Development and Emplacement of Ana Slide, Eivissa Channel, Western Mediterranean Sea

Submarine landslides can destroy seafloor infrastructures and generate devastating tsunamis. In spite of decades of research into the functioning of submarine landslides there are still numerous open questions, in particular how different phases of sliding influence each other. Here, we re-analyze Ana Slide—a relatively small (<1 km3) landslide offshore the Balearic Islands, which is unique in the published literature because it is completely imaged by high-resolution 3D reflection seismic data. Ana Slide comprises three domains: (a) a source area that is almost completely evacuated with evidence of headscarp retrogression, (b) an adjacent downslope translational domain representing a by-pass zone for the material that was mobilized in the source area, and (c) the deposit formed by the mobilized material, which accumulated downslope in a sink area and deformed slope sediment. Isochron maps show deep chaotic seismic units underneath the thickest deposits. We infer that the rapid deposition of the landslide material deformed the underlying sediments. A thin stratified sediment unit between three lobes suggests that Ana Slide evolved in two failure stages separated by several tens of thousands of years. This illustrates the problem of over-estimating the volume of mobilized material and under-estimating the complexity even of relatively simple slope failures without high-quality 3D reflection seismic data

Characterisation of Submarine Landslides on the Eastern Slopes of the Eivissa Channel, Western Mediterranean Sea

Submarine landslides pose a risk to offshore infrastructure and can generate devastating tsunamis. There exist a range of hypotheses about pre-conditioning factors and trigger mechanisms, yet the understanding of landslide development and emplacement processes has been limited to a few case studies. On the one hand, this is partly because the study of sub-seafloor structures is limited to sediment echosounder profiles and 2D and rarely 3D reflection seismic data. However, because these data are expensive to acquire, data coverage is sparse and often of low resolution. On the other hand, historical records of landslide- generated tsunamis are rare, and can often not be differentiated from tsunamis generated exclusively by earthquakes. In addition, the study of active slope failures is mainly based on the 2D and 3D reflection seismic analysis of their remnants – the mass transport deposits (MTDs) – meaning that slope failure development and emplacement processes can only be interpreted from these remains. Interpretation of reflection seismic data is a highly ambiguous task and depends on the data which includes resolution and penetration depth and the experience of the interpreter. Monitoring of active failure processes has only been successful to a limited extent, for instance by measurements of landslide velocities deduced from telecommunication cable breaks. Because of the large size of submarine landslides, it is becoming more and more evident that they can deform seafloor sediments to significant depth. Furthermore, stations and platforms installed directly on the seafloor can be damaged or buried by landslide material or even be incorporated into the slope failure

Characterisation of Submarine Landslides on the Eastern Slopes of the Eivissa Channel, Western Mediterranean Sea

Submarine landslides pose a risk to offshore infrastructure and can generate devastating tsunamis. There exist a range of hypotheses about pre-conditioning factors and trigger mechanisms, yet the understanding of landslide development and emplacement processes has been limited to a few case studies. On the one hand, this is partly because the study of sub-seafloor structures is limited to sediment echosounder profiles and 2D and rarely 3D reflection seismic data. However, because these data are expensive to acquire, data coverage is sparse and often of low resolution. On the other hand, historical records of landslide- generated tsunamis are rare, and can often not be differentiated from tsunamis generated exclusively by earthquakes. In addition, the study of active slope failures is mainly based on the 2D and 3D reflection seismic analysis of their remnants – the mass transport deposits (MTDs) – meaning that slope failure development and emplacement processes can only be interpreted from these remains. Interpretation of reflection seismic data is a highly ambiguous task and depends on the data which includes resolution and penetration depth and the experience of the interpreter. Monitoring of active failure processes has only been successful to a limited extent, for instance by measurements of landslide velocities deduced from telecommunication cable breaks. Because of the large size of submarine landslides, it is becoming more and more evident that they can deform seafloor sediments to significant depth. Furthermore, stations and platforms installed directly on the seafloor can be damaged or buried by landslide material or even be incorporated into the slope failure

Sediment echosounder processed data (TOPAS 18 working area dataset) of RV HESPERIDES during cruise HERMESIONE, Eivissa Channel, Mediterranean Sea

Spanish research vessel BIO Hespérides using a SIMRAD TOPAS PS018 system. During the cruise, TOPAS was used mainly in Chirp mode, but a Ricker pulse was partly used in the Eivissa channel. Real-time processing of the data consists of basic filters and gain control. Data were recorded both in the native system and in SEGY format. The data is projected to UTM WGS 1984 31N

2D multichannel seismic reflection processed data (Airgun working area dataset) of RV HESPERIDES during cruise HERMESIONE, Eivissa Channel, Mediterranean Sea

