Transitional Fontainebleau sand 35° Bulging PlexiglassMAS 35 Transitional Fontainebleau+oil 35° Bulging PlexiglassMAS 36 Transitional Fontainebleau+oil 35° Bulging PlexiglassMAS 37 Transitional Fontainebleau+oil 35° Bulging Plexiglass

Fig. 7. Accumulation experiment (33 MAS). a) To trigger the landslide movement inthe model, 10 g of material has been inserted, which corresponds to a volume of6.25 m3 of material in real conditions. At time 00:02.42, a scarp forms and remains sta-ble for the experiment's duration and occurs during the first landslide movement. Thislandslide, which is 32 cm high on the slope, affects the entire slope and it continuesuntil 00:03.74. At time 00:03.79, a second landslide starts at 7 cm up the slope. Thematerial, mobilized by the first landslide movement, remains in the lower part of themodel and creates an instability in this area and generates the second landslide move-ment. Afterward, a series of progressive landslides were triggered and were all local-ized approximately 10 cm up the slope. b) Relationship between the trigger time andthe surface involved in the landslide (33 MAS, accumulation experiment). The blueline marks the extent that adding additional material causes no additional movement.(For interpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)

7T. Nolesini et al. / Geomorphology xxx (2012) xxx–xxx

submarine landslides are complex, and we reproduce the pressure de-rived from the sub-aerial landslide or lava accumulation (lava-deltaforming event) with additional material inserted on the slope to breakthe slope balance.

In the subaqueous model, the development of a subaerial land-slide in the upper part of the slope triggers the submarine landslidebody (Fig. 9). It is interesting to note that at a certain depth (approx-imately the middle of the depositional part of the submarine slope),the speed of the sliding material reaches a threshold where thewater carries the shallow part of the sediment away. This processtriggers the formation of several minor and faster landslides thatcould be assimilated to small turbidity currents. This phenomenastops when all of the material that is arriving at the breaking pointof the slope composed of the first landslide body loses velocity and re-constructs a unique landslide body.

5. Discussion

5.1. Slow ground deformations

The Sciara del Fuoco is a depression that is filled by more than a200 m-thick deposit mainly composed of irregular alternations ofthick sequences of thin lava flows and reverse-graded grain-supported volcaniclastic layers, which result from the continuous slid-ing of slope materials and successive redistribution by small debris av-alanches and grain flows (Boldini et al., 2009). Continuous levels ofloose granularmaterials that extend over large areas can represent pref-erable initiation paths for slip surfaces at different depths (Boldini et al.,

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

2009). The low deformation rate observed by the GBInSAR system ismuch less than the typical velocity of “earth flows and small debris av-alanches” occurring on the Sciara del Fuoco (Baldi et al., 2008), whichsuggests a creep behavior for the volcaniclastic material. Moreover,the erosion and accumulation processes are generally recorded in theexternal flank of the NW craters, and they have been observed in theSciara del Fuoco area in the period following the lava flow on the 1stand 2nd August 2011 (Fig. 3). These movements lasted for severalweeks and involved the exact area affected by the 2011 lava, withmuch faster ground deformation than the movements detectable bythe GBInSAR system. Planar deep creep occurs on long slopes whenthe strata dip parallel to the slope and the rocks have different rheolog-ical characteristics (Ter-Stepanian, 1966). One of the possible mecha-nisms could be the sliding of lava material on volcaniclastic productsat many planar discontinuities, which has been observed by Boldini etal. (2009). Slopes subjected to creep are prone to failure because the

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Fig. 8. Bulging experiments (31 MAS). a) The balloons positioned under the sieved material triggered the first landslide that involved an increasing material volume with time. The redballoon emerged at time 00:0.41, and afterward, the downward part of the slope is not considered valid for the experiment. At time 00:0.41, the second landslidemovement related to thesecondballoon positioned on the slope begins. Themovement continues until 00:0.42 (F2)when thematerial reaches the end of the slope. After 46 s, the area below the inflated balloon isalso considered invalid. At this time, a retrogressive landslide that moves to the end of the slope is observed, even if nomore air is injected into the balloons. b) Relationship between thetrigger time and the area involved in the two landslides generated in the bulging experiments (31MAS). The first landslide, F1, is the upper part of the figure and the second landslide, F2,is the lower part of the figure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

8 T. Nolesini et al. / Geomorphology xxx (2012) xxx–xxx

slowmotionmay reduce strength at shear zones (Voight and Elsworth,1997).

