United States

Shelves surrounding volcanic islands represent only a small portion of the entire submarine edifice that can range a few thousand meters below sea level. Despite their small area, the erosional, depositional, tectonic and volcanic processes affecting the subaerial island leave a significant imprint on the shelf morphology when compared to the slope or the submarine base of the islands (Quartau et al., 2014). Therefore, shelf morphology can be used to improve our knowledge about the island evolution, especially the more recent events. In this study, we use multibeam bathymetry, side scan sonar and seismic reflection data of the southern shelf of Porto Santo Island in Madeira Archipelago to map its seabed morphology and seismic stratigraphy from the nearshore to the shelf edge (~100m b.s.l.), aiming to better understand the evolution of mass-movements affecting the area. In general, the shelf consists of an erosive rocky surface mostly covered by sediments, and locally outcropping on the seafloor. The sedimentary cover on the shelf is highly variable both in thickness and internal architecture, mainly due to different onshore sediment supply and available accommodation space. Two main seismic units have been recognized: the lower one (U1) is interpreted as a transgressive deposit, formed during sea level rise after the Last Glacial Maximum (LGM); the upper one (U2) is interpreted as present-day highstand deposits. Based on the morphological and seismo-stratigraphic differences the study area has been divided in three sectors (Western, Central and Eastern). On the Western and Eastern sectors two sets of submerged erosional terraces have been mapped at different depths (35-45 m, 50-75 m b.s.l.). The overall arcuate shape of the southern shelf edge of Porto Santo Island strongly matches the coastline shape, suggesting the occurrence of an old large-scale landslide event (LS1),  prior to the shelf formation (Quartau et al., 2018a). Further incisions on the shelf edge of the Central sector, mapped at and upslope the shelf edge, imply more recent mass-movement processes, LS2 at least older than seismic unit U1 and LS3 formed on the late Holocene, indicating a retrogressive evolution of the instability process. The absence of erosional terraces in the Central sector supports this hypothesis, being probably removed by the LS2 event. The terraces depths and their destruction by the LS2 event suggests that these were formed/modified at least by the MIS 5a-5d stillstands or even by older ones. The estimated volume of the LS2 event (0.6 km3) suggests that it might have caused a hazardous tsunami if it failed as a single event. The recent LS3 events, due to their small magnitude were probably not hazardous. In summary, three different mass-movements were defined according to their dimension and position on the shelf: from LS1 to LS3, the scars are increasingly smaller and further upslope. These observations provided a more comprehensive understanding of the recent evolution of Porto Santo insular shelf in terms of mass-movement processes.

SGI 2021

Small-scale instability processes affecting volcanic island shelves: the case study of the southern shelf of Porto Santo Island (Madeira Archipelago)

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L'Istituto Nazionale di Oceanografia e Geofisica Sperimentale - OGS e il Dipartimento di Matematica e Geoscienze dell'Università di Trieste sono orgogliosi di ospitare il 90° Congresso della Società Geologica Italiana a Trieste dal 14 al 16 settembre 2021. La modalità del congresso è interamente online.

Con il motto "Geologia senza frontiere" questo evento mira a promuovere le Scienze della Terra come disciplina scientifica proiettata nel futuro, oltre i confini geografici, politici e disciplinari, per comprendere meglio l'interno del Pianeta Terra e la struttura del sottosuolo, difendere la società da pericolosi eventi geologici, comprendere meglio la variabilità climatica, pianificare l'utilizzo delle georisorse in modo etico e nel rispetto dell'ambiente.


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The National Institute of Oceanography and Experimental Geophysics - OGS and the Department of Mathematics and Geosciences of the University of Trieste are proud to host the 90th Congress of the Italian Geological Society in Trieste on 14-16 September 2021.  The Congress will be held entirely online.

