Alessandro Bosman

Comment on “On the cause of the 1908 Messina tsunami, southern Italy” by Andrea Billi et al.

[1] The December 28th 1908 Messina Earthquake has been ranked as one of the most destructive events of the last centuries [Davison, 1936] The damages produced by ground shaking were aggravated by the effects of a remarkable tsunami, with up to 11 m of run-up height, that followed the earthquake [Omori, 1909; Baratta, 1910; Tinti et al., 2004]. The location of the causative fault is still a matter of debate [e.g., Argnani et al., 2009], the modelling of the associated tsunami [Tinti and Armigliato, 2003] allowing uncertainty. [2] Billi et al. [2008] have recently proposed that the tsunami that stroke the coast of the Strait in December 1908 originated from a large submarine landslide (20 km) located offshoreGiardini-Naxos. The hypothesis is based on the study of tsunami arrival times [Platania, 1909; Baratta, 1910] and is supported, according to Billi et al. [2008], by inspection of: a multibeam morphobathymetry [Marani et al., 2004] and of a crustal-scale seismic profile [Scrocca et al., 2004]. [3] The arguments of Billi et al. [2008], however, have weak points that cast doubt on their interpretation. But most importantly, data collected in the last few years by the authors of this comment (A. Argnani and F. Chiocci), and work on tsunami modelling (S. Tinti and his group) cast further doubt on the proposed hypothesis that a large submarine landslide that was caused by the 1908 earthquake is located offshore Giardini-Naxos. These issues will be discussed in the following.

Cannibalization of a Continental Margin By Regional Scale Mass Wasting: An Example From the Central Tyrrhenian Sea

A morpho-stratigraphic study carried out on the Western Pontine Archipelago continental slope (Eastern Tyrrhenian Margin) revealed a suite of instability/erosional features producing the complete cannibalization of a whole span of continental slope, from the shelf break down to the abyssal plain (located at 3600 m w.d.); only 2% of the whole area (2.000 sq. km) is not affected by these processes. One of the main controlling factors seems to be the slope gradient that produce different instability features, ranging from gravity-driven failure (simple and complex slides) to gravity-flow (debris and grain flows).

Morphology of Lipari offshore (Southern Tyrrhenian Sea)

High-resolution multibeam bathymetry was recently collected around Lipari, the largest and most densely populated island of the Aeolian Archipelago (Southern Tyrrhenian Sea). The data were acquired within the context of marine geological studies performed in the area over the last 10 years. We present the first detailed morphological map of the Lipari offshore at 1:100,000 scale (Main Map). A rugged morphology characterizes the submarine portions of Lipari volcano, reflecting both volcanic and erosive-depositional processes. The volcanic features include cones, lava flows and bedrock outcrops. Erosive-depositional features include an insular shelf topped by submarine depositional terraces related to Late-Quaternary sea-level fluctuations, as well as landslide scars, channelized features, fan-shaped deposits and wavy bedforms. The different distribution of volcanic and erosive-depositional features on the various sectors of Lipari is mainly related to the older age of the western flank with respect to the eastern one. The map also provides insights for a first marine geohazard assessment of this active volcanic area.

Coastal and Submarine Landslides in the Tectonically-Active Tyrrhenian Calabrian Margin (Southern Italy): Examples and Geohazard Implications

High-resolution multibeam bathymetry is used as basis for detecting instability processes on the Tyrrhenian margin offshore Calabria (Italy). Among the many evidence at different spatial scale, we focus on selected cases that may represent a potential geohazard because of their scale, shallow/proximal location and state of activity. These include failures along coastal cliffs and canyon headwalls indenting narrow shelves. Coastal rocky failures impacting shallow water offshore Scilla and Palmi retain significant tsunamigenic potential, as proved by the 1783 Scilla event. Slide scars at canyon headwalls offshore Bagnara Calabra and Gioia Tauro indicate retrogressive failure active at water depth <10 m, just few hundreds of meters from the coast and the settlements and infrastructures there present. Geohazard related to these features is therefore relevant as testified by the failure induced during construction of the Gioia Tauro harbour in 1977.

Mass Wasting Features on the Submarine Flanks of Ventotene Volcanic Edifice (Tyrrhenian Sea, Italy)

High-resolution multibeam bathymetry acquired around Ventotene and S. Stefano islands (eastern Pontine Archipelago, Italy) enabled us to map main mass wasting features affecting their submarine portions. Large-scale instability morphological features are absent (apart from a 4 × 2.5 km caldera in the western sector), whereas 126 landslide scars of 100-m of length scale were identified between 130 and 1,150 m of water depth (wd). Two main groups of scars can be distinguished: the first one affects the edge of the insular shelf between 130 and 260 m wd. The second group affects the lower slope and surrounding basins, representing cases of retrogressive failure at the heads of channelized features. The different morphological relief of the scars coupled with the recognition of crescent-shaped bedforms made it possible to distinguish two mass-wasting/erosive stages and consequently to map the more active sectors of the edifice. The future evolution of the mass wasting processes will produce the enlargement of erosive sectors with possible formation of large channels, which will carve wide sectors of the edifice, as suggested by available geological constraints and by comparison with the nearby and older western sector of the Pontine archipelago, where a more mature organization of mass wasting processes is observed. The present study can provide useful insights for hazard assessment and future planning of risk mitigation in such islands that are densely populated and touristically exploited during the summer months.