5.2. Overpressure-driven sheet intrusions

Deformations documented by the GBInSAR are the consequence oflateral sheet propagation due to an increase in magma overpressure(Casagli et al., 2009). During the period 2009–2011 (Fig. 4), Strombolivolcano experienced different periods characterized by the occurrenceof major explosions, high CO2 plume- and soil-flux and lava overflows(Aiuppa et al., 2011; Di Traglia et al., 2012), while other periods werecharacterized by a high explosion frequency, spattering, lava overflowsas well as high-energy Strombolian explosions without the occurrenceof major explosions (Coppola et al., 2012). The GBInSAR registered an

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

increase in the deformation rate in CR, SdF1 and SdF2 in a range of 5to 82 days before the onset of these periods. Syn-explosive deformationrates of the crater flanks are separated with respect to the CR data. Thedeformations in the crater flanks coincide with major explosions andhigh-energy “ordinary” Strombolian explosions, but some bursts donot produce “spikes”. This result could imply that the various deforma-tion patterns of the high-intensity explosions (paroxysms, major andhigh-energy strombolian explosions) could be related to the variable or-igin of the gas slugs, which was proposed by Burton et al. (2007b). It isinteresting to note that the increase in the CO2 soil-flux generally oc-curred during the same period as the GBInSAR anomalies (Aiuppa etal., 2011). Passive degassing, puffing and explosions during “ordinary”Strombolian activity are the result of the dynamic equilibrium betweenthe continuous refilling of deep volatiles exsolved from the magma

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

Fig. 9. Sub-aqueous experiment. a) The red line shows the material inserted and part ofthe superficial deposits of the slope. The initial shape of the sliding body is a small lobethat initially tends to expand, and this lobe occupies a larger part of the slope as itpropagates along the maximum gradient and increases the landslide speed. b) In ap-proximately the middle of the depositional parts of the slope, the speed of the slidingmaterial reaches a threshold where the water carries the shallow part of the sedimentaway and forms several minor and faster bodies that are deposited downslope. Thismechanism produces a progradation of the deposit and an erosion of the lobes formedearly. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of the article.)

Fig. 10. Instability-related features in the NW flank of the Stromboli volcano. a) 2007flank/paroxysmal eruption vents with the associated landslide morphologies and de-posits. The opening of vent 1 and vent 2 was associated with a small-scale slope insta-bility, which caused a breach in the summit crater area and a small landslide in theSciara del Fuoco. b) The opening of vent 2 produced a retrogressive landslide, whichis demonstrated by a small collapse scar. c) and d) The scar produced during the2007 effusive phase in the Sciara del Fuoco continues to suffer erosion due to the accu-mulations of materials (1st–2nd August 2011 lava flow). The morphology of the exter-nal portion of the summit crater area changed dramatically during the last few yearsbecause the rolling of the ejected pyroclasts cannot produce large instability phenom-ena due to the flat area at the base of the craters produced during the 2007 eruption(Pianoro collapse; see Casagli et al., 2009 for details).

9T. Nolesini et al. / Geomorphology xxx (2012) xxx–xxx

batch and superficial degassing (Burton et al., 2007a; Aiuppa et al.,2010). Likewise, variations recorded during the paroxysmal eruptionsand major explosion-dominated periods indicate an overpressure ofthe volatile degassing from new volatile-rich LP magma batches(Aiuppa et al., 2011; Inguaggiato et al., 2011). The increase in explosionfrequency, tremor amplitude and the long periods of gravity anomaliesthat were registered during these periods were related to the increasein the open-system degassing (Carbone et al., 2012). Increasing theamount of gas from the deep implies an increment in themagma bubblecontent (vesicularity) and, consequently, a decrease in themagma den-sity (Carbone et al., 2012). The decrease in the magma density has alsobeen confirmed by the abundance of the low-density (LD) componentof the HP magma and the presence of LP magma (D'Oriano et al., 2011).