With the motto "Geology Without Borders" this event aims to promote Earth Sciences as a scientific discipline projected into the future, beyond geographical, political and disciplinary boundaries, to better understand the interior of Planet Earth and the structure of the subsurface, defend society from dangerous geological events, better understand climatic variability, plan the use of geo-resources in an ethical way and respecting the environment.


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technical paper

The topic of my work is a seismic tomography which has as object the investigation of Southern Tyrrhenian. This tomography was obtained by means of inversion of teleseismic data and an iterative computation of them to obtain the so-called residual that is the difference between the observed travel times and these theoretical travel times. The final tridimensional velocity model corresponds to that having the minimum residual. The entire process of operation of a seismic tomography is very relevant in order to investigate subduction zones. This is the case of the Southern Tyrrhenian oceanic back-arc basin. The subducting lithosphere has been mostly consumed along the Tyrrhenian-Apennine system has been consumed with the exception of the Calabrian arc sector. The new seismic tomography, which derived from the inversion of travel times of teleseismic ray paths, travelling in the upper mantle at high depths. The adopted database consists of 1929 teleseisms  recorded in period 1990-2012 by 122 southern Italian seismic station directly connected to ISC (International Seismological Centre). The software FMTT was employed for the inversion of these arrival times. I have made 10 horizontal sections of final model from 50 km of depth to 500 km of depth, with an interval of 50 km of depth from each other. I have made 8 vertical sections and 3 transversal sections, choosing as traces the same illustrated by Montuori (2007), for her teleseismic tomography of Southern Tyrrhenian. This work, together with contributions of Chiarabba (2008) and Calò (2012). Summarising, the horizontal sections show an evolution of the high velocity body that represents the Ionian slab. At depth of 250 km, the tomography evidences a sort of “transition” due to the absence of the Southern Tyrrhenian HVA and the occurrence of a low velocity region with maximum of -0.5 km/s scattered between the Aeolian Islands and Calabria. This low velocity region practically splits the Tyrrhenian slab into two parts, in Neapolitan region and in the southern Calabria-northern Sicily region. The presence of this “window slab” could be interpreted as a tear in which unperturbed mantle insert itself.

Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

In the past, volcanic hazard assessment was typically based on one or a few, subjectively chosen representative eruptive scenarios, using a specific combination for the values of the parameters related to the eruptive size and intensity to represent a particular eruptive size class. Recently, in modelling tephra load hazard, the natural variability in terms of eruptive size and intensity, vent position and meteorological conditions, has been taken into account (e.g., Sandri et al., 2016; Selva et al., 2018). Here, we adopted a probabilistic methodology for a novel long-term volcanic hazard analysis which accounts for intrinsic eruptive variability, to quantify tephra load hazard in Southern Italy, from Somma-Vesuvius and Campi Flegrei, on a regional target domain (ca. 600km x 600km, covering Campania, Basilicata, Apulia, Calabria, Sicily) at a 3-km resolution. Numerical simulations were performed by FALL3D model (Folch et al., 2020) in which the values for the eruption parameters are sampled within plausible ranges for given vents and size classes (Small, Medium, Large). Wind profiles have been collected from the ECMWF ERA5 from 1991 to 2020, with a spatial resolution of 0.03° x 0.03°. The results of this tephra fallout hazard scheme provided absolute hazard curves showing how the tephra load exceedance probability varies on the target domain, in 50 years. The results achieved are perfectly similar, in format. To those commonly achieved with the classic approach for probabilistic seismic hazard assessment, and perfectly compatible with fragility analysis in order to compute and rank the associated tephra load risk with the seismic and other natural risks whose hazards are calculated in the same way. Moreover, the proposed method can be extended to larger temporal intervals to quantify the volcanic hazard due to less frequent events that are relevant for the risk quantification on critical infrastructures.

