Flank Instability Research Articles

Long‐term stress‐strain analysis of volcano flank instability: The eastern sector of Etna from 1980 to 2012

AbstractWe investigated the evolution of seismicity and deformation in the unstable eastern flank of Etna volcano over a 30 year period (from 1980 to 2012). A significant temporal correlation has been revealed between periods of flank acceleration and intensified seismic activity by comparing seismicity along the northern border (Pernicana fault system) of the sliding flank and the deformation of the eastern flank. Two marked phases have been observed in 1984–1986 and in the years following 2002. These two phases are separated by an intermediate phase from 1987 to 2001, in which the flank sliding slowed down and the seismicity dropped drastically. This common temporal evolution in the deformation rate and seismic release supports the hypothesis that the seismicity in the northern border can be viewed as a marker of the response to accommodate the stress exerted by the traction of the eastern flank sliding. This interplay has also been corroborated by finite element method numerical computations that highlight a good correlation between the seismicity pattern and areas of positive stress changes induced by the sliding surface. The two intense phases of flank acceleration are representative of two main different sources: volcano flank instability stretching the eastern sector in the first 1984–1986 phase and magmatic intrusions pushing the eastern flank seaward since the 2002–2003 eruption. Establishing the relationship between flank acceleration and seismic activation therefore contributes to understanding Etna's mechanical behavior and provides insights into the processes regulating the unstable flank response.

Seismo‐tectonic behavior of the Pernicana Fault System (Mt Etna): A gauge for volcano flank instability?

Flank instability at basaltic volcanoes is often related to repeated dike intrusions along rift zones and accompanied by surface fracturing and seismicity. These processes have been mostly studied during specific events, and the lack of longer‐term observations hinders their better understanding. Here we analyze ~20 years of deformation of the Pernicana Fault System (PFS), the key structure controlling the instability of the eastern flank of Mt. Etna. We exploit East‐West and vertical components of mean deformation velocity, as well as corresponding time series, computed from ERS/ENVISAT (1992–2010) and COSMO‐SkyMed (2009–2011) satellite radar sensors via Synthetic Aperture Radar Interferometry techniques. We then integrate and compare this information with field, seismic, and leveling data, collected between 1980 and 2012. We observe transient displacements accompanied by seismicity, overprinted on a long‐term background eastward motion (~2 cm/yr). In the last decades, these transient events were preceded by a constant amount of accumulated strain near the PFS. The time of strain accumulation varies between a few years and a few decades, also depending on magma emplacement within the nearby North East Rift, which may increase the strain along the PFS. These results suggest that the amount of deformation near the PFS may be used as a gauge to forecast the occurrence of instability transients on the eastern flank of Etna. In this context, the PFS may provide an ideal, small‐scale structure to test the relations between strain accumulation, stress loading, and seismic energy release.

Flank instability of Stromboli volcano (Aeolian Islands, Southern Italy): Integration of GB-InSAR and geomorphological observations

Stromboli is characterized by frequent explosions of variable energy and periodically interrupted by more energetic blasts emitting large volumes of material. The pressurization of a volatile-poor, high-porphyritic magma column that is gas-recharged by the deep-seated, volatile-rich, low-porphyritic magma precedes such events and produces deformations on the NW flank of the volcano, Sciara del Fuoco. By integrating geomorphological observations with long-term displacements from ground-based interferometric radar since December 2007, we identified two landslides whose movements are strongly related with volcanic activity. Movement patterns obtained through a novel long-term analysis of GB-InSAR data permitted us to hypothesize the type of movement and depth for both landslides. Furthermore their position allowed us to affirm that the effusive vent formed in 2007 at 400ma.s.l., was the result of the deflection of a feeder dike caused by landslide fractures, thus showing the important role of geomorphological discontinuities in volcanic environments.