The increase in the CO2 soil-flux is a proxy for the closed toopen-system degassing transition that occurs at certain depths(Burton et al., 2007b; Aiuppa et al., 2010). Gas percolation from LPand HP magmas produces a magma density decrease through an in-crease in vesiculation (Lautze and Houghton, 2005; Burton et al.,2007b). The ordinary activity is fed by a denser magma column,while the magma became less dense before paroxysmal eruptions(Lautze and Houghton, 2005). The low-density component of theHP magma is also present during the major explosions (Andronicoand Pistolesi, 2010). An increased degassing rate, an increased explo-sion frequency, major explosions and lava overflows are related to thegas coalescence and slug formation (Carbone et al., 2012; Del Bello etal., 2012). This high-intensity activity at mafic volcanoes was general-ly followed by a reduction in the magma density (Scandone et al.,2008; Di Traglia et al., 2009; Cimarelli et al., 2010), and hence, low de-formation rate values that were observed in the Sciara del Fuoco canbe related to a conduit system filled by the high-density componentof the HP magma, i.e., by less overpressurized magma.

Therefore, we interpret the increase in the deformation rates priorto the major explosion-dominated periods as a lateral expansion ofthe dike-conduit system due to the increase in the gas volume inthe shallow-seated magma. Shallow intrusive events are commonduring the construction of mafic volcanoes that outwardly commonlyappear to be dominated by eruptions producing pyroclastic deposits(e.g. Németh and Cronin, 2008, 2011). The magma flow in sheetscan be driven by excess magma pressure at the source, the magmabuoyancy and the gradients of the "tectonic" stress normal to thesheet plane (Rubin, 1995). We considered the "tectonic" stress

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

normal to the sheet plane throughout the 2009–2011 to be almostconstant because strong earthquakes have not been registered duringperiods of high deformation rates, while magma buoyancy can beneglected because it generally affects the vertical propagation ofmagma-filled cracks (Rubin, 1995).

Considering these assumptions and limitations and using the de-formation rate as a proxy for the order of magnitude of the sheetpropagation rate (U), the magma overpressure (P0) needed to gener-ate the registered deformation rates can be evaluated by:

U ¼ lP30=3ηM

2 ð1Þ

whereM is the host rock elastic stiffness, η is themagma viscosity and l isthe sheet length (Rubin, 1995). Using the host rock's elastic stiffness(10–20 GPa; Apuani et al., 2005; Boldini et al., 2009) and the sheet lengthover the interval 100–500 m (Mattia et al., 2004; Chouet et al., 2008), weobtain an overpressure on the order of 10−2 MPa for a low deformationrate (0.01 mm/day), 10−1 MPa for deformation rates typical for themajor explosion-dominated periods (0.36–12.9 mm/day) and 100–101 MPa for the high deformation rate (300 mm/h) that characterizedthe 2007 paroxysmal flank eruption (Casagli et al., 2009). It is importantto note that the direction of the GBInSAR line-of-sight is fairly parallel tothe direction of the dike that propagates from the central conduit(NE-SW as reported by Tibaldi et al., 2009).

Magma rheology is also a critical factor because it limits sheetpropagation (Scaillet et al., 1998; Takeuchi, 2004). We can neglectthe effect of the gas bubbles on the magma rheology and only consid-er the melt+crystals contributions because, in a T junction (i.e., mainconduit+lateral dike) of the volcanic conduit, bubbles tend to followa vertical path (Pioli et al., 2009). Therefore, the viscosity of the HPmagma (>105 Pa s; Giordano et al., 2008; Vona et al., 2011) is onthe same order of magnitude (106 Pa s) that was indicated byScaillet et al. (1998) and is within the viscosity interval (105–

107 Pa s) indicated by Takeuchi (2004) as the “dike-propagationlimit” (Corazzato et al., 2008). This implies that, even if magma over-pressure is the driving force of lateral sheet injection, the rheologicalcontrol could also limit the sheet propagation in the Sciara del Fuocoduring the major explosion-dominated periods. Therefore, for a flankeffusive eruption, the overpressure in the conduit-dike system should

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10 T. Nolesini et al. / Geomorphology xxx (2012) xxx–xxx

be large enough (on the same order of magnitude as 100–101 MPa) toovercome the “dike-propagation limit” imposed by magma rheology.

