The last 1000 years of eruptive activity at the Fossa Cone (Island of Vulcano, Southern Italy)

The last 1000 years of eruptive activity of the Fossa cone have been characterised by a wide spectrum of eruptive styles, from mafic ash eruptions (violent Strombolian), to sub-Plianian, toveffussive, to steam-blast explosions and eruptions related to the ascent of viscous plug, asvVulcanian explosions. Tephra and lava sequences on the cone-flanks have covered almost completely the older products, forming an “onion-like” structures of this small composite cone. Given the nature of the eruptions of the Fossa cone, being mainly of small- to medium intensity, the importance of the ring plain, as a record of the chronostratigraphy of volcanic and sedimentary activity is crucial to a correct reconstruction of the evolution of the Fossa cone. In general, the first activities of the investigated period (XIII sec.), namely Palizzi eruption (Chapter 3), were characterised by frequent eruptions of small intensity (mafic ash eruptions, sensu Ono 1995) as reported by historical accounts (De Fiore 1922). These eruptions have been spaced by rest period, during which erosion occurred, producing a re-distribution of the volcanic material from the cone to the neighbouring areas. The mafic ash eruptions were punctuated by two more violent, sub-Plinian, eruptions affecting a wide area of the Island of Vulcano with a large volume (3.75x106-6.40x106 m3 and 3.65x106-3.67x106 m3 for the first and the second, respectively) of tephra fall deposits. After the second sub-Plinian eruption, a phase of effusive eruptions occurred, signing the end of this period characterised by magmatic activity only. The style of the eruptions drastically changed and a large, probably long-lasting, steam-blast eruption, called Commenda eruption (Chapter 4), occurred, ejecting ballistic blocks in a large range (at least 2.2 km from the summit crater area) and producing wide-spread PDCs (>4x105 m3) and non-juvenile ash emission (>1x107 m3 m3). The climactic phase of the eruption consumed a large quantity of energy (3,75X1012J), forming the current crater and ejecting lithic material with a overpressure of ~2 MPa. This eruption took place simultaneously with the Mt. Pilato-Roche Rosse rhyolitic eruption in the Island of Lipari, triggered by a mafic injection. The Commenda eruption was driven by the sudden uprising of a large amount of magmatic gas (2,8x106 m3), produced by a degassing shoshonitic melt (~10-1 km3). After a rest period (at least one century, based on the analysis of the historical chronicles), the activity renewed (Chapter 5) in the AD1444 with a second, high-energetic (1.25x1013 kJ) steamblast eruption (Forgia 1), that occurred eccentrically with respect to the summit crater area of the Fossa cone, in its northern flank and affecting an area currently occupied by the Vulcano Porto town. This eruption “opened” a new stage of the activity of the Fossa cone, characterised by the ascent of viscous plugs of magma that powered at least 8 discrete eruption lasted from days to years. These eruptions were mainly Vulcanian in styles, including the first described Vulcanian eruption (AD1888-1890), but frequent steam-blast explosions occurred, generally before the onset of the main magmatic phase. One effusive phase characterised the long-lasting, complex eruption that occurred on AD1731-39. The erupted volume ranged between 106 and 107 m3, affecting an wide area that comprised at least the northern Sicilian coast and probably reached the Mt. Etna (De Fiore et al 1922), more or less 70 km far from the island. Comparing the pre- and post-AD1888-90 topography of the summit crater area, a minimum value (16.5x106 m3) of deposited material was produced. This volume approximately corresponds the 88% of the associated tephra volume (18.5x106 m3 calculated by the methods of Bonadonna and Houghton 2005), confirming the evidence of high-deposited volume in the in the summit crater area during these stage of eruptions. During this eruptive crise, the summit crater area of the volcano growth at an average rate of 0,34 m3s-1 in a period of ~20 months. This value is something less but in the same order of magnitude of the rate of cone grew evaluated for the AD1986 eruption at Izu-Oshima (Sumner 1998) and the AD1954 eruption at Ngaurouhoe (Hobden 2002) volcanoes, while is one order of magnitude greater than the value calculated for the AD1783 eruption of the Asama volcano (Yasui and Koiaguchi 2004). The data on emitted tephra volumes during the investigated period suggested that the Fossa cone experienced eruptions ranging bewteen 2 and 3 in the VEI classification (Volcanic Explosivity Index, Newhall and Self 1982), without a clear evidence of waning. These data are useful for future “short-term” hazard evaluations for the Island of Vulcano, that need to consider an eruption of VEI 3 as the most likely scenario (Chapter 6). Diluted PDCs and ballistic showers have been considered so far as the most recurrent hazardous events of La Fossa volcano (e.g. Dellino et al. 2010). Nonetheless, this work suggests that widespread and long-lasting fallout events as well as lahars represent a frequent phenomenon and, therefore, fundamental processes to be considered for a comprehensive hazard assessment of the island of Vulcano