The structure and morphology of the Basse Terre Island, Lesser Antilles volcanic arc

Basse Terre Island is made up of a cluster of composite volcanoes that are part of the Lesser Antilles volcanic arc. The morphology of these volcanoes and the onshore continuation of the grabens and strike–slip faults that surround the island are poorly documented due to erosion and rainforest cover. Therefore, we conducted a morphological analysis of the island using Digital Elevation Model (DEM) data integrated with field observations to document erosional, constructional, and deformational processes. A DEM-based analysis of 1,249 lineaments and field structural measurements of 16 normal faults, 3,741 veins and fractures, and 46 dykes was also carried to document the structures that predominate in sub-surface rocks. The results indicate that the over 1-My-old and elongated Northern Chain volcano, which makes up the northern half of the island, was built by high eruption rates and/or a low viscosity magma injected along the N–S to NNW–SSE-striking extensional structures formed by the flexure of the lithosphere by the overall subduction regime. After 1 Ma, the southern half of the island was shaped by an alignment of conical volcanoes, likely built by a more viscous magma type that was guided by the NW–SE-striking Montserrat-Bouillante strike–slip fault system. These N to NNW and NW structural directions are however poorly expressed onshore, possibly due to slow slip motion. The sub-surface rocks mostly contain E–W-striking structures, which have likely guided the many flank instabilities documented in the studied area, and guide hydrothermal fluids and shallow magmatic intrusions. These structures are possibly part of the E–W-striking Marie-Gallante offshore graben.

Submarine structure of Vulcano volcano (Aeolian Islands) revealed by high-resolution bathymetry and seismo-acoustic data

A new definition of the morpho-structural submarine setting of the Vulcano volcano is presented based on the integration of multibeam swath bathymetry, high-resolution seismic profiles and sea-floor sampling. A number of unknown volcanic features have been mapped on the submarine flanks of Vulcano Island as volcanic outcrops and isolated cones. For most of them a relatively old age is suggested (especially to the west of the island), while evidence of relatively recent volcanic activity characterizes the north-eastern submarine sector, offshore Vulcanello. The morphology and distribution of these features, either aligned along NNW–SSE trends or radially elongated, suggest the prevalence of regional structural control or local stresses related to the volcano structure, respectively, in the different sectors of the volcanic edifice. Shallow-water (< 120 mbsl) insular shelves, related to wave action during Late Quaternary sea-level fluctuations, were recognized in correspondence of the oldest sectors of the edifice, while they are lacking in the younger sectors. Beyond the shelf edge, a strong increase of slope gradients characterizes the submarine flanks, promoting the formation of erosive-depositional and mass-wasting features at different scale, such as landslide scars, gullies, headless channelized features, crescent-shaped bedforms and volcaniclastic fans. In particular, two large volcaniclastic fans were identified on the north-eastern and south-western flanks, where marine retrogressive processes are very active and sediment is funnelled from the coasts to the foot of the volcano. These fans are created by the gradual stacking of gravity flows fed by repeated small- and medium-scale mass-wasting events, instead of being related to large-scale flank instability events. The lack of large-scale lateral collapse on the submarine flanks of Vulcano volcano has been related to the morpho-structural and volcano-tectonic setting of the volcanic edifice, characterized by the occurrence of summit caldera collapses limiting its total height and by the absence of well-defined rift zones. ► The submarine portions of Vulcano account for ≈ 85% of its areal extent. ► Multibeam and seismic data show new details of the submarine flanks. ► Interacting constructional and erosive-depositional processes are documented. ► Insular shelf development gives constraints on the evolution of the edifice. ► No large-scale lateral collapse deposits have been recognized on submarine flanks.

Chapter 3 Characteristics and morphological evolution of the Aeolian volcanoes from the study of submarine portions

Abstract The seven Aeolian Islands exist in much greater volumes below sea level than above sea level; study of the submarine portions is therefore fundamental for a better understanding of the development of volcanism in the area, tectonic control from the regional structural setting and the morphological evolution of volcanic edifices. The interplay between destructive processes (marine erosion, mass-wasting and flank instability) and constructive processes in the growth and evolution of the Aeolian volcanoes is discussed based on a study of their present-day morphology. In particular, the distribution and characteristics of insular shelves in the shallow-water sectors of volcanic edifices are compared and discussed, considering that variable geological/local factors combined with sea-level fluctuations controlled the development and morphology of these features. It is proposed that submerged shelves may furnish relative chronological constraints for the development of the Aeolian volcanoes, enhancing present knowledge which is mostly based on subaerial portions.

