Susceptibility of intrusion-related landslides at volcanic islands: the Stromboli case study

Susceptibility of intrusion-related landslides in an active volcano was evaluated coupling the landslide susceptibility estimation by random forest (RF), and the probabilistic volcanic vent opening distribution, as proxy for magma injection, using the QVAST tool. In order to develop and test the method proposed here, the RF/QVAST approach was adopted for Stromboli volcano (Southern Italy) since it experienced moderate to huge instability events, it is geomorphologically prone to instability events, and it is affected by active intense volcanic activity that can produce slope instability. The main destabilizing factors of the volcanic flanks are the slope, the aspect, the terrain roughness, the land cover and the litho-technical features of the outcropping rocks. Estimation of volcanic susceptibility shows that the areas with high probability of new vent opening are located in the north-western unstable volcano flank (Sciara del Fuoco), in the volcano summit and the north-eastern volcano flank coherent with the possible re-activation of the eruptive fissures related to the regional tectonic setting. The areas with higher probability of intrusion-related landslides are located in the upper part of the Sciara del Fuoco, while the rest of the island show moderate to low probability of intrusion-related landslide occurrence.     

Deformation at Stromboli volcano (Italy) revealed by rock mechanics and structural geology

We approach the reconstruction of the recent structural evolution of Stromboli volcano (Italy) and the analysis of the interplay between tectonics, gravity and volcanic deformation. By tying together structural, lithostratigraphic and rock mechanics data, we establish that since 100 ka BP, the edifice has faulted and jointed mainly along NE-striking planes. Faults mostly dip to the NW with normal displacement. Taking also into account the presence of a NW-trending regional least principal stress and of tectonic earthquake hypocenters inside the cone, we suggest that this fracturing can be related to the transmission of tectonic forces from the basement to the cone. Dyking concentrated along a main NE-trending weakness zone (NEZ) across the volcano summit, resembling a volcanic rift, whose geometry is governed by the tectonic field. In the past 13 ka, Stromboli experienced a reorganisation of the strain field, which was linked with the development of four sector collapses affecting the NW flank, alternating with growth phases. The tectonic strain field interplayed with dyking and fracturing related to unbuttressing along the collapse shoulders. We propose that tectonics control the geometry of dykes inside the cone and that these, in turn, contribute to destabilise the cone flanks.     

Landslides in the Rumphi District of Northern Malawi: characteristics and mechanisms of generation

Landslides are a common phenomenon in all the regions of Malawi. A number of historical landslides have been documented and are summarized here. This paper examines the occurrence of landslides in the Rumphi District of Northern Malawi. The study is based on a detailed study of 98 landslides that occurred in 2003 at Ntchenachena and Chiweta areas in the Rumphi District. This paper examines factors that contributed to and caused these landslides. The paper suggests that landslides were triggered by 206 mm of rain received in 2 days. The high percentage of medium to fine sand and abrupt rise in pore pressure accelerated the process of liquefaction at Ntchenachena. Cleft water pressure at the point between regolith and soil mass caused a number of slides in the Chiweta areas. High slope angles, deep weathering of the basement and high annual total rainfall contributed to the slope instability. Human activities through cultivation on steep slope, slope remodeling, and deforestation significantly altered the conditions of the slopes, thereby increasing the degree of landslide hazard present in these areas.     

Ground‐based InSAR reveals conduit pressurization pulses at Stromboli volcano

At Stromboli volcano (Southern Italy), only few minor precursors (gas output) have been identified for ‘major’ Strombolian explosions. We use ground‐based interferometric synthetic aperture radar (GBInSAR) technology to monitor the displacement rate of the summit area of Stromboli. We analysed the 2009–2011 period. The analysis of the displacement rate has been performed by dividing the summit zone monitored by the GBInSAR into three regions, corresponding to the edge of the crater area (1 and 2), and to the slope of the NE crater (3). Pulses of rapid expansion of sector 3, of variable duration and amplitude, appear in coincidence with periods of intense activity that include lava flows and major explosions. We associate this expansion with the pressurization of the magma column that is recharged by deep‐derived gas, promoting the onset of ‘major explosion‐dominated’ activity.     