A statistical-based approach for determining the intensity of unrest phases at Stromboli volcano (Southern Italy) using one-step-ahead forecasts of displacement time series

The evaluation of the intensity of unrest phases at active volcanoes is a crucial topic in volcano hazard studies. This is particularly troublesome in the case of persistently active volcanoes like Stromboli (Southern Italy), where intense eruptive summit activity (overflows, strong spattering, powerful explosions) has in some cases anticipated a flank eruption. In this context, a new approach for the analysis of displacement data is introduced. Daily displacements of the Stromboli crater terrace measured between January 1, 2010, and August 7, 2014, by a ground-based interferometric synthetic aperture radar system were compared, in retrospect, to displacement predictions provided by an autoregressive integrated moving average-based model. The methodology consisted in assessing when the actual displacements exceeded a fixed probability threshold for the forecasts (*95 %). Two sets of data were consequently produced: (1) series of residuals between actual displacements and model threshold (‘‘anomalies’’) and (2) series of normalized residuals between actual displacements and model threshold (‘‘normalized anomalies’’). This permitted to statistically identify and quantify the anomalous deformation at the crater terrace over the reference time interval of the analysis. Anomalies started to occur before each period of intense volcanic activity, highlighting the possibility to discern between background activity and unrest. Moreover, results indicated that the inflation of the crater terrace during the preparatory phase of the 2014 flank eruption was characterized by the greatest amount of anomalous deformation

Semi-automatic delimitation of volcanic edifice boundaries: Validation and application to the cinder cones of the Tancitaro–Nueva Italia region (Michoacán–Guanajuato Volcanic Field, Mexico)

The shape and size of monogenetic volcanoes are the result of complex evolutions involving the interaction of eruptive activity, structural setting and degradational processes. Morphological studies of cinder cones aim to evaluate volcanic hazard on the Earth and to decipher the origins of various structures on extraterrestrial planets. Efforts have been dedicated so far to the characterization of the cinder cone morphology in a systematic and comparable manner. However, manual delimitation is time-consuming and influenced by the user subjectivity but, on the other hand, automatic boundary delimitation of volcanic terrains can be affected by irregular topography. In this work, the semi-automatic delimitation of volcanic edifice boundaries proposed by Grosse et al. (2009) for stratovolcanoes was tested for the first time over monogenetic cinder cones. The method, based on the integration of the DEM-derived slope and curvature maps, is applied here to the Tancitaro–Nueva Italia region of the Michoacán–Guanajuato Volcanic Field (Mexico), where 309 Plio-Quaternary cinder cones are located. The semiautomatic extraction allowed identification of 137 of the 309 cinder cones of the Tancitaro–Nueva Italia region, recognized by means of the manual extraction. This value corresponds to the 44.3% of the total number of cinder cones. Analysis on vent alignments allowed us to identify NE–SW vent alignments and cone elongations, consistent with a NE–SW σmax and a NW–SE σmin. Constructing a vent intensity map, based on computing the number of vents within a radius r centred on each vent of the data set and choosing r = 5 km, four vent intensity maxima were derived: one is positioned in the NW with respect to the Volcano Tancitaro, one in the NE, one to the S and another vent cluster located at the SE boundary of the studied area. The spacing of centroid of each cluster (24 km) can be related to the thickness of the crust (9–10 km) overlying the magma reservoir

Decrypting geophysical signals at Stromboli Volcano (Italy): Integration of seismic and Ground-Based InSAR displacement data

We present the integration of seismic and Ground‐Based Interferometric Synthetic Aperture Radar system (GBInSAR) displacement data at Stromboli Volcano. Ground deformation in the area of summit vents is positively correlated with both seismic tremor amplitude and cumulative amplitudes of very long period (VLP) signals associated with Strombolian explosions. Changes in VLP amplitudes precede by a few days the variations in ground deformation and seismic tremor. We propose a model where the arrival of fresh, gas‐rich magma from depth enhances gas slug formation, promoting convection and gas transfer throughout the conduit system. At the shallowest portion of the conduit, an increase in volatile content causes a density decrease, expansion of the magmatic column and augmented degassing activity, which respectively induce inflation of the conduit, and increased tremor amplitudes. The temporal delay between increase of VLP and tremor amplitudes/conduit inflation can be interpreted in terms of the different timescales characterizing bulk gas transfer versus slug formation and ascent