Chapter 4 Bathy-morphological setting of the Aeolian Islands

Abstract An updated bathy-morphological setting of the Aeolian Islands is presented, based on new detailed bathymetric maps of the western, central and eastern sectors of the archipelago. In recent years, the acquisition of multibeam swath bathymetry has greatly expanded knowledge of the submarine portions of the Aeolian volcanic edifices, revealing that their submarine extension is much wider than that of the islands. Indications given by the submarine setting are fundamental for better understanding of the evolution of volcanism and the control exerted by main structural lineaments, as well as to locate large-scale flank instability events and recent submarine eruptive activity. DVD: Bathymetric maps of the eastern, central and western sectors are included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic

Volcanic deformation and flank instability due to magmatic sources and frictional rheology: the case of Mount Etna

SUMMARY The overall picture of Mount Etna deformation emerging since a couple of decades of geodetic surveys shows effects of magma accumulation, characterized by inflation/deflation cycle, accompanied by a sliding instability of the southeast flank, whose manifestation is an increase in the horizontal deformation away from the volcano summit. This is a very interesting case to test whether advanced models, taking into account topography, internal structure and frictional rheology, may contribute to a better understanding of the complex interplay among mechanical response, magmatic activity and gravitational load occurring in a volcanic system. Using finite element numerical models we make predictions of surface displacements associated with a simple expansion source and with a dike-like vertical discontinuity. A new methodology is developed to initialize the lithostatic stress field according to the material and geometrical complexities of the models considered. Our results show that, while an amplification of the horizontal displacement can be easily obtained up to a maximum distance of 10 km from the source, we have not been able to find any configuration to extend further this signal. For the case of Mount Etna this suggests that the large horizontal displacements observed in the east flank along the coast cannot be directly related to magma accumulation below the volcano’s summit.

How do volcanic rift zones relate to flank instability? Evidence from collapsing rifts at Etna

Volcanic rift zones, characterized by repeated dike emplacements, are expected to delimit the upper portion of unstable flanks at basaltic edifices. We use nearly two decades of InSAR observations excluding wintertime acquisitions, to analyze the relationships between rift zones, dike emplacement and flank instability at Etna. The results highlight a general eastward shift of the volcano summit, including the northeast and south rifts. This steady‐state eastward movement (1–2 cm/yr) is interrupted or even reversed during transient dike injections. Detailed analysis of the northeast rift shows that only during phases of dike injection, as in 2002, does the rift transiently becomes the upper border of the unstable flank. The flank's steady‐state eastward movement is inferred to result from the interplay between magmatic activity, asymmetric topographic unbuttressing, and east‐dipping detachment geometry at its base. This study documents the first evidence of steady‐state volcano rift instability interrupted by transient dike injection at basaltic edifices.

Automatic Recognition of Landslides Based on Neural Network Analysis of Seismic Signals: An Application to the Monitoring of Stromboli Volcano (Southern Italy)

In the last 9 years, the amount and the quality of geophysical and volcanological observations of Stromboli's' activity have undergone a marked increase. This new information highlighted that the landslides on the Sciara del Fuoco flank are tightly linked to the volcanic activity. Actually, at the beginning of the December 28, 2002, effusive eruption, the seismic monitoring network was less dense than now, and therefore it is not known if there was an increase in the landslide rate before the eruption. Despite this, it is known that a big landslide occurred 2 days after the beginning of the eruption which caused a tsunami (December 30, 2002). More recently, the effusive eruption in February 2007 was preceded by an increase in landslides on the Sciara del Fuoco flank, which were recorded by the seismological monitoring system that had been improved after the 2002–2003 crisis. These episodes led us to believe that monitoring the Sciara del Fuoco flank instability is an important topic, and that landslides might be significant short-term precursors of effusive eruptions at the Stromboli volcano. To automatically detect landslide signals, we have developed a specialized neural algorithm. This can distinguish between landslides and the other types of seismic signals usually recorded at the Stromboli volcano (i.e., explosion quakes and volcanic tremor). The discrimination results show an average performance of 98.67 %. According to the experience of the crisis of 2007, to identify changes that can be considered as precursors of effusive eruptions, we set up an automatic decision-making method based on the neural network responses. This method can operate on a continuous data stream. It calculates a landslide percentage index (LPI) that depends on the number of records that are classified by the net as landslides over a given time interval. We tested the method on February 27, 2007, including the beginning of the effusive phase. The index showed an increase as early as at 09:00 UTC on that day and reached its maximum value (100 %) at 12:00, about 40 min before the onset of the eruption. After the beginning of the effusive phase, the index remains high due to the blocks that roll down along the slope from the front of the lava flow. On the basis of these tests, we propose a decision-making method that is able to recognize a trend in the LPI similar to that of 2007 eruption, allowing the identification of precursors of effusive phases at the Stromboli volcano.

