Abstract
‘Geological events and probabilities’ is one of the datasets available as geographical information system (GIS) maps on the European Marine Observation and Data network (EMODnet) Geology Portal (http://www.emodnet-geology.eu/map-viewer/). It collates existing data on earthquakes, landslides, tsunamis, fluid emissions and volcanic eruptions in European seas. Active faults have also been considered. Here we focus on Italian seas to tentatively infer mutual relationships between geological events and their connections with underlying geology and will comment on occurrences in three key geographical areas: the Ligurian Sea, the Southeastern Tyrrhenian Sea and the Aeolian Archipelago. This analysis can be of great support in the field of civil protection, as well as in the planning of human activities in marine-coastal areas.
Thematic collection: This article is part of the Mapping the Geology and Topography of the European Seas (EMODnet) collection available at: https://www.lyellcollection.org/cc/EMODnet
The European Marine Observation and Data network (EMODnet) project and the work of EMODnet Geology has been described in detail by Moses and Vallius (2020) and Vallius et al. (2020). The ‘Geological events and probabilities’ layer is one of the datasets available as geographical information system (GIS) maps on the EMODnet Geology Portal (http://www.emodnet-geology.eu/map-viewer/). This dataset, coordinated by the Geological Survey of Italy–ISPRA, collates existing data on geological events recorded in European seas, such as submarine landslides, volcanic structures, tsunamis and fluid emissions, gathered by national research institutes and governmental offices and validated by national geological surveys (Vallius et al. 2020). Active faults have also been included in the dataset to provide more comprehensive information. To provide information on earthquakes the portal also includes a link to the European–Mediterranean Seismological Centre (EMSC). The criteria for the identification and classification of geological events and the database structure have been described by Battaglini et al. (2020). Events are represented in different layers, subdivided by type of event and by GIS shapefile geometry (points, lines and polygons), complemented by detailed attribute tables containing significant amounts of relevant information (see Battaglini et al. 2020).
These data allow regional to local-scale analysis of the spatial extent of documented geological events, as well as structural features that provide controls on their location and nature. This paper focuses on Italian seas to tentatively infer mutual relationships between geological events and will comment on occurrences in three key geographical areas: the Ligurian Sea, the Southeastern Tyrrhenian Sea and the Aeolian Archipelago. All figures presented here are GIS representations of portions of selected digital layers displayed on the EMODnet Geology Portal. The authors recommend that readers visit the EMODnet Geology Portal to browse through the available datasets, downloading those of interest for a better visualization and understanding of the information contained within the layers.
Commonly, geological hazards are treated as isolated phenomena. Their occurrences, however, are often interlinked, with one event triggering others in often unpredictable yet related ways (Budimir et al. 2014; Mignan et al. 2014). These diverse geological processes may overlap, both in space and time, and have potential to interact, which may lead to exacerbation of consequences beyond that of hazards in isolation (Gill and Malamud 2016). For instance, the compounded effects of coincident events may be far greater than the effects of individual ones, or individual events may precondition or trigger cascades of other hazards (e.g. earthquake triggering a submarine landslide that triggers a regional tsunami; Sassa and Takagawa 2019). The assessment of risk posed by a combination of hazards is not simply the sum of individual risks.
Geological events in Italian seas
Brief outline of the geological setting of Italian seas
The Mediterranean region has had a complex tectonic past and has been shaped by, and continues to be influenced by, a diverse range of dynamic geological processes (Stanley and Wezel 1985) many of which pose a risk to the heavily populated areas around the region, as well as onshore and offshore infrastructure. The Mediterranean Sea, aligned in an east–west direction, is the result of continuing late Mesozoic and younger geodynamic events (Wezel 1985; Mascle and Mascle 2012), connected with convergence of the Africa and Eurasia plates and closure of the Tethys Ocean. Tethyan oceanic crust was consumed along subduction zones as volcanic arcs, back-arc basins and the Alpine orogenic chain formed in Eurasia. Active tectonics appears to be the major control on the overall geomorphology of the Italian region (Fig. 1). Figure 1 represents the main geomorphological features (a product also realized within the EMODnet Geology Project) together with Quaternary tectonics (a layer within the Geological events and probabilities dataset), on top of a land–sea digital terrain model (DTM) accessible through the EMODnet Bathymetry Portal. The relationships shown in these data highlight that many features (such as intraslope basins, seamounts and canyons) are associated with and outlined by faults. Important geomorphological features include the offshore Tyrrhenian back-arc basin and, to the east on the mainland, the Apennine Mountains (Bosellini 2017). About 2 myr ago, the Tyrrhenian basin reached the highest interpreted rate of ocean-floor spreading, estimated to have been about 20 cm a−1 (Nicolosi et al. 2006). Such expansion led to the emplacement of numerous submerged volcanic seamounts, with magmatic activity becoming younger from NW to SE (D'Angelo et al. 2019). Offshore geological events associated with this active tectonism are widespread in the entire area, most dense on Italy's SW flank and less common to the NE in the Adriatic Sea (Fig. 2).