Multi-Temporal Evaluation of Landslide Movements and Impacts on Buildings in San Fratello (Italy) By Means of C-Band and X-Band PSI Data

This work provides a multi-temporal and spatial investigation of landslide effects in the San Fratello area (Messina province within the Sicily region, Italy), by means of C-band and X-band Persistent Scatterer Interferometry (PSI) data, integrated with in situ field checks and a crack pattern survey. The Sicily region is extensively affected by hydrogeological hazards since several landslides regularly involved local areas across time. In particular, intense and catastrophic landslide phenomena have recently occurred in the San Fratello area; the last event took place in February 2010, causing large economic damage. Thus, the need for an accurate ground motions and impacts mapping and monitoring turns out to be significantly effective, in order to better identify active unstable areas and to help proper risk-mitigation measures planning. The combined use of historical and recent C-band satellites and current X-band Synthetic Aperture Radar sensors of a new generation permits spatially and temporally detection of landslide-induced motions on a local scale and to properly provide a complete multi-temporal evaluation of their effects on the area of interest. PSI ground motion rates are cross-compared with local failures and damage of involved buildings, recently recognized by in situ observations. As a result, the analysis of landslide-induced movements over almost 20 years and the validation of radar data with manufactured crack patterns, permits one to finally achieve a complete and reliable assessment in the San Fratello test site

Updated landslide inventory of the area between the Furiano and Rosmarino creeks (Sicily, Italy)

A 1:10,000 scale landslide inventory map has been prepared for the area between the Furiano and Rosmarino creeks, in the Nebrodi Mountains (north-eastern Sicily, Italy), a territory highly prone to slope failures, due to the local geological and geomorphological settings and intense rainfall. The landslide inventory database included within the Hydrogeological Setting Plan of the Sicily Region has been used as a starting point for this work. The updated inventory map has been compiled through a combination of conventional approaches (i.e. aerial photo-interpretation and field surveys) and new remote sensing techniques (ground deformation measurements obtained by interferometric analysis of satellite Synthetic Aperture Radar images). The new landslide inventory consists of 566 events, classified according to their typology and state of activity

Shifts in the eruptive styles at Stromboli in 2010-2014 revealed by ground-based InSAR data

Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR) is an efficient technique for capturing short, subtle episodes of conduit pressurization in open vent volcanoes like Stromboli (Italy), because it can detect very shallow magma storage, which is difficult to identify using other methods. This technique allows the user to choose the optimal radar location for measuring the most significant deformation signal, provides an exceptional geometrical resolution, and allows for continuous monitoring of the deformation. Here, we present and model ground displacements collected at Stromboli by GBInSAR from January 2010 to August 2014. During this period, the volcano experienced several episodes of intense volcanic activity, culminated in the effusive flank eruption of August 2014. Modelling of the deformation allowed us to estimate a source depth of 482 ± 46 m a.s.l. The cumulative volume change was 4.7 ± 2.6 × 10(5) m(3). The strain energy of the source was evaluated 3-5 times higher than the surface energy needed to open the 6-7 August eruptive fissure. The analysis proposed here can help forecast shifts in the eruptive style and especially the onset of flank eruptions at Stromboli and at similar volcanic systems (e.g. Etna, Piton de La Fournaise, Kilauea)

Guidelines on the use of inverse velocity method as a tool for setting alarm thresholds and forecasting landslides and structure collapses

Predicting the time of failure is a topic of major concern in the field of geological risk management. Several approaches, based on the analysis of displacement monitoring data, have been proposed in recent years to deal with the issue. Among these, the inverse velocity method surely demonstrated its effectiveness in anticipating the time of collapse of rock slopes displaying accelerating trends of deformation rate. However, inferring suitable linear trend lines and deducing reliable failure predictions from inverse velocity plots are processes that may be hampered by the noise present in the measurements; data smoothing is therefore a very important phase of inverse velocity analyses. In this study, different filters are tested on velocity time series from four case studies of geomechanical failure in order to improve, in retrospect, the reliability of failure predictions: Specifically, three major landslides and the collapse of an historical city wall in Italy have been examined. The effects of noise on the interpretation of inverse velocity graphs are also assessed. General guidelines to conveniently perform data smoothing, in relation to the specific characteristics of the acceleration phase, are deduced. Finally, with the aim of improving the practical use of the method and supporting the definition of emergency response plans, some standard procedures to automatically setup failure alarm levels are proposed. The thresholds which separate the alarm levels would be established without needing a long period of neither reference historical data nor calibration on past failure events