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

To understand the relationship between volcano deformations, magma overpressure and flank instability, the results of analog experiments of slope instability and ground deformations, recorded by the GBInSAR system on the western flank of the Stromboli volcano during the period 2009–2011 have been analyzed. Analog experiments that consider both the external (accumulation on the slope) and the endogenous (intrusion-related bulging) trigger phenomena. The effect of accumulation on the sub-aqueous slope on the initiation of sub-marine mass movements has been analyzed. By combining the monitoring data with analog modeling, the observed deformations from the combined action of overpressure in the volcanic system and gravity have been related. The results suggest that the superficial movements observed by the GBInSAR system represent the response of the Stromboli volcano to overpressure changes in the conduit. The movements observed on the Sciara del Fuoco were slope instability phenomena where the gravitational component produced a constant creep, while changes in the magma overpressure explain why certain periods are characterized by accelerations, which induce instability on the external flanks of the crater area and in the Sciara del Fuoco and eventually promote failure of the volcanic slopes.

Triggering mechanisms of static stress on Mount Etna volcano. An application of the boundary element method

In the last 30years, numerous eruptions and associated deformation episodes have occurred at Mt. Etna volcano. Datasets recorded by continuous monitoring of these episodes provide a unique opportunity to study the relationships between volcanism, flank instability and faulting activity. We have investigated the stress triggering mechanism between magmatic reservoir inflation, intrusive episodes and flank dynamics. Using three-dimensional numerical Boundary Elements Models we simulated volcano-tectonic events and calculated Coulomb stress changes. Using this modeling approach, we analyzed four realistic scenarios that are representative of recent kinematics occurring at Mt. Etna. The main results obtained highlight how (1) the inflation of a deep spherical magma source transfers elastic stress to a sliding plane and faults, (2) the opening of the NE Rift and S Rift (to a less efficient extent) favors movements of the instable sector and may encourage seismicity on the eastern flank faults, and (3) flank instability may trigger the uprising of magma. Defining the effects of the elastic stress transfer and relationships among the main forces acting on volcano, may help to forecast possible eruption scenarios during future episodes of unrest at Mount Etna and provide an important tool for decision makers during volcanic emergencies involving the highly populated areas of the volcano.

Understanding Etna flank instability through numerical models

As many active volcanoes, Mount Etna shows clear evidence of flank instability, and different mechanisms were suggested to explain this flank dynamics, based on the recorded deformation pattern and character. Shallow 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 40×60km, 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.

An overview of experimental models to understand a complex volcanic instability: Application to Mount Etna, Italy