Assessing long-term tephra fallout hazard from Neapolitan volcanoes on Southern Italy  

Volcanic ash is the result of extensive magma fragmentation during eruptions. It depends upon a combination of magma properties (rheology, vesicularity, permeability, gas overpressure) and the possible involvement of external fluids during magma ascent. However, fragmentation processes during the ash-dominated, low-to-mid intensity eruptions are still a matter of debate. Combination of morpho-textural data of the erupted ash with geophysical data on the activity can inform about the energy of the activity as well as on the mechanisms of ash production and dispersal. The phase of heavy ash emission occurred during March 2016 at Copahue volcano (Argentina) generated a very low infrasonic amplitude, corresponding to low exit velocity and low magma overpressure. The apparent unbalance between measured geophysical parameters and the intensity of the activity raises a number of questions concerning the links among acoustic pressure, gas overpressure and efficiency of magma fragmentation. Generally, magma-water interaction is supposed to be the main process of magma fragmentation at Copahue. Conversely, our data on ash texture indicate that a process of magma volatile exsolution under low overpressures actively controlled the fragmentation during the investigated eruptive stage. Ash componentry also revealed that a large fraction of deposit consisted of recycled material. Therefore, comparing these results with geophysical information, we interpreted the activity as influenced by a process of energy buffering, operated by the passage of the erupting mixture through a thick layer of granular, ash-bearing crater infilling produced by the ash recycling processes. The importance of this abundant hot, vent-hosted material for the convective stability of the eruptive plume and for the ash dispersal was highlighted, with direct consequences for the assessment of the related hazard and management of eruptive crises,

Mechanisms of ash production and recycling during low-energy, mid-intensity eruptions at Copahue volcano (Argentina)

New generations of Earth Observation (EO) satellites, such as the Copernicus Sentinel missions, are now providing freely available imagery with unprecedented spatial and temporal resolutions on a global scale. Importantly, the diversity of sensors on-board the spacecrafts provide the potential for a comprehensive monitoring of volcanic unrest. In particular, the synergetic use of radar (Sentinel-1 SAR), short-wave infrared (Sentinel-2 MSI) and ultraviolet (Sentinel-5P TROPOMI) payloads, are particularly promising for surveillance of surface deformation and topographic changes, emplacement of volcanic deposits, detection of thermal anomalies, and emission of volcanic SO2. MOUNTS (Monitoring Unrest from Space, www.mounts-project.com) is a monitoring system which intends to demonstrate the potential of such an integrated interdisciplinary approach, where systematic processing and visualization of EO products can support both scientific and operational communities for volcanic risk assessment and management. This presentation will present a number of recent volcanic crisis captured by MOUNTS, and thereby illustrate how a variety of hazards can be detected and monitored from space.

Volcanic hazard monitoring from space using MOUNTS

The European Marine Observation and Data Network (EMODnet) is a network of organizations supported by the EU’s integrated maritime policy. These organizations work together to observe the sea, process the data according to international standards and make that information freely available as interoperable data layers and data products (accessible from www.emodnet.eu). It is subdivided into Lots concerning Bathymetry, Geology, Biology, Chemistry, Physics, Seabed Habitats and Human activities. The Geology Lot (www.emodnet-geology.eu) is carried out by a consortium of Geological Surveys and includes data on seafloor sediments grainsize, sedimentation rates, Quaternary geology, pre‐Quaternary geology and stratigraphy, coastal behaviour, geological events, mineral resources, submerged landscapes. Datasets available from the “Geological events and probabilities” layers, coordinated by the Geological Survey of Italy – ISPRA, include submarine landslides, earthquakes, volcanic centers, tsunamis, fluid emissions and Quaternary faults. Data gathered by Partners have been conveyed into single Europe-wide shapefiles, complemented by detailed attribute tables. Mud-volcanoes and fluid emissions of non-volcanic origin have been initially considered separately from volcanic emission, in order to prevent overlaps and confusion, because terms adopted to describe morphological types of mud-volcanoes, especially in the Black Sea, are the same used for magmatic structures. However, during later phases of the project, with increasing resolution and scale of representation, all types of fluid emissions have been gathered together in the same layer. The major component of volcanic fluid emissions is CO2, while the major component of non-volcanic fluid emissions is methane. These latter have been characterized by type, height, region, composition, morphological and activity types, age of activity. Part of the Italian results has been collected into monographs published by the Geological Survey of Italy, one dedicated to a systematic inventory of submerged volcanic structures (D’Angelo et al., 2019) and one collating contributions on occurrences of fluid emissions (D’Angelo et al., 2020) which is presented in the workshop on september 13th. In the volume dedicated to volcanoes, hydrothermal occurrences are reported wherever information is available. A specific focus is dedicated to mineral deposits associated with fluid emissions in the southern Tyrrhenian Sea and highlights the widespread occurrence of sulfate and sulfide minerals in the surroundings of volcanic structures.