5.3. Slope instability

Slope angle is a crucial factor in the failure of volcanic slope, whichis indicated by the high frequency of collapses of steep-sided

Fig. 11. Deformation rates observed prior to the increase in the explosive intensity is related tocrease in the gas volume in the shallow-seated magma during a) “ordinary”, b) “major explosirelated to themagma overpressure at the base of the sheet (L= sheet length; η=magma visco

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

volcanoes (Voight and Elsworth, 1997), but Boldini et al. (2009) pro-vided evidence that the peak friction angle of the volcaniclastic mate-rials filling the Sciara del Fuoco is greater than the average dip of boththe subaerial slope (35°–36°) and the slip surface of the 2002 land-slide (33°; Boldini et al., 2009). These conclusions have been con-firmed by the experimental approach proposed here. Our analogmodels reveal that the internal friction angles alone are not able to

the lateral expansion of the upper part of the shallow dike-conduit system due to the in-on-dominated period” and c) flank/paroxysmal eruptions. The observed deformation ratesity). Deformation rate related to paroxysmal eruptions is reported in Casagli et al. (2009).

bility on Stromboli volcano: Integration of GBInSAR data and analogh.2012.10.014

11T. Nolesini et al. / Geomorphology xxx (2012) xxx–xxx

generate the slope instability. However, the bulging experiments orthe accumulation of material in the summit part of the slopereproduced the effect of lava loading on the crater flanks and/or onthe Sciara del Fuoco slope and were able to produce landslides. Thisresult implies that the Sciara del Fuoco instability is related to themagma intrusions rather than gravity force alone.

Themagmatic pressure is considered to be responsible for the failureof the volcanic slope by increasing the shear stress and reducing theshear strength (Voight and Elsworth, 1997; Elsworth and Day, 1999).This pressure consists of two components: a magmastatic componentand an excess-pressure (overpressure) component (Elsworth andVoight, 1996; Voight and Elsworth, 1997). Our data on the conduit over-pressure are on the same order of magnitude of the overpressure usedas triggers in the numerical models developed for the analysis of flankinstability (Apuani et al., 2005, Apuani and Corazzato, 2009; Casagli etal., 2009; Tibaldi et al., 2009). To repeat these analyses, consideringthe three order of magnitude characteristics of each activity type, isstrongly suggested. The “ordinary” activity characterized by creep onthe Sciara del Fuoco (overpressure in the OM 10−2 MPa; Fig. 10), theperiods dominated by high deformation rate without flank instability(overpressure in the OM 10−1 MPa; Fig. 10), and flank eruptions dom-inated by high flank instability and possible landslide generation (over-pressure in the order of magnitude of 100–101 MPa; Fig. 10). Therefore,it is rather suitable that, after a period of creep movements, sheet em-placement can occur during a stage of increased magma pressure, asproposed by Tibaldi et al. (2009).