Volcanic edifices are often unable to support their own load, triggering the instability of their flanks. Many analogue models have been aimed, especially in the last decade, at understanding the processes leading to volcano flank instability; general behaviors were defined and the experimental results were compared to nature. However, available data at well-studied unstable volcanoes may allow a deeper understanding of the specific processes leading to instability, providing insights also at the local scale. Etna (Italy) constitutes a suitable example for such a possibility, because of its well-monitored flank instability, for which different triggering factors have been proposed in the last two decades. Among these factors, recent InSAR data highlight the role played by magmatic intrusions and a weak basement, under a differential unbuttressing at the volcano base. This study considers original and recently published experimental data to test these factors possibly responsible for flank instability, with the final aim to better understand and summarize the conditions leading to flank instability at Etna. In particular, we simulate the following processes: a) the long-term activity of a lithospheric boundary, as the Malta Escarpment, separating the Ionian oceanic lithosphere from the continental Sicilian lithosphere, below the most unstable east flank of the volcano; b) spreading due to a weak basement, with different boundary conditions; c) the pressurization of a magmatic reservoir, as that active during the 1994–2001 inflation period; d) dike emplacement, as observed during the major 2001 and 2002–2003 eruptions. The experimental results suggest that: 1) the long-term activity of a lithospheric tectonic boundary may create a topographic slope which provides a differential buttressing at the volcano base, a preparing factor to drive longer-term (>105years) instability on the east flank of the volcano; 2) volcano spreading (<104years) has limited effect on flank instability at Etna; 3) magmatic intrusions (<101years), both in the form of Mogi-like sources or dikes, provide the most important conditions to trigger flank instability on the shorter-term.

Interaction of volcano-tectonic fault with magma storage, intrusion and flank instability: A thirty years study at Mt. Etna volcano

We investigated the relationship between the occurrence of earthquakes along the main volcano-tectonic structures and periods of volcanic unrest at Mt. Etna. We focused our study on the Pernicana Fault System (PFS), one of the most outstanding tectonic structures delineating the northern border of the sliding eastern flank of Etna volcano. During recent decades several flank eruptions have occurred at Mt. Etna and sometimes PFS released seismicity before the eruptive events, while in other cases there have been earthquakes that did not precede any eruption. To highlight a possible relation between PFS ruptures and volcanic unrest, we took into account the most energetic earthquakes (M≥3.5) occurring in the last three decades (1980–2010), and considered the volcano deformation sources previously inferred by inverting geodetic data recorded during the several flank eruptions in this time interval. The estimates of stress redistribution on the PFS due to different volcano sources, such as the magma storage, the dike intrusions and the sliding eastern flank, were studied by implementing 3D numerical models that also consider the presence of topography and medium heterogeneity. Our results show that the pressurization of an intermediate storage and the traction exerted by the eastern flank sliding contribute to the seismicity along the PFS even without preceding an immediate eruption. Instead, the seismicity along the PFS related to the intrusions inside the northern sector of the volcano would represent a potential early-warning for an impending eruption at Mt. Etna.

Flank instability structure of Mt. Etna inferred by a magnetotelluric survey

This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt. Etna. We take thirty soundings along two profiles oriented in the N‐S and NW‐SE directions, and from these data recover two 2D resistivity models of the subsurface. Both models reveal three major layers in a resistive‐conductive‐resistive sequence, the deepest extending to 14 km bsl. The shallow layer corresponds to the volcanic cover, and the intermediate conductive layer corresponds to underlying sediments segmented by faults. These two electrical units are cut by ∼ E‐W‐striking faults. The third layer (basement) is interpreted as mainly pertinent to the Apennine‐Maghrebian Chain associated with ∼ SW‐NE‐striking regional faults. The detailed shapes of the resistivity profiles clearly show that the NE Rift is shallow‐rooted (∼0–1 km bsl), thus presumably fed by lateral dikes from the central volcano conduit. The NW‐SE profile suggests by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Toward the SE, the resistive basement dramatically dips (from ∼3 km to ∼10 km bsl), in correspondence with the Timpe Fault System. Several high‐conductivity zones close to the main faults suggest the presence of hydrothermal activity and fluid circulation that could enhance flank instability. Our results provide new findings about the geometry of the unstable Etna flank and its relation to faults and subsurface structures.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores Archipelago-North Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3°N, 160.7°E, K= 33 and A95= 3.4°) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 ± 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNW-SSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNW-SSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNW-ESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Analysis of satellite and in situ ground deformation data integrated by the SISTEM approach: The April 3, 2010 earthquake along the Pernicana fault (Mt. Etna - Italy) case study

