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We present the characterization of geohazard-related features of the Ligurian Sea with the map of its physiographic domains (1:250,000 scale) and five maps (1:100,000 scale) of the morphological and morpho-bathymetric elements. These were realized in the framework of the MaGIC (Marine Geohazard along Italian Coasts) project, promoted by the Italian Civil Protection Department. The characterization of submarine geohazards along continental margins, together with mapping of marine geological features are critical for the management of coastal risks. Multibeam bathymetric data combined with high-resolution 2D reflection seismic data, allowed to identify and map the main tectono-sedimentary features related to mass movements along the Ligurian continental margin. A dense grid of seismic profiles allowed to improve the comprehension of the margin structure as well as its recent tectonic activity providing new insights concerning mass wasting processes and active tectonics relationships to plan further, focalized investigations in areas of maximum risk. [ABSTRACT FROM AUTHOR]

The review of recent bathymetric and geophysical data collected in the framework of several research and cartographic projects have allowed a detailed reconstruction of the morpho-structural setting and the (neo)tectonic evolution for both the Alpine and Apennine margins of the Ligurian Sea (Italy). The widespread occurrence of erosional processes and sediment mass movements along the steep continental slope and within the system of submarine canyons reflect the close correlation between the active tectonics and the recent morpho-dynamic evolution of the Ligurian Margin. This relation is better constrained in the western sector (Alpine) of the Ligurian Sea, where the recent uplift of the continental margin is associated to a well-developed system of inherited structures reactivated under a compressive/transpressive regime and widespread seismicity. In the eastern sector, where the seismicity is lower or absent, the mass movements are limited to few areas (e.g., the Portofino slope) coinciding with seismic clusters. Additionally, this sector is characterized by moderate and episodic fault reactivations under a compressive regime. The evidence of compressive deformation along the inherited fault systems has been revealed in some areas of the Ligurian Sea where the post-drifting extensional tectonics is interrupted by episodic tectonic inversion (at least) during the Middle–Upper Miocene and the Plio–Pleistocene until present. [ABSTRACT FROM AUTHOR]

Tephra layers preserved in marine sediments are strong tools to study the frequency, magnitude and source of past major explosive eruptions. Thirty‐seven volcanoes from the Ecuadorian and Colombian arc, in the northern Andes, experienced at least one eruption during the Holocene. The volcanic hazard is therefore particularly high for the populated areas of the Andes and in particular cases for the coastal region, and it is crucial to document such events to improve hazard assessment. The age and distribution of deposits from major Holocene eruptions have been studied in the Cordillera, but no descriptions of distal fallouts have been published. In this study, we focused on 28 Holocene tephra layers recorded in marine sediment cores collected along the northern Ecuador—Southern Colombia margin. New lithological, geochemical and isotope data together with 14C datings on foraminifers allow us to determine the age and volcanic source of marine tephra, and to propose a first land‐sea correlation of distal tephra fallouts. We show that at least seven explosive eruptions from Guagua Pichincha, Atacazo‐Ninahuilca, Cotopaxi, and Cerro Machín volcanoes left tephra deposits recorded in marine cores over 250 km away from their source. Volume estimates of emitted tephra range between 1.3 and 6.0 km3 for the tenth century Guagua Pichincha, ∼5 ka Atacazo‐Ninahuilca, ∼6.7 and ∼7.9 ka Cotopaxi events, suggesting that they were eruptions of Volcanic Explosivity Index of 5. The distribution of these deposits also brings new constraints for a better evaluation of the volcanic hazard in Ecuador. Plain Language Summary: During major explosive eruptions, large volumes of gases and tephra (lapilli and ash particles) are thrown into the atmosphere and can be spread by winds over 100 km and more. Tephra fallouts can impact the population, infrastructures and climate. It is therefore essential to document the age and magnitude of past major eruptions to better assess the volcanic hazards. In this study, we use the mineralogy, glass shard morphology, and the geochemical composition of tephra settled in marine sediments off Ecuador and Colombia to investigate their source. Thickness of tephra layers and radiocarbon ages performed on under‐ and over‐lying marine fauna allow us to determine the age of the eruptions, whereas the distribution of tephra yields constraints on the volume of fallout deposits. We show that the largest explosive eruptions from Ecuadorian and Colombian volcanoes reached the Pacific Ocean with a recurrence rate of about 1.5 events per millennium over the past 8 kyr. Key Points: We propose a first land‐sea correlation of distal Holocene tephra off Ecuador based on 14C age and geochemical dataProducts from at least seven explosive Holocene eruptions in Ecuador and south Colombia reached the Pacific OceanVolumes of tephra emitted by largest eruptions vary between 1.3 and 6.0 km3, suggesting they were VEI‐5 eruptions [ABSTRACT FROM AUTHOR]