Volcanic and non-volcanic fluid emissions: data from EMODnet Geology

Sediment cores and boreholes, high-resolution sub-bottom and multi-channel seismic reflection profiles allowed to study origin and recurrence of mass-transport deposits (MTDs) emplaced along the eastern slope of the Gela Basin in the Strait of Sicily. 27 stacked MTDs were mapped with volumes between < 1 km3 and 630 km3 and related to the different phases of margin evolution since the Pliocene. Sedimentary prograding wedges, fed from the north, developed from the late Pliocene and extended eastward. In the Pleistocene, progradation extended to the southern basin with oblique shelf-edge clinothems (Gauchery et al., 2021a). From the Pliocene to the Middle Pleistocene Transition (MPT, 0.8 Ma), the sediment wedge increased in thickness by 150 m/Myr, afterwards the margin developed an aggradational motif, with sediment rates of 900 m/Myr (Gauchery et al., 2021a). The MPT represented not only a drastic change in margin outbuilding, but also in the volume of submarine landslides, with large tectonic-related MTDs replaced by smaller contourite- and clinothem-related MTDs due to faster bottom currents and increased sedimentation rates. Contourite deposits formed in the Pliocene but grew significantly during MIS 5 and late MIS 2 (Gauchery et al., 2021a). The presence of erosional surfaces downcutting the contourite deposits favoured the emplacement of the South Gela Basin Slide (SGBS) around 17 kyr cal. BP (Gauchery et al., 2021b). Slope stability analysis indicates that even strong seismic shaking (Mw = 6.5) at the offshore continuation of the Scicli-Ragusa fault line, located < 10 km from the SGBS, was insufficient to mobilized it (Zaniboni et al., 2021). SGBS was promoted by high sedimentation rates (up to 1300 cm kyr-1) during the last phase of LGM and early phase of Heinrich Stadial 1 (HS1), driven by a combination of inner-mid shelf sediment input, lateral advection by strong Levantine bottom-current and an abrupt and brief intensification of the surface water speed. During the post-glacial sea-level rise pulses, salinity changes at the interface between surface and intermediate waters lead to enhanced bottom-current erosion and margin instability along the outer shelf and upper slope.

Submarine landslides in the Strait of Sicily: relation with tectonics and climate forcing, contourite deposits and high sedimentation rates, post-LGM sea-level fluctuations