Study of Recent Small-Scale Landslides in Geologically Active Marine Areas Through Repeated Multibeam Surveys: Examples from the Southern Italy

Repeat multibeam surveys are used to investigate recent submarine landslides in three different areas on southern Italy, i.e. Stromboli, Southern Messina Strait and Punta Alice. High-resolution Digital Terrain Models (DTMs) depict the morphology of mass-wasting events, while residual maps of the difference between successive bathymetric surveys indicate failure volumes ranging from tens of thousands to millions of cubic meters. A tentative estimate of recurrence time for the slope failures was based on chronological constraints and historical reports, and gave values ranging from tens to hundreds of years. Characterizing mass-wasting events and their recurrence time is a necessary step for geo-hazard assessment of densely populated coastal areas.

Marine geological and archaeological evidence of a possible pre-Neolithic site in Pantelleria Island, Central Mediterranean Sea

Abstract Recent underwater archaeological surveys recovered hundreds of flint artefacts between depths of 18 and 21 m at Cala Tramontana, a small bay located in the eastern part of Pantelleria Island. Most of the flint artefacts indicate debitage, and are characterized by cores and flakes without any specific morphology. Different lithic tools were also identified, such as fragments of blades, truncations, end-scrapers, points and crested blades. An initial hypothesis is that this lithic industry represents the oldest traces of human visitation to the island, possibly related to the exploitation of the nearby obsidian source, and favoured because of the sheltered coastal configuration of Cala Tramontana and Cala Levante with respect to the dominant winds and related storms. However, the present-day coastal setting in the bay is rather inhospitable, with high cliffs and difficult marine access. In contrast, palaeo-landscape reconstructions by means of high-resolution multibeam bathymetry reveal the possible presence of a small palaeo-beach in the inner part of the bay when the sea level was 15 m lower than at present. By comparing this palaeo-sea level with the eustatic curve (and by excluding possible vertical movements), we roughly estimate an age of the lithic industry of 9.6–7.7 cal ka BP.

Integrated Subaerial‐Submarine Morphological Evolution of the Sciara del Fuoco after the 2002 Landslide

In December 2002, an effusive eruption at Stromboli triggered a complex instability phenomenon, which affected both the subaerial and submarine portion of the Sciara del Fuoco slope, causing destructive landslide(s) and tsunami waves. Among the monitoring activities coordinated by the Italian Civil Defence Department, systematic photogrammetric and bathymetric surveys were carried out. Digital photogrammetry technique and multibeam soundings were used to obtain high-resolution digital elevation model of land and sea-floor surface of the NW flank of Stromboli (Sciara del Fuoco depression). Merging the subaerial and submarine data, and comparing multitemporal digital models, we first estimated the mass volumes involved in the failures of the subaerial and shallow submarine slope to be about 24 x 10 6 m 3 , and then, we monitored the continuous and relevant morphological changes induced by erosional―depositional processes during the various syneruptive and posteruptive stages. Filling processes of the scar by lava flow and debris and the morphological evolution of the slope in the 2 years after the event were described.

Submarine Mass-Movements on Volcanic Islands: Examples from the Aeolian Archipelago (Italy)

High-resolution bathymetry collected around the Aeolian islands allowed the recognition of a large spectrum of geomorphic features referable to mass-movements. Besides large-scale sector collapse scars (recognized only at Stromboli), two main groups of medium-scale submarine slides are distinguished based on their size, geometry and location. The first group is defined by semi-circular slide scars at hundred-meter scale, indenting the outer edges of insular shelves and overlying sedimentary deposits in the first hundred(s) m of water depth (wd). The second group is defined by downslope-elongated scars carving the steep submarine flanks of volcanic edifices and interpreted as retrogressive slope failures. Within this latter group, particular attention has been devoted to the frequent slope failures affecting the submarine portion of the Sciara del Fuoco at Stromboli, and to the nested slide scars affecting the old and extinct Banco del Bagno submarine volcanic center at Lipari. The main predisposing and triggering factors, recurrence times and tsunamigenic potential of the recognized mass-movements are briefly discussed to provide some insights into geohazard assessment of the area.

Cyclic Steps at the Head of Channelized Features Along the Calabrian Margin (Southern Tyrrhenian Sea, Italy)

High-resolution multibeam bathymetry have revealed coaxial trains of crescent-shaped bedforms within the heads of channelized features lying in shallow-water sectors along the tectonically-controlled Calabrian Margin. These bedforms have wavelengths of tens or a few hundreds of metres and wave heights of a few metres, and their crest-lines trend perpendicular to the maximum slope gradients. Repeated multibeam surveys carried out in 2007, 2008, 2012 and 2013 showed a rapid and significant morphological evolution of the channel’s floors, with the generation or upslope migration of the bedforms. Based on their size, upslope migration and similarities to crescent-shaped bedforms recognized in other active canyon heads, these features can be interpreted as cyclic steps. The bedforms are, in fact, formed or modified by frequent slope failures and related sedimentary flows, whose occurrence is favoured by the concurrent presence of several predisposing and triggering mechanisms, such as a high sedimentation rate due to a steep coastal creek, severe storms and seismic events.