Monitoring, prediction, and early warning using ground-based radar interferometry

In order to define adequate prevention measures and to manage landslide emergencies, real-time monitoring is required. This paper presents two different applications of the remote sensing technique: the ground-based synthetic aperture radar interferometry, here proposed as a monitoring and early warning support for slope instability. Data acquisitions carried out through a ground-based synthetic aperture radar interferometer, operating in K_u band, installed in front of the observed slopes, are discussed. Two case studies, based on the use of the same apparatus (formerly developed by the Joint Research Center of the European Commission and by Ellegi-LiSALab srl), are reported: the first one concerns the monitoring of a large landslide, named Ruinon (Valfurva, Italy). The second one deals with the monitoring of the NW unstable slope in the Stromboli island aimed to implementing an early warning system. Acquired interferometric data are processed to provide displacements and velocity maps of the monitored area. The monitoring services ongoing on the Ruinon landslide and on Stromboli demonstrate the capability of this technique to operate in different operative settings (i.e., different phenomena and geological framework) and for different aims (monitoring for prevention, early warning, and emergency assessment). This methodology has also been proved by national and regional authorities of civil protection in order to provide a real-time monitoring for emergency management.     

Application of large-scale ring shear tests to the analysis of tsunamigenic landslides at the Stromboli volcano, Italy

The island of Stromboli (Southern Italy) is a 4,000-m-high volcanic edifice about 900 m above sea level. Most of the NW flank is formed by a wide scar (Sciara del Fuoco) filled by irregular alternations of volcaniclastic layers and thin lava flows. Between 29 and 30 December 2002, a submarine and a subaerial landslide involved the northernmost part of the Sciara del Fuoco slope and caused two tsunami waves with a maximum run-up of 10 m. Mechanisms of the rapid submarine landslide and the preceding deformation of the subaerial and submarine slope were investigated using large-scale ring shear tests on the saturated and dry volcaniclastic material. The shear behaviour of the material under different drainage conditions was analysed during tests conducted at DPRI, Kyoto University. Pore pressure generation, mobilised shear strength and grain crushing, within a range of displacements encompassing the different stages of evolution of the slope, were considered. Experimental results suggest that even at larger displacements, shear strength of the dry material explains the virtual stability of the slope. Conversely, full or partial liquefaction can be invoked to explain the submarine failure and the subsequent long runout (more than 1,000 m) of the failed materials.     

Seismological constraints on the 2018 Mt. Etna (Italy) flank eruption and implications for the flank dynamics of the volcano

3D earthquake locations, focal mechanisms and stress tensor distribution in a 16‐month interval covering the 2018 Mt. Etna flank eruption, enabled us to investigate the relationship between magma intrusion and structural response of the volcano and shed light on the dynamic processes affecting the instability of Mt. Etna. The magma intrusion likely caused tension in the flanks of the volcano, leading to significant ground deformation and redistribution of stress on the neighbouring faults at the edge of Mt. Etna's unstable sector, encouraging the ESE sliding of the eastern flank of the volcano. Accordingly, FPSs of the post‐eruptive events show strike slip faulting mechanisms, under a stress regime characterized by a maximum compressive σ₁, NE‐SW oriented. In this perspective, any flank eruption could temporarily enhance the sliding process of both the southern and eastern flanks of the volcano.     

Major changes in volcano behaviour after a sector collapse: insights from Stromboli, Italy

In recent years, increasing numbers of volcanoes are being recognized as having undergone large sector collapses. How far such events can change the succeeding behaviour of the volcano is poorly known, despite the importance for natural hazards and economic applications. By analysing the last 100‐kyr geological history of Stromboli (Italy), which has involved multiple collapses, this paper illuminates the feedback effect between sector collapse and magma upwelling and emplacement. The collapsed zone of Stromboli is episodically unbuttressed, draining magma preferentially with formation of eruptive centres in the collapse amphitheatre and dyking along the collapse shoulders. After a collapse, there were no further linear eruptions across the cone on opposing middle–lower slopes, but these occurred again after rebuilding of the cone. The time‐averaged eruption rate increased immediately before the first collapse and remained higher during the succeeding period of frequent collapses.     

Effect of antecedent-hydrological conditions on rainfall triggering of debris flows in ash-fall pyroclastic mantled slopes of Campania (southern Italy)