In the last decade many studies have focussed on small-scale (≤1 km3 ) submarine landslide, since they are more frequent with respect to the larger ones and pose a potential threat for the coastal strip and infrastructures. The potential geohazard of a marine landslide is also related to the timing of the event and therefore is of paramount importance to define, as far as possible, its age. This study deals with the Taranto Landslide, an impressive landslide located 12 km off the coast, in the northeastern sector of the Gulf of Taranto, on the southwest-facing Apulian Continental Slope, with the aim to learn more on its geohazard potential. The dataset used in this study consists of multibeam echo sounder (MBES) data and high resolution Chirp Sonar profiles, dip- and strike-oriented which has served to define the morphometric parameters of the landslide and the thickness of the post slide sediment drape. The failure occurred across a slope gradient ranging between 3° and 4°. The evacuated scar surface extends from 400 m below sea level (mbsl) down to 1000 m deep and is bounded by a well-defined headscarp, up to 75 m high, and by two subparallel lateral scarps 4-6 km away from each other. The total inferred volume of the landslide was derived by the scar dimensions and resulted as being about 0,3 km3 (Meo et al., 2018). The slump exhibits a simple geometry, with a single arcuate headscarp and no evidence of retrogressive failure process - such as multiple scars merging or upslope modifications in the form of stepped profiles of the basal evacuation surface – was observed. Besides, the scar surface depicts a sub-parallel profile to the adjacent undisturbed slope, suggesting a single slope-parallel plane of failure along which the sediment and the cohesive blocks glided towards the base of the slope. Therefore a single failure event was inferred to the Taranto landslide.

The Taranto Landslide (North Ionian Sea): further morphological and stratigraphical observations and timing of the event by indirect stratigraphic evidences

The Research Institute for Geo-Hydrological Protection of the Italian National Research Council (IRPI CNR) has developed in cooperation with Civil Protection of Sardinia Region, a regional early warning system for the possible occurrence of rainfall-induced landslides (SARF Sardinia), which currently compares the national rainfall thresholds with the rainfall measured by national and regional rain gauge networks and with rainfall forecasts provided by COSMO-I5 and BOLAM models. In the framework of the current Collaboration Agreement between the Sardinia Region and IRPI CNR, a catalogue containing meteorological and hydrogeological information related to rainfall-induced landslides in Sardinia has been compiled. The catalogue lists 261 landslides that occurred in the period 2007-2020. The information includes: (i) event identifier; (ii) rainfall data (name and geographic coordinates of the selected rain gauge, duration, rainfall mean intensity, and cumulated rainfall responsible for triggering); (iii) landslide geographic data (region, province, municipality, location affected by the event, geographic coordinates of the landslide, spatial accuracy of the landslide); (iv) landslide type; (v) landslide temporal data (year, month, day, time, temporal accuracy of the landslide); and (vi) source of data/information. Rainfall series used to reconstruct the landslide triggering conditions include measurements of 186 rain gauges from a Regional Agency network (SASI), 186 additional stations provided by the Environmental Protection Agency of the Region of Sardinia (ARPAS) and the former Hydrological Service (ExSI), and 96 rain gauges from the National Department for Civil Protection network. Information on rainfall-induced landslides comes from digital and printed newspapers, blogs, technical documents, landslide event reports, and fire fighter reports. The information in the catalogue is used to define regional rainfall thresholds to be implemented in the SARF Sardinia. Thresholds are defined using the frequentist method, which allows the calculation of cumulated event rainfall–rainfall duration (ED) threshold curves corresponding to different non-exceedance probability levels and including uncertainty in the parameters that define the thresholds. Currently, the Civil Protection of Sardinia Region is using the implemented system to support decisions in order to issue warning advices.