A broadband seismological station (PRIMA) installed offshore Nice airport (southeastern France) reveals a strong amplification effect of seismic waves. PRIMA station was in operation for 2 years (9/2016 to 10/2018) on the outer shelf at a water depth of 18 m. Situated at the mouth of the Var River, this zone is unstable and prone to landslides. A catastrophic landslide and tsunami already occurred in 1979, causing 10 casualties. Given the level of seismicity of the area, it is important to infer the impact of an earthquake on this zone. We analyze the recordings of earthquakes and seismic noise at the PRIMA station by comparing them to nearby inland stations. We find that the seismic waves are strongly amplified at PRIMA at some specific frequencies (with an amplification factor greater than 10 at 0.9 Hz). Using geological and geophysical data, we show that the main amplification frequency peak (at 0.9 Hz) is due to the velocity contrast between the Pliocene sedimentary layer and fine-grained sediments dated from the Holocene, at about 100 m depth. This velocity contrast is also present along the Var valley, but the level of amplification detected on PRIMA station is larger. Using numerical simulations of seismic waves in a 2D model that accounts for the pinch-out geometry related to the termination of the Holocene sedimentary layer, we can partially explain this amplification. This offshore site effect could have a crucial impact on the triggering of a submarine landslide by an earthquake in this region. More generally, this effect should be taken into account for the modeling of landslides and induced tsunamis triggered by seismic waves. [ABSTRACT FROM AUTHOR]

We present a new numerical surface process model allowing us to take into account submarine erosion processes due either to submarine landslides or to hyperpycnal currents. A first set of models show that the frequency of hyperpycnal flows influences the development of submarine canyons at the mouth of continental rivers. Further experiments show that an increase in submarine slope leads to faster regressive canyon erosion and a more dentritic canyon network, whereas increasing the height of the unstable sediment pile located on the shelf break leads to wider and less dendritic canyons. The models are then applied to the western segment of the north Ligurian margin (northwestern Mediterranean), which displays numerous submarine canyons with various sizes and morphologies. From west to east, canyon longitudinal profiles as well as margin-perpendicular profiles progressively change from moderately steep, concave-up shapes to steeper linear to convex-up shapes suggesting increasing eastward margin uplift. Moreover, the foot of the margin is affected by a marked slope increase with evidences of mass transport due to landslides. Numerical models which reproduce well the North Ligurian margin morphologic features indicate that the western part of the margin is submitted to rather low (i.e. 0.4 mm yr−1) uplift and intense submarine erosion due to frequent hyperpycnal currents, whereas the eastern part bears more rapid (i.e. 0.7 mm yr−1) uplift and has little or no hyperpycnal currents. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present here new data from the MALISAR surveys that focus on two areas of the northern Ligurian margin where we reveal recent and active deformation. A set of N60°E scarps that are oblique to the margin is seen at the foot of the continental slope. These correspond to cumulated reverse-strike slip faulting that is consistent with the present-day kinematics of earthquakes. At the north-western margin, the Marcel Fault appears at the surface as a 10-km-long scarp trending N60°E. Several earthquakes of moderate magnitude (3.8 < M < 4.6) have focal mechanisms consistent with the geometry and the kinematics of, and have been located around, the Marcel Fault. At the north-eastern margin, offshore of Imperia, the major feature is a large promontory that is bound by a network of N60°E faults on its southern side. The structures and the deflection of the drainage network are consistent with a recent uplift of the Imperia Promontory. No surface ruptures have been identified in the epicentre area of the 1887 Ligurian earthquake (the major historical regional event: M ~6.5-6.7), although the Imperia Fault network has the necessary characteristics to account for this earthquake. Therefore, from the present study, we propose that the rupture, as a reverse-strike slip faulting of a few or all of the segments belonging to the Imperia Fault network, was the source of the 1887 event. These MALISAR data have allowed identification of an 80-km-long transpressive system of Plio-Quaternary faults. The question of the potential activation of all of these faults during a single event is now of major concern, which has led to reappraisal of the regional seismic hazard as moderate to high. [ABSTRACT FROM AUTHOR]

