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The Rock Engineering System (RES) applied to landslide susceptibility zonation of the northeastern flank of Etna: methodological approach and results
No full textGround deformations in the northeastern flank of Etna are well known. Despite only a few landslide events have been documented, these have significantly involved and damaged lifelines and buildings. These events are mainly related to the activity of the volcano-tectonic structures and associated seismicity, as in the case of the 2002 reactivation of the Presa landslide during an increased activity of the Pernicana fault system. In order to highlight the areal distribution of potentially unstable slopes based on a detailed, site-specific study of the factors responsible for landslide, and to ultimately contribute to risk management, a landslide susceptibility analysis of the northeastern flank of Etna in the Pernicana area was carried out, and a susceptibility map at 1:10.000 scale was produced, extending over an area of 168 km2. Different methods are proposed in the literature to obtain the regional distribution of potentially unstable slopes, depending on the problem scale, the slope dynamic evolution in the geological context, and the availability of data. Among semi-quantitative approaches, the present research combines the Rock Engineering System (RES) methodology with parameter zonation mapping in a GIS environment. The RES method represents a structured approach to manage a high number of interacting factors involved in the instability problem. A numerically coded, site-specific interaction matrix (IM) analyzes the cause-effect relationship in these factors, and calculates the degree of interactivity of each parameter, normalized by the overall
interactivity of the system (weight factor). In the specific Etna case, the considered parameters are: slope attitude, lithotechnical properties (lithology, structural complexity, soil and rock mass quality), land use, tectonic structures, seismic activity (horizontal acceleration) and hydrogeological conditions (groundwater and drainage). Thematic maps are prepared at 1:10.000 scale for each of these parameters, and instability-related numerical ratings are assigned to classes. An instability index map is then produced by assigning, to each areal elementary cell (in our case a 10 m pixel),
the sum of the products of each weight factor to the normalized parameter rating coming from each input zonation map. This map is then opportunely classified in landslide susceptibility classes (expressed as a percentage), enabling to discriminate areas prone to instability. Overall, the study area is characterized by a low propensity to slope instability. Few areas have an instability index
of more than 45% of the theoretical maximum imposed by the matrix. These are located in the few steep slopes associated with active faults, and strongly depending on the seismic activity. Some other areas correspond to limited outcrops characterized by significantly reduced lithotechnical properties (low shear strength). The produced susceptibility map combines the application of the RES with the parameter zonation, following methodology which had never been applied up to now in in active volcanic environments. The comparison of the results with the ground deformation evidence coming from monitoring networks suggests the validity of the approach
Numerical Model of the Stromboli Volcano (Italy) Including the Effect of Magma Pressure in the Dyke System.
No full textThe Stromboli island, in the Aeolian archipelago (Italy), is one of the most active volcanoes in Europe. In the last 13,000 years, its growth has been complicated by four sector collapses affecting the NW flank, the latest of which resulting in the formation of Sciara del Fuoco (SdF) horseshoe-shaped depression. Slope instability phenomena are represented not only by giant deep-seated gravitational slope deformations, but also by more frequent large landslides, such as occurred in December 2002-January 2003, and shallow landslides, involving loose or weakly cemented deposits, that constitute a natural hazard and affect residential and tourists safety. It is noteworthy that in volcanic environment the instability factors are manifold and much more complex than in other non-volcanic contexts. This paper deals with the Stromboli NW flank instability, and focuses on the effects of magma pressure in the feeding system. Two main objectives have been pursued: (1) to test a methodological approach, in order to evaluate a complex instability process; (2) to contribute to the understanding of volcano deformation and collapse mechanisms and associated hazard. A numerical model was developed by the Finite Difference Method and the FLAC 4.0 code, considering a cross-section of the entire volcano, orthogonal to the SdF and including both subaerial and submerged slopes. The stability of the volcano was analysed under gravity alone, and by introducing the magma pressure effect, both related to magmastatic and overpressure components. The results indicate that gravity alone is not sufficient to affect the stability of the volcano slopes, nor is the magmastatic pressure component. If an excess magma pressure component is introduced, instability is produced in accordance with field evidences and recent slope dynamics
Understanding Etna flank instability through numerical models