Rainfall thresholds for landslide early warning system in Sardinia

Landslides are frequent and destructive geo-hydrological events that cause harm to people every year worldwide (Petley, 2012). In the 13-year period 2004-2016, Froude and Petley (2018) recorded 55, 997 landslide fatalities in 4,862 distinct landslide events at the global scale. Catalogues of landslide fatalities are also available at national and regional level. Nevertheless, a small number of studies have investigated the causes and circumstances that have led to the loss of lives. In Italy, a country where landslides cause significant societal and economic damage every year, a catalogue of landslide events with human consequences is continually updated, and used to determine the individual and the societal risk posed by landslides (Guzzetti 2000; Salvati et al., 2010, 2013; Rossi et al., 2019). Despite the efforts, little is known on the circumstances of landslides fatalities, and their dependence on the age, gender and time of the day. For this purpose, efforts were spent to update the national catalogue (Salvati et al., 2003; 2017) for the 51-year period 1970-2020. We looked specifically to the hour of occurrence grouping them in by day or by night and considering the daytime duration differences during the year.  The new version of the catalogue lists 336 fatal landslides that have caused 1099 fatalities. We determined the gender (from the name) of 1,040 landslide fatalities (94.5%), and the age for 1,027 fatalities (93.5%) occurred in Italy in the period 1970 and 2020. Overall, males (587) account for 56.7% and females (453) for 43.3% of the landslide fatalities for which the gender is known in the catalogue. The new collected data allowed determining possible relations between the light condition (day or night), the places (indoor or outdoor), the gender (male, female) and the age (using 4 age categories) of landslide fatalities. We were able to identify the exact times for 948 fatalities (91%) and the exact place indoor or outdoor for 649 fatalities (62.4%).  Then, for each fatality, we related the times of occurrence with the place (outdoor and indoor) were people lost their life and with the gender and age. Males lost their life frequently outdoor, mainly inside their vehicles, representing a dangerous death contingence, most of them occurring in daylight. Differently, female landslide fatalities occurred more frequently indoor and in the majority by night revealing for them, the dangerous condition of darkness. Using the national census data and a multinomial distribution we estimated the expected fatalities by gender and age and we compared the results with the observed landslide fatalities distribution. Landslide male fatalities compared to the female ones are significantly higher than those expected by census data, indicating both a diverse propensity towards the risk taking and a different degree of exposure between males and females.

Conditions and behaviours influencing people vulnerability to landslides

Shallow landslides susceptibility assessment usually relies on the parametrization of hydraulic and geotechnical features of soils. Among these, saturated hydraulic conductivity (Ks) represents one of the most important soil properties that influences infiltration rates, runoff, groundwater recharge and drainage processes, which makes it of particular concern in the prediction of natural hazards including catastrophic floods and shallow landslides. On the other hand, it is well known that vegetation may play a positive role towards slope stability and over the last decades many efforts have been done in order to quantify the role of the vegetation towards protection from shallow landsliding (Giadrossich et al., 2017). Basically, there are two main vegetation effects: hydrological (e.g., reduction of the water pore pressure through tree rainfall interception) and mechanical ones (increase of the soil strength due to the presence of roots and increase of the soil shear stress due to the vegetation load). Soil Ks is expected to be an important factor that influences plant growth, plant-available water, root system and root biomass and, therefore, this work focuses on the quantitative assessment of the relationship between vegetation characters and hydraulic conductivity. Study areas affected by shallow landslides were chosen in the Garfagnana and Cardoso basin (Northern Apennines, Italy) and in Mt. Amiata volcano (Southern Tuscany, Italy) where field measurements (about 200) of below-ground vegetation (Root Area Ratio - RAR), above-ground vegetation (Leaf Area Index-LAI and vegetation load) and Ks were carried out inside, in the neighbour and far outside shallow landslide locations. Acquisition of data within landslides areas was supported by means of a multi-temporal landslide inventory already available for study areas. Below-ground vegetation data were collected in trench profiles, while above-ground vegetation data were acquired using digital relascope and digital cover photography. Measurements of Ks were carried out by means of both constant and falling head permeameters. Results showed that RAR and soil depth affected Ks, with a decrease of Ks as the depth increases and roots decrease. Moreover, Ks varied for different vegetation types, indicating that shifts in aboveground vegetation features may strongly impact the water dynamics of soil. Above-ground vegetation plays a slight negative role on slope stability. Instead, root reinforcement to soil in terms of a root-related cohesion parameter plays a relevant role for depths involved in shallow landslides.

Small-scale instability processes affecting volcanic island shelves: the case study of the southern shelf of Porto Santo Island (Madeira Archipelago)

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Small-scale instability processes affecting volcanic island shelves: the case study of the southern shelf of Porto Santo Island (Madeira Archipelago)

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Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

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