Mountainous areas surrounding the Campanian Plain and the Somma-Vesuvius volcano (southern Italy) are among the most risky areas of Italy due to the repeated occurrence of rainfall-induced debris flows along ash-fall pyroclastic soil-mantled slopes. In this geomorphological framework, rainfall patterns, hydrological processes taking place within multi-layered ash-fall pyroclastic deposits and soil antecedent moisture status are the principal factors to be taken into account to assess triggering rainfall conditions and the related hazard. This paper presents the outcomes of an experimental study based on integrated analyses consisting of the reconstruction of physical models of landslides, in situ hydrological monitoring, and hydrological and slope stability modeling, carried out on four representative source areas of debris flows that occurred in May 1998 in the Sarno Mountain Range. The hydrological monitoring was carried out during 2011 using nests of tensiometers and Watermark pressure head sensors and also through a rainfall and air temperature recording station. Time series of measured pressure head were used to calibrate a hydrological numerical model of the pyroclastic soil mantle for 2011, which was re-run for a 12-year period beginning in 2000, given the availability of rainfall and air temperature monitoring data. Such an approach allowed us to reconstruct the regime of pressure head at a daily time scale for a long period, which is representative of about 11 hydrologic years with different meteorological conditions. Based on this simulated time series, average winter and summer hydrological conditions were chosen to carry out hydrological and stability modeling of sample slopes and to identify Intensity-Duration rainfall thresholds by a deterministic approach. Among principal results, the opposing winter and summer antecedent pressure head (soil moisture) conditions were found to exert a significant control on intensity and duration of rainfall triggering events. Going from winter to summer conditions requires a strong increase of intensity and/or duration to induce landslides. The results identify an approach to account for different hazard conditions related to seasonality of hydrological processes inside the ash-fall pyroclastic soil mantle. Moreover, they highlight another important factor of uncertainty that potentially affects rainfall thresholds triggering shallow landslides reconstructed by empirical approaches.     

The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy

We report the occurrence of unusual, high-magnesium (Fo₉₆) olivinephenocrysts in a basaltic lava and an ejected lithic block fromthe Upper Vancori period (13 ka) and the recent activity (2002–2003)of Stromboli volcano, Italy. The samples that contain this distinctivemineral chemistry are a shoshonitic basalt and a basaltic andesitewith anomalous bulk-rock chemical characteristics in which theiron is highly oxidized (6–8 wt % Fe₂O₃ and <1 wt %FeO). In other respects these samples are similar to the majorityof Stromboli basalts, characterized by the coexistence of olivine,clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts,and clinopyroxene, plagioclase and Fe–Ti oxides in thegroundmass. In the high-magnesium olivine samples, Fe–Tioxides (pseudobrookite) typically occur as symplectitic intergrowthswith the olivine phenocrysts, indicating simultaneous growthof the two phases. We propose, as a paragenetic model, thatthe Fo₉₆ olivine phenocrysts crystallized from a highly oxidizedbasaltic magma in which most of the iron was in the ferric state;hence, only magnesium was available to form olivine. The highlyoxidized state of the magma reflects sudden degassing of volatilephases associated with instantaneous, irreversible, transientdegassing of the magma chamber; this is postulated to occurduring periods of sudden decompression induced by fracturingof the volcanic edifice associated with paroxysmic activityand edifice collapse. KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes     

Deterministic estimation of hydrological thresholds for shallow landslide initiation and slope stability models: case study from the Somma-Vesuvius area of southern Italy

Rainfall-induced debris flows involving ash-fall pyroclastic deposits that cover steep mountain slopes surrounding the Somma-Vesuvius volcano are natural events and a source of risk for urban settlements located at footslopes in the area. This paper describes experimental methods and modelling results of shallow landslides that occurred on 5–6 May 1998 in selected areas of the Sarno Mountain Range. Stratigraphical surveys carried out in initiation areas show that ash-fall pyroclastic deposits are discontinuously distributed along slopes, with total thicknesses that vary from a maximum value on slopes inclined less than 30° to near zero thickness on slopes inclined greater than 50°. This distribution of cover thickness influences the stratigraphical setting and leads to downward thinning and the pinching out of pyroclastic horizons. Three engineering geological settings were identified, in which most of the initial landslides that triggered debris flows occurred in May 1998 can be classified as (1) knickpoints, characterised by a downward progressive thinning of the pyroclastic mantle; (2) rocky scarps that abruptly interrupt the pyroclastic mantle; and (3) road cuts in the pyroclastic mantle that occur in a critical range of slope angle. Detailed topographic and stratigraphical surveys coupled with field and laboratory tests were conducted to define geometric, hydraulic and mechanical features of pyroclastic soil horizons in the source areas and to carry out hydrological numerical modelling of hillslopes under different rainfall conditions. The slope stability for three representative cases was calculated considering the real sliding surface of the initial landslides and the pore pressures during the infiltration process. The hydrological modelling of hillslopes demonstrated localised increase of pore pressure, up to saturation, where pyroclastic horizons with higher hydraulic conductivity pinch out and the thickness of pyroclastic mantle reduces or is interrupted. These results lead to the identification of a comprehensive hydrogeomorphological model of susceptibility to initial landslides that links morphological, stratigraphical and hydrological conditions. The calculation of intensities and durations of rainfall necessary for slope instability allowed the identification of deterministic hydrological thresholds that account for uncertainty in properties and observed rainfall intensities.     