No full textShallow and deep deformations, mainly associated with both eruptive and seismic events, are concentrated along recognised fracture and fault systems, mobilising the eastern and south-eastern flank of the volcano. Several interacting causes were postulated to control the phenomenon, including gravity force, magma ascent along the feeding system, and a very complex local and/or regional tectonic activity. Nevertheless, the complexity of such dynamics is still an open subject of research and being the volcano flanks heavily urbanised, the comprehension of the gravitative dynamics is a major issue for public safety and civil protection. The present research explores the effects of the main geological features (in particular the role of the subetnean clays, interposed between the Apennine–Maghrebian flysch and the volcanic products) and the role of weakness zones, identified by fracture and fault systems, on the slope instability process. The effects of magma intrusions are also investigated. The problem is addressed by integrating field data, laboratory tests and numerical modelling. A bi- and tri-dimensional stress–strain analysis was performed by a finite difference numerical code (FLAC and FLAC3D), mainly aimed at evaluating the relationship among geological features, volcano-tectonic structures and magmatic activity in controlling the deformation processes. The analyses are well supported by dedicated structural–mechanical field surveys, which allowed to estimate the rock mass strength and deformability parameters. To take into account the uncertainties which inevitably occur in a so complicated model, many efforts were done in performing a sensitivity analysis along a WNW–ESE section crossing the volcano summit and the Valle del Bove depression. This was mainly devoted to evaluate the effect of topography, geometry and rheological behaviour of the structural units. The 3D numerical model, extended 40 × 60 km, was implemented to simulate the volcano deformation pattern. First, the role of the Pleistocene subetnean clays was investigated, then, two “structural weakness zones” – the Pernicana Fault system and the NE rift – were introduced and their effects on the flank instability evaluated. Two extreme hydrogeological conditions, drained and undrained, were analysed. The results are expressed in terms of stress–strain field, displacement pattern, plasticity states and shear strain increments. Two main instability mechanisms were identified: one at shallow depth, with the sliding surface located inside the subetnean Quaternary clay, and another deep-seated mechanism with a not continuous and less evident sliding surface, developed inside the Apennine–Maghrebian Chain flysch, bordered by active structures. Both mechanisms contribute to explain the present deformation pattern and some of the main structures of the Etna flank. The effect of magma pressure exerted on the active dyke walls during eruptions was then simulated and relations between magmatic activity and flank instability were preliminarily investigated
Stability of a collapsing volcano (Stromboli, Italy) : Limit equilibrium analysis and numerical modelling
No full textStromboli is an active island volcano, belonging to the Aeolian Archipelago (Italy). In the last 13 ka it experienced four lateral collapses affecting its northwestern flank, with its most recent volcanic crisis (December 2002) associated with landslides and related tsunami events. This paper presents the first stability analysis of the Stromboli volcanic edifice. The main input is the geotechnical model of the volcano, defined on the basis of stratigraphical, lithological, material properties, and structural data, collected from in-situ surveys and laboratory tests. Two-dimensional stability analysis was performed by limit equilibrium methods (LEM) and finite difference modelling (FLAC 4.0 code), mainly focusing on the subaerial part of Stromboli's NW flank (Sciara del Fuoco). The variability of the Safety Factor was studied by deterministic, sensitivity and probabilistic analysis focusing on the effect of external forces, such as magma pressure and seismicity, as potential triggering mechanisms of lateral collapse. The LEM analyses were developed considering a maximum depth of 150-350 in for the sliding surface, to which correspond collapse volumes of 95,000-185,000 m(3)/m, respectively. The study shows that, without external forces, the investigated rock mass is stable and that the tectonic seismicity of the area alone does not destabilize the studied slope. On the contrary, magma pressure in dykes can represent a destabilizing factor. Numerical modelling results are concordant with those from LEM. In addition, the simulation reveals that deformations and superficial landslides, preannounce and contribute to retrogressive plasticization and maybe to failure surface deepening. Shallow submarine landslides represent a possible triggering mechanism, and the landslide events of 30 December 2002 consistently fit the simulated evolution. FLAC has revealed to be a useful tool for modelling such a complex system, it allowed to calibrate the response of the geotechnical model, test the validity of the assumptions, simulate the stress-strain evolution, and prepare a possible model for future scenarios. (c) 2004 Elsevier B.V. All rights reserved
Deformation at Stromboli volcano (Italy) revealed by rock mechanics and structural geology
No full textWe 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
Physical and mechanical properties of rock masses at Stromboli : a dataset for volcano instability evaluation
No full textStromboli island has a complex geological history with repeated changes in the volcanic activity alternating with destructive events, caldera collapses and flank landslides. The last activity resulted in the creation of the Sciara del Fuoco depression which was modified by the recent 2002-2003 landslide. The variation in lithology, degree of tectonization and disturbance has resulted in the presence of a wide spectrum of geotechnical materials. This paper summarises the physical and mechanical properties of Stromboli's intact rocks, rock masses and loose deposits, based on field surveys and laboratory tests. A new classification of the rock succession is introduced and four lithotechnical units defined: Lava, Lava-Breccia, Breccia and Pyroclastic deposit. The range of variability in bulk volume, porosity, intact rock compressive strength and geological strength index is presented. The Hoek and Brown's failure criterion was applied for each lithotechnical unit and the rock mass friction angle, apparent cohesion, tensile and compressive strength, global strength and modulus of deformation calculated in a specified stress range
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Mineralogical and geotechnical characterization of a clay unit that underlies the unstable flanks of Mount Etna – Sicily
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