GIS map of tectonics and geomorphological elements in Italian seas showing structural control on many features; Quaternary tectonics is represented according to the type of tectonic element; geomorphology is represented by selected features mostly associated with faults (underlying land–sea DTM from EMODnet Bathymetry Consortium 2018, https://www.emodnet-bathymetry.eu/).
GIS map of all geological events layers in Italian seas, characterized according to EMODnet Geology WP6 Guidelines; volcanic centres are represented by morphological type, landslides by type of movement, Quaternary tectonics by type of element, tsunamis by type of origin event and mud volcanoes are distinguished from other types of fluid emissions (underlying land–sea DTM from EMODnet Bathymetry Consortium 2018, https://www.emodnet-bathymetry.eu/).
Ligurian Sea
The Ligurian Sea is approximately shaped as a triangle, with the base going from Nice to La Spezia and the apex near Genova (Fig. 3). On its western side, the continental shelf is very narrow (1–2 km) and the adjacent slope dips steeply (average slope 8°) towards the abyssal plain of the Liguro-Provençal basin. By contrast, the continental shelf on the eastern side of the Ligurian Sea has an average width of 10 km. The Genova Canyon, one of the largest submarine canyons of the Mediterranean basin (Scafidi et al. 2017), is located in the central part of the Ligurian Sea. Its width reaches 50 km and its overall length is more than 70 km. The canyon axis extends from 200 to 2400 m depth, and the canyon walls reach and locally exceed slopes of 50°. The Genova Canyon system includes several tributaries, including the east–west-trending Levante Canyon from the eastern Ligurian Sea (Fig. 3). The Levante Canyon has steep sides, a flat bottom and an average depth of 150 m below the surrounding seafloor. The axes of these canyons are fault controlled; moreover, frequent earthquakes recorded in the area indicate that these faults are still active (Morelli 2008).
GIS map of tectonics, geomorphology and geological events in the Ligurian Sea; volcanic centres are represented by morphological type (the composite structures form the Ligurian Volcanic Seamount Sector), landslides by type of movement, Quaternary tectonics by type of element; only linear elements are displayed for geomorphology to avoid polygon overlaps (underlying land–sea DTM from EMODnet Bathymetry Consortium 2018, https://www.emodnet-bathymetry.eu/).
Volcanic products were emplaced during the Miocene opening of the Liguro-Provençal back-arc basin, and now form the Ligurian Volcanic Seamount Sector (Pensa et al. 2019).
Late Pleistocene extensional tectonics caused an increase in slope steepness across the seafloor and within canyons, erosion within canyons and widespread gravitational mass movements (Morelli 2008), such as canyon gravitational mobilization, gravitational slope collapse, and rotational or translational landslides (for classification of landslides, see Battaglini et al. 2020). The heads of some canyons extend very close to the coastline and influence coastal stability, eroding and conveying large amounts of sediments to the slope, through mass-wasting processes, including debris flows and turbidity currents. Some of these sediments have been accumulated on the floor of the basin in a Late Pleistocene mega-turbidite (Fig. 2) described by Rothwell et al. (1998).