Structural and geomorphological aspects of the Kat landslides (Tokat—Turkey) and susceptibility mapping by means of GIS

Kat County, which is located in a slope of hilly region and constructed in the side of a mountain along the North Anatolian Fault Zone, is frequently subject to landslides. The slides occur during periods of heavy rainfall, and these events cause destruction to property, roads, agricultural lands and buildings. In the last few decades, a lot of houses and buildings have been damaged and destroyed. Settlement areas have remained evacuated for a long time. The slope instabilities in the study area are a complex landslide extending from north to south containing a lot of landslides. Field investigations, interpretation of aerial photography, analyses of geological data and laboratory tests suggest that some factors have acted together on the slopes to cause the sliding. In the wet season, the slopes became saturated. As the saturation of the earth material on the slope causesa rise in water pressure, the shear strength (resisting forces) decreases and the weight (driving forces) increases; thus, the net effect was to lower the safety factor. Previous failures have affected the rock mass, leading to the presence of already sheared surfaces at residual strengths. The relation between the joint planes and the instability of the slope in the study area was discussed and it was found that the potential slope instabilities are mainly in the directions of NW–SE, NE–SW and N–S. The landslide susceptibility map obtained by using the geographical information system showed that a large area is susceptible and prone to landslides in the northern part of the study area.An erratum to this article can be found at     

基于重力剖面的长白山天池火山地壳岩浆囊建模

长白山天池火山是中国最具潜在喷发危险的多成因复式火山,在近2000年来,曾经发生过世界上最大规模的喷发。为了进一步研究长白山天池火山的潜在危险,有必要研究火山的岩浆囊位置和分布。为此,在长白山天池火山开展了一条南北方向的重力剖面测量。结合前人工作,如地震勘探P波速度反演和大地电磁测深（MT）电阻率反演,以及地质信息,采用人机交互的形式,建立了一条长约150km的密度模型。从建模结果图中可以发现：（1）长白山天池下方存在地壳岩浆囊;（2）长白山天池北坡地壳存在一个高阻、低密度体,深度在7~15km,距离天池2~10km,可能是富含气体的岩浆囊;（3）在南坡和北坡大约3km深度处普遍存在一个岩浆岩层;（4）在天池下存在一个已经塌陷堵塞的火山颈。     

Subaqueous density flow processes and deposits of an island volcano landslide (Stromboli Island, Italy)

Stromboli is a 3000 m high island volcano, rising to 900 m above sea-level. It is the most active volcano of the Aeolian Archipelago in the Tyrrhenian Sea (Italy). Major, large volume (1 km³) sector collapses, four occurring in the last 13 kyr, have played an important role in shaping the north-western flank (Sciara del Fuoco) of the volcano, potentially generating a high-risk tsunami hazard for the Aeolian Islands and the Italian coast. However, smaller volume, partial collapses of the Sciara del Fuoco have been shown to be more frequent tsunami-generating events. One such event occurred on 30 December 2002, when a partial collapse of the north-western flank of the island took place. The resulting landslide generated 10 m high tsunami waves that impacted the island. Multibeam bathymetry, side-scan sonar imaging and visual observations reveal that the landslide deposited 25 to 30 × 10⁶ m³ of sediment on the submerged slope offshore from the Sciara del Fuoco. Two contiguous main deposit facies are recognized: (i) a chaotic, coarse-grained (metre-sized to centimetre-sized clasts) deposit; and (ii) a sand deposit containing a lower, cross-bedded sand layer and an upper structureless pebbly sand bed capped by sea floor ripple bedforms. The sand facies develops adjacent to and partially overlying the coarse deposits. Characteristics of the deposits suggest that they were derived from cohesionless, sandy matrix density flows. Flow rheology and dynamics led to the segregation of the density flow into sand-rich and clast-rich regions. A range of density flow transitions, both in space and in time, caused principally by particle concentration and grain-size partitioning within cohesionless parent flows was identified in the deposits of this relatively small-scale submarine landslide event.     

Can rainfall trigger volcanic eruptions? A mechanical stress model of an active volcano: ‘Piton de la Fournaise’, Reunion Island

This paper relates the hydrology of a volcano to its eruptive activity, which seem to be correlated. Based on the groundwater pattern previously determined by hydrologic and thermal modelling, we formulate hypotheses for the mechanisms linking rainfall and volcanic eruptions. We focus on: mechanical loading by the weight of the infiltrated rain, which might favour magma overpressure or slope instability on the eastern volcanic flank; unloading of the infiltrated water, discharged toward the ocean, which could increase the tangential tensile stress above the magma chamber. We test the sensitivity of the model to the magma chamber geometry and the rainfall distribution. The mechanical unloading of the infiltrated water is the mechanism most likely to trigger eruptions. However, the presence of a sliding surface and rainfall loading could favour the development of tensile stress in the 'shell' surrounding the magma chamber but with a lower intensity than in the previous case.     