Etna is known worldwide as one of the most studied and monitored active volcanoes. Flank instability along the eastern and southern portions of Mt. Etna has been observed and measured thanks to geodetic networks and InSAR data analysis. The spreading area is bordered to the north by the east–west Pernicana Fault System (PFS) whose dynamics is often linked with the eruptive activity, as recently observed during the 2002–2003 eruption. A seismic sequence occurred from April 2–3, 2010, along the PFS with very shallow (a few hundred meters) mainshocks of magnitude 4.3 and 3.6. Explosions and ash emissions at the summit craters followed this swarm culminating a few days later (April 7–8). Despite their small magnitude, the earthquakes caused damage and significant surface fracturing along the PFS. In order to investigate and measure the deformations in the near field of the earthquakes, the SISTEM integration approach has been exploited. The SISTEM enabled integrating geodetic in situ ground deformation measurements (GPS and leveling) with satellite interferometric measurements (ENVISAT and ALOS), in order to obtain high resolution 3D displacement maps, allowing to overcome the limitations of each technique and take advantage of the particular features of each of them. The integrated ground deformation field evidenced that the medium behave elastically. We inverted the SISTEM results using an optimization algorithm based on the Genetic Algorithm (GA) in order to model the kinematics of the PFS associated to seismic swarm; the results are in good agreement with the field evidence and improved the knowledge on the kinematics of the PFS and Mt. Etna's unstable flank.

Long-term record of CO2degassing along Mt. Etna's flanks and its relationship with magma dynamics and eastern flank instability

A record of soil CO2 flux data on the lower flanks of Mt. Etna, measured from 1989 to 2008, is discussed in the framework of multidisciplinary observations, including seismicity, ground deformations, and flank instability. A huge increase in soil CO2 fluxes appears to be related to the dynamics of magma ascent in the upper portions of the volcano (0–3 km) and the intrusion of dykes along the southern rift, as mainly observed before the 1991–93 eruption. Even after the 1991–93 eruption, the recharge of the shallow/central reservoir was accompanied by a long-term increase in soil CO2 degassing mainly in the southwestern area. The 2001 eruption marked dramatic changes in the areal distribution of seismicity, the deformation pattern, and the soil CO2 degassing. Indeed, while the soil CO2 degassing showed background values in the southwestern area, it progressively increased in the eastern sector and along the Pernicana fault. This has been related to the marked sliding of the eastern flank since the 2002–03 eruption and the associated seismicity. This study provides evidence that the extent of soil CO2 degassing on the lower flanks of Mt. Etna is controlled by (1) the volume of involved magma, (2) the intrusion of dykes in the upper parts of the volcano, and (3) fault movement and seismicity. This implies that different degassing structures must be monitored simultaneously when attempting to understand the behavior of the volcano as a whole.

Geology and tectonics of the southwestern boundary of the unstable sector of Mt. Etna (Italy)

The Etna volcano is known to be unstable and gradually moving downslope toward east and south. The study of the local geology and structure of these unstable flanks and of their boundaries is important to understand the evolution of the flank instability and the stress field driving the spreading of the edifice. In this paper we present the geological evolution of the lower south-western sector of Mount Etna base on a detailed field survey at 1:10,000 scale. This area located at the boundary of the unstable south-eastern flank of the volcano is affected by the active Ragalna Fault and by distinctive eruptive fissures (Monte Calvario formation, 18–15 ka). The Monte Calvario formation is made of viscous lavas often associated to breccia and autoclastic lava facies fed by a set of eruptive fissures NE–SW oriented. Their emplacement occurred immediately before the Ellitico caldera collapse, marking a previously poorly constrained shift in the feeding system of the volcano. The orientation of these eruptive fissures and lineaments, and the geometry of the sedimentary substratum modeled from subsurface data, indicate that the area was dominated by a local stress field consistent with the lateral spreading of Mount Etna since at least from the final Ellittico phase (about 18 ka) with a nearly constant mean displacement rate. These data give new insights into the geology and structure of the flank instability at Etna. The main geologic hazard of the studied area is not related to the opening of a new eruptive fissures, but it is posed by the presence into Il Calvario quarry of fluoro-edenite, an asbestiform mineral that caused an high mortality rate for respiratory system cancer in Biancavilla town. In fact the Monte Calvario formation groups other eruptive fissures with similar volcanological and geochemical characteristics, where hydrothermal alteration occurred with the likely formation of health-threatening minerals.