Southeastern Tyrrhenian Sea
The Paola basin is one of the peri-Tyrrhenian intra-slope basins (Fig. 1), located between the Marsili back-arc basin and the uplifting Apennine chain (Fig. 4). It is bordered on its western side by volcanic seamounts with the narrow (5–10 km wide) continental shelf along the Italian coast forming the eastern basin margin. Tectonically forced uplift of the Calabria area, together with the opening of the Tyrrhenian basin, resulted in very high sedimentation rates (Corradino et al. 2020). Fluid emissions (vents, pockmarks, seeps) have been interpreted based on analysis of multibeam backscatter data (Loreto et al. 2011). Gases in this area are produced by bacterial degradation of organic material; their ascent is favoured by intense faulting. They increase the interstitial overpressure and lead to a deterioration of the physical properties of the deposits, enhancing instability on the continental margin (Dugan 2012; Urgeles and Camerlenghi 2013). Landslides are consequently widespread in the basin. However, the morphological barrier represented by the seamounts prevents gravitational flows from transferring sediments into the deeper Marsili basin. As a result, the Paola basin contains the thickest (>4 km) section of Plio-Quaternary sediments of the eastern Tyrrhenian margin (Trincardi et al. 1995).
GIS map of tectonics, geomorphology and geological events in the Southeastern Tyrrhenian Sea; volcanic centres are represented by morphological type, landslides by type of movement, Quaternary tectonics by type of element; only linear elements are displayed for geomorphology to avoid polygon overlaps (underlying land–sea DTM from EMODnet Bathymetry Consortium 2018, https://www.emodnet-bathymetry.eu/).
Aeolian Archipelago
The Aeolian Islands volcanic arc is located in the southeastern portion of the Tyrrhenian Sea, between the Calabrian western coast and northern Sicily (Fig. 5). The volcanic arc correlates with the fast retreat of the Ionian slab and the associated spreading of the Marsili basin (2–1 Ma). All of the seamounts emerging as islands are stratovolcanoes, whereas other edifices attributed to the Aeolian Eastern Tyrrhenian Volcanic Seamount Sector (Pensa et al. 2019) are of either fissural or composite origin. A few deeply incised canyons collect and transport mass-wasting detritus to the deeper basin. The area is characterized by intense tectonic activity and has experienced several destructive, historical earthquakes (the best known is the 1908 Messina event). Earthquakes generally have extensional focal mechanisms; transpressive mechanisms are less common (Scarfì et al. 2013; Barreca et al. 2014). Volcanic eruptions are also frequent: the most recent event, explosions of the Stromboli volcano, occurred in the summer of 2019.
GIS map of tectonics, geomorphology and geological events in the Aeolian Archipelago area; volcanic centres (all of them belonging to the Aeolian Eastern Tyrrhenian Volcanic Seamount Sector) are represented by morphological type, landslides by type of movement, Quaternary tectonics by type of element, tsunamis by type of origin event; only linear elements are displayed for geomorphology to avoid polygon overlaps (underlying land–sea DTM from EMODnet Bathymetry Consortium 2018, https://www.emodnet-bathymetry.eu/).
On the Aeolian Islands, landslides can involve both volcanic deposits accumulated over time or the collapse of parts of the volcanic structures, as was the case for the December 2002 Stromboli event. In this event, a series of terrestrial landslides and one submarine landslide (estimated volume over 10 × 106 m3) (Tinti et al. 2005), occurred around Stromboli Island owing to the collapse of a crater ridge on the NW side of the volcano following a particularly violent eruptive episode. The landslides gave rise to a series of tsunami waves (maximum height was 10 m), which caused severe damage and destroyed a few houses on Stromboli Island (Maramai et al. 2005; Tinti et al. 2005).
Sciara del Fuoco is the currently active volcanic area, where a large amount of coarse-grained volcaniclastic material, derived from the persistent Strombolian activity, is collected and spread radially as turbidity currents as far as 20 km from the volcano edifice to depths of 2400 m (Pensa et al. 2019). Many tsunamis have previously occurred in this area (Maramai et al. 2014). The most destructive is associated with the 1908 Messina earthquake, which had an up to 11 m run-up height and caused more than 80 000 deaths (http://www.protezionecivile.gov.it/risk-activities/seismic-risk/emergencies/reggio-calabria-messina-1908).