Landslide triggers along volcanic rock slopes in eastern Sicily (Italy)

A new dataset of landslides, occurred in a tectonically active region, has been analysed in order to understand the causes of the slope instability. The landslides we have dealt with took place along the volcanic rock cliff of S. Caterina and S. Maria La Scala villages (eastern Sicily, Italy), a densely inhabited area located on the eastern margin of Mt. Etna, where some seismogenic faults, locally named Timpe system, slip during moderate local earthquakes and also move with aseismic creep mechanisms. The results show that landslides are triggered by heavy rainfalls, earthquakes and creep fault episodes. Indeed, they occur along discrete fault segments, exhibiting a combination of both brittle failure, indicated by the earthquake occurrence, and aseismic creep events. The analysis of seismicity occurred on the Timpe fault system has shown that the active Acireale fault, in its southernmost segment, is subject to an aseismic sliding, which increases after the stick–slip motion in the nearby faults. Therefore, aseismic creep seems to concur in the predisposition of a rock to fail, since strains can increase the jointing of rock masses leading to a modification in the slope stability. Understanding the factors concurring to the slope instability is a useful tool for future assessments of the landslide hazard in densely settled areas, located on a volcanic edifice, such as Etna that is slowly sliding seawards, and where active faults, seismicity and heavy rains affect the deeply fractured slopes.     

顺层岩质边坡溃屈变形机制及失稳判定方法

溃屈型破坏是一种常见的顺层岩质边坡破坏模式,溃屈变形发展机制及失稳破坏的定量研究对滑坡工程勘查和防治指导十分重要。本文根据边坡的地质环境和力学作用机制,建立了三维受压板简化模型,其能够考虑岩层自身重力、地震力、静水压力的共同作用和岩体材料塑性变形的影响。基于弹塑性受压板稳定理论,利用能量法推导得到了边坡溃屈变形破坏的临界方程。对于溃屈型边坡的结构失稳和滑动失稳分别提出了相应的稳定性判定方法,并针对不同状态的边坡提出了相应的防治措施建议。以四川省甘孜藏族自治州巴塘县下归哇边坡为例,对所提判定方法的正确性进行了验证。计算结果表明,边坡的临界溃屈长度（a1）为483.8 m,说明下归哇边坡能够发生溃屈变形;现场勘查得知边坡实际溃屈长度（a′）为530.0 m,a′＞a1,可知边坡是稳定的。这与实际情况相吻合,由此证明本文所提出的判定方法可行。     

Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors

Detailed geomorphological mapping carried out in 5 sample areas in the North of Lisbon Region allowed us to collect a set of geological and geomorphological data and to correlate them with the spatial occurrence of landslide. A total of 597 slope movements were identified in a total area of 61.7 km², which represents about 10 landslides per km².The main landslide conditioning factors are: lithology and geological structure, slope angle and slope morphology, land use, presence of old landslides, and human activity.The highest landslide density occurs in Cretaceous marls and marly limestones, but the largest movements are in Jurassic clays, marls and limestones.The landslide density is higher on slopes with gradients above 20 °, but the largest unstable area is found on slopes of 10 ° to 15 °, thus reflecting the presence of the biggest slope movements. There is a correlation between landslides and topographical concavities, a fact that can be interpreted as reflecting the significance of the hydrological regime in slope instability.Concerning land use, the highest density of landslides is found on slopes covered with shrub and undergrowth vegetation.About 26% of the total number of landslides are reactivation events. The presence of old landslides is particularly important in the occurrence of translational slides and complex and composite slope movements.20% of the landslide events were conditioned by anthropomorphic activity. Human's intervention manifests itself in ill-consolidated fills, cuts in potentially unstable slopes and, in a few cases, in the changing of river channels.Most slope movements in the study area exhibit a clear climatic signal. The analysis of rainfall distribution in periods of recognised slope instability allows the distinction of three situations: 1) moderate intensity rainfall episodes, responsible for minor slope movements on the bank of rivers and shallow translational slides, particularly in artificial trenches; 2) high intensity rainfall episodes, originating flash floods and most landslides triggered by bank erosion; 3) long-lasting rainfall periods, responsible for the rise of the groundwater table and triggering of landslides with deeper slip surfaces.     

Degassing, crystallization and eruption dynamics at Stromboli: trace element and lithium isotopic evidence from 2003 ashes

During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ⁷Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ⁷Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that ^melt/fluidD_Li > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.