5.4. Hazard implications on collapsing volcanoes

Slope failure is a common phenomenon in the volcanic environ-ment, and it affects edifices of different sizes, from cinder cones(Riggs and Duffield, 2008; Di Traglia et al., 2009; Németh et al.,2011) to stratovolcanoes (Siebert, 1984; Acocella, 2005; Procter etal., 2009; Zernack et al., 2009; Roverato et al., 2011; Zernack et al.,2011) and lava shield volcanoes (Voight and Elsworth, 1997;Hurlimann et al., 2004). Flank failure may be triggered by an increasein the pore pressure (magma and/or gas overpressure), deposition oferupted products (over-steepening or overloading), shear strengthreduction due to hydrothermal alteration or tectonic displacement(Evans et al., 2006 and references therein). Intrusion-related col-lapses are very frequent in large stratovolcanoes and in lava domes,and the generation of retrogressive landslides (as in Fig. 11a, b) hasbeen demonstrated by direct observations and geological studies(Voight, 2000; Baldi et al., 2008). The breaching of cinder cones isgenerally observed at slope bulging, lava flow accumulation or tocone rafting due to the lava flow movement at the base of the cone(Tibaldi, 1995; Valentine and Gregg, 2008; Di Traglia et al., 2009;Németh et al., 2011). The superimposed effects of lava accumulationand magma intrusion were observed at Mount Etna during the col-lapse of the southeast crater (in the summit craters area), and thegeneration of a debris avalanche was observed up to 1.2 km awayfrom the source (Norini et al., 2009). Lava accumulation on the coneflank caused rapid erosion and down-slope propagation of the de-pression, while the propagation of the eruptive fissure below the col-lapsing flank produced the main collapse and the associated debrisavalanches, which were followed by a series of retrogressive land-slides (Norini et al., 2009) and are in good agreement with our exper-imental results. The accumulation experiments demonstrate that theaccretion of a portion of the slope produces alters the flank instabilityand triggers small landslides. Breaking the balance that exists in theslope forms a successive series of landslides that propagate upwardsthe slope.

These phenomena were observed by the GBInSAR after the 1st–2ndAugust 2011 lava flow on the Sciara del Fuoco slope (Fig. 3). During theperiod following this flow, the area affected by the lava emplacementshowed a very low coherence for a long period (1–2 month) even if

Please cite this article as: Nolesini, T., et al., Deformations and slope instamodeling, Geomorphology (2012), http://dx.doi.org/10.1016/j.geomorp

themain lava bodywas not yet eroded on the Sciara del Fuoco. This im-plies that there was prolonged flank erosion related to the presence ofthe small lava remnant in the upper part of the Sciara del Fuoco(Figs. 3, 11). Direct observations of the GBInSAR data and the experi-mental results are in agreement with the recent outcomes in whichlava loading creates a general modification of the stress field in theSciara del Fuoco, which causes an increase in the deformation over theentire upper and central part of the slope, which increases with thethickness of the lava layer (Apuani et al., 2005; Corazzato et al., 2008).

The experimental results confirmed that both set-ups used (accu-mulation and bulging) are not able to produce large-scale slope insta-bility, and hence, neither the lava accumulation nor the localizedintrusion are triggers of the large flank collapses and the associatedtsunamis. The lack of tsunamigenic landslides during the 2007paroxysmal/flank eruption has been related to the lower magmaticpressures that developed at the tip of the NW–SE-striking dike,which is compared with the 2002–03 eruption (Neri et al., 2008).This interpretation highlights the crucial role of the GBInSAR monitor-ing system. Currently, the unique apparatus installed on the StromboliIsland is capable of detecting ground deformations related to the accu-mulating pressures below Sciara del Fuoco (within the conduit) and is,thus, able to forecast any catastrophic collapse.

6. Conclusions

Combining the monitoring data with analog modeling, the super-ficial movements observed by the GBInSAR system represent the re-sponse of Stromboli volcano to changes of overpressure in theconduit. This suggests that the movements observed on the Sciaradel Fuoco are slope instability phenomena where the gravitationalcomponent produces a constant creep (e.g., slow movements duringthe 2011 event), while changes in the magma overpressure explicateswhy certain periods are characterized by accelerations, which induceinstability in the external flanks of the crater area and in the Sciara delFuoco, and eventually promote the failure of the volcano slopes.

Acknowledgments

This work has been sponsored in part by the National Civil Protec-tion Department (DPC) in the framework of the SAR.net2 and by theINGV in the framework of Paroxysm projects. The DPC is acknowl-edged for supporting the project and permitting this publication.The authors are grateful to Ellegi-Lisalab for providing the systemsused for data acquisition and processing. We thank Y. Nestola(Università di Parma) and A. Vona (Università Roma Tre) for helpfuldiscussions . We also thank K. Németh (Massey University) and A.Tibaldi (Università di Milano-Bicocca) for the constructive revisionsthat helped to improve the manuscript.

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