Based on new multibeam bathymetric data, seismic-reflection profiles and side-scan sonar images, a great number of submarine failures of various types and sizes was identified along the northern margin of the Ligurian Basin and characterized with 3 distinct end-members concerning their location on the margin, sedimentary processes and possible triggering mechanisms. They include superficial landslides mainly located in the vicinity of the main mountain-supplied rivers and on the inner walls of canyons (typically smaller that 10 m in volume: Type 1), deep scars 100-500 m high along the base of the continental slope (Type 2), and large-scale scars and Mass Transport Deposits (MTDs) affecting the upper part of the slope (Type 3 failures). The MTDs are located in different environmental contexts of the margin, including the deep Var Sedimentary Ridge (VSR) and the upper part of the continental slope in the Gulf of Genova (Finale Slide and Portofino Slide), with volumes of missing sediment reaching up to 1.5 × 10 m. High sedimentation rates related to hyperpycnal flows, faults and earthquake activity, together with sea-level fluctuations are the main factors invoked to explain the distribution and sizes of these different failure types. [ABSTRACT FROM AUTHOR]

This paper presents an overview of the evolution of the Nile deep-sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep-sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large-scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum ( ca 25 to 14·8 ka) the central and eastern parts of the Nile deep-sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea-level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood-derived deposits occurred across the shallower part of the submarine delta during sea-level rise between ca 14·8 and 5 ka. The most recent deep-sea channel–lobe system was very active during this period of rising sea-level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large-scale slope failure. The Nile deep-sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep-sea turbidite system may be more active during periods of rising and high sea-level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea-level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large-scale sedimentary system responded to monsoon-driven climate change and sea-level change in a system-wide and contemporaneous manner. [ABSTRACT FROM AUTHOR]

Abstract: Clastic mud beds rich in continental organic matter are observed recurrently in the Nile deep-sea turbidite system. They formed during flooding periods of the river similar to those that induce sapropel formation and occurred during periods of increased density stratification of the eastern Mediterranean. The very fine-grained flood deposits are intercalated within pelagic sediments, sapropels and Bouma-type turbidites. These flood deposits form by the successive reconcentrations of surface (hypopycnal) plumes by convective sedimentation, which in turn generate a fine-grained low-energy hyperpycnal flow. Sea-level high stands seem also to favor hypopycnal plume formation and increase clastic mud bed formation. Consequently, these muddy clastic beds provide a direct link between deep-marine sedimentary records and continental climatic change through flood frequency and magnitude. [Copyright &y& Elsevier]

The Var turbidite system is a small sandy system located in the Ligurian Basin. It was deposited during the Pliocene-Quaternary in a flat-floored basin formed during the Messinian salinity crisis. The system was fed through time by the Var and Paillon canyons that connect directly to the Var and Paillon rivers. It is still active during the present sea-level highstand. Two main mechanisms are responsible for gravity-flow triggering in the Var turbidite system: (1) mass-wasting events affect mainly the upper part of the continental slope, in areas where volumes of fresh sediment delivered by rivers are highest, and result from the under-consolidation state of slope sediments and earthquakes, and (2) high-magnitude river floods resulting from melting of snow and convective rainfall during fall and spring seasons, and generating hyperpycnal turbidity currents at river mouths when the density of freshwater transporting suspended particles exceeds that of ambient seawater. Failure- and flood-induced gravity flows are involved through time in the construction of the Var Sedimentary Ridge, the prominent right-hand levee of the Var system, and sediment waves. Processes of construction of both the Var Ridge and sediment waves are closely connected. Sandy deposits are thick and abundant in the eastern (downchannel) part of the ridge. Their distribution is highly constrained by the strong difference of depositional processes across the sediment waves, potentially resulting through time in the individualization of large and interconnected sand bodies. [ABSTRACT FROM AUTHOR]