Final considerations
The standardized and harmonized inventory of information relating to geology and geological events available via the EMODnet Geology Project Portal (Battaglini et al. 2020) provides a state-of-the-art database of knowledge and therefore provides a solid baseline for further investigations. Easily accessible data, which can be visualized as digital maps on the EMODnet Geology Portal, complemented by comprehensive attribute tables, allow end users to consider mutual relationships between geological events, as well as to identify information gaps.
Geological events in submerged areas are more common in tectonically active and geologically young areas, as can be observed by looking at their overall distribution in European seas on the EMODnet Geology Portal.
All of the types of geological events considered in this paper occur across the seas surrounding the Italian peninsula (Fig. 2). A dense network of faults highlights the active tectonics. Earthquakes are frequent along the Apennine chain and around the Tyrrhenian Sea (https://www.seismicportal.eu/). Volcanic structures are present all along Italy's western side, from the older (5 Ma) seamounts identified in the northwestern Ligurian Sea to the currently active volcanoes of the Aeolian Islands, marking the progressive opening of the back-arc basins.
High sedimentation rates are common on the continental margin of the Tyrrhenian Sea, owing to the erosion of rapidly uplifting mountainous terrains (Corradino et al. 2020). This continental margin shows areas of intense erosion to the north and east, from which originate gravitational processes and large mass wasting, especially where the margin is characterized by a very narrow continental shelf and a steep slope (Figs 3 and 4).
The presence of fluid emissions, as hydrothermal vents related to volcanism or as gases produced by chemical–physical alterations of organic material, enhances the potential for submarine landslides (Fig. 4). When gases trapped in sedimentary layers, mainly in massive deposits in palaeo-delta environments or in fault-controlled basins, migrate to the seabed they may greatly contribute to induce instability on the continental margins leading to the formation of unstable sedimentary bodies (Rothwell et al. 1998; Dugan 2012).
Tsunamis can be generated by earthquakes, volcanic eruptions and landslides. Tsunamis of volcanic origin are mainly caused by explosive eruptions (submarine and subaerial) and by landslides of volcanic material that may be associated with the eruptions (Fig. 5). Subaerial volcanoes can produce far-reaching pyroclastic flows that plunge at high speed into the sea, displacing large volumes of water and generating tsunamis (Paris 2015).
Mutual relationships between geological events have been identified within the database developed for the EMODnet Geology Project and connections with the underlying geology have been inferred by considering their distribution and attributes against an outline of the geological setting.
Information concerning geological events is highly relevant for geohazards assessment, infrastructure planning and mitigation against future events. It is therefore important to understand when, where and how such hazards may affect any given region. The identification of possible interacting hazard networks would allow improved planning of possible changes in vulnerability during successive events. In turn, this could help to improve preparedness efforts (Gill and Malamud 2016). Consequently, the analysis of previous geological events is of great support in the field of civil protection as well as for land management and planning, particularly regarding marine-coastal areas.
Acknowledgements
We would like to thank all reviewers for their fruitful comments, which helped us to improve our paper. We also want to thank the EMODnet Bathymetry Consortium (2018): EMODnet Digital Bathymetry (DTM) for the DTM displayed in the figures (https://doi.org/10.12770/18ff0d48-b203-4a65-94a9-5fd8b0ec35f6).
Author contributions
AF: data curation (equal), funding acquisition (equal), methodology (equal), project administration (lead), writing – original draft (equal), writing – review & editing (equal); LB: data curation (lead), funding acquisition (supporting), methodology (equal), project administration (supporting), writing – original draft (equal), writing – review & editing (equal); SD: data curation (equal), funding acquisition (lead), methodology (equal), project administration (equal), writing – original draft (equal), writing – review & editing (equal)
Funding
This work was funded by the Executive Agency for Small and Medium-sized Enterprises (EASME/EMFF/2016/1.3.1.2 - Lot 1/SI2.750862).
Data availability
The datasets generated during and/or analysed during the current study are available in the EMODnet Geology Portal repository, https://www.emodnet-geology.eu/map-viewer/
Scientific editing by Cherith Moses; Andrew Hart
- © 2021 The Author(s). Published by The Geological Society of London. All rights reserved
References
EMODnet collation of geological events data provides evidence of their mutual relationships and connections with underlying geology: a few examples from Italian seas