2D reflection seismic data were acquired between September 14th to October 9th 2009, using the BIO Hespérides in the framework of the HERMESIONE cruise with Miquel Canals, University of Barcelona as the responsible PI. The seismic source used in the Eivissa Channel was a string of two BOLT® 1900LL guns in a cluster configuration towed 42.5 m beneath the stern of the vessel. The capacities of the guns used in this campaign are as follows: cluster of 40+40 cubic inches (cu. in.). The gun spacing is 0.8 metres between guns in the same cluster. The seismic source array was located at 2.5 m water depth. The firing frequency was 6 seconds. The single-channel streamer model 16.3x40.175 manufactured by SIG France®, with a length of 150 meters of active section, is divided into three active sections of 50 meters each. Each of the active sections is configured to form one channel, adding the 40 hydrophones that make up each of them, obtaining 50 metres of separation between the three groups. The separation between hydrophones is 1.25 metres. The streamer was at 0.5 – 1.5 m beneath the sea surface. The data is projected to UTM WGS 1984 31N

Assessment of Submarine Landslide Volume

Submarine landslides pose major geohazards as they can destroy seafloor infrastructure such as communication cables and cause tsunamis. The volume of material displaced with the landslide is one factor that determines its hazard and is typically estimated using bathymetric and/or seismic datasets. Here, we review methods to determine the initial failed volume based on a well-constrained case study, the Ana Slide, a small slope failure in the Eivissa Channel off the eastern Iberian Peninsula. We find that not only the availability and quality of datasets but also the emplacement mechanism determines the quality of the volume estimation. In general, the volume estimation based on comparison of modern and reconstructed pre-failure seafloor topographies yields conservative, yet robust volumes for the amount of material that was mobilized. In contrast, volume estimated from seismic data may be prone to overestimation if no detailed constraints on the nature of the chaotic, transparent, or disrupted seismic facies commonly used to identify landslide material are available

Multibeam bathymetry processed data (EM12 S-120 echosounder working area dataset) of RV HESPERIDES during cruise BIG95, Eivissa Channel, Mediterranean Sea

Multibeam bathymetry data were collected during the BIG'95 cruise in 1995 with the Spanish research vessel BIO Hespérides using a SIMRAD EM-12S (13 kHz, 81 beams). The data were processed using SwathEd software to a 50x50 m grid. The data is fully processed from raw files, which are available upon request. The data is projected to UTM WGS 1984 31N

A new methodology to assess the potential of conjectural trigger mechanisms of submarine landslides exemplified by marine gas occurrence on the Balearic Promontory

Highlights • full constitutive model parameter set of marine soil for MCC. • workflow for studies of potential trigger mechanisms exemplified by marine gas occurrence. • quantitative estimation of slope stability (by FOS) on the Balearic Promontory. • Gassy soil can be considered a preconditioning factor, not a trigger mechanism. Abstract The destructive potential of submarine landslides for populated coastal areas and maritime infrastructure has been described many times. However, the geological processes that can trigger such landslide events have not yet been fully established. In order to be able to conclusively assess the trigger potential of these processes, a quantification of the slope stability is indispensable. This requires a precise knowledge of the geotechnical and geological boundary conditions before and after the investigated landslide event, as well as the change in these boundary conditions caused by the alleged trigger mechanism. In order to make these described preconditions and the work process generally applicable, a universally adaptable methodology for the identification of trigger mechanisms was developed. Here it is successfully applied to marine gas occurrence, which has recently been considered as a trigger mechanism due to the negative influence of enclosed gas bubbles on the shear strength of fine-grained soils. The constitutive model by Sultan and Garziglia (2014) is applied to simulate the gas-influenced undrained shear strength of a marine soil from a sediment starved margin on the Balearic Promontory and a range of Finite Element Limit Analyses (FELA) are conducted to determine the resulting loss of stability of different slope geometries. Within the scope of these calculations, the first set of Modified Cam Clay model parameters for a soil from the western Mediterranean is introduced. Based on the simulations, it can be concluded that marine gas occurrence decreases the overall stability of a slope compared to the saturated state. However, it also becomes obvious that the prevailing slope geometries with low inclinations are stable with a substantial capacity reserve for all simulated scenarios. Conclusively, gassy soil can be designated as a preconditioning factor decreasing the slope stability, as the state of failure can only be reached if the slope was in a precarious state before, or in combination with other impacts

A new methodology to assess the potential of conjectural trigger mechanisms of submarine landslides exemplified by marine gas occurence on the Balearic Promontory

The destructive potential of submarine landslides for populated coastal areas and maritime infrastructure has been described many times. However, the geological processes that can trigger such landslide events have not yet been fully established. In order to be able to conclusively assess the trigger potential of these processes, a quantification of the slope stability is indispensable. This requires a precise knowledge of the geotechnical and geological boundary conditions before and after the investigated landslide event, as well as the change in these boundary conditions caused by the alleged trigger mechanism. In order to make these described preconditions and the work process generally applicable, a universally adaptable methodology for the identification of trigger mechanisms was developed. Here, it is successfully applied to marine gas occurrence, which has recently been considered as a trigger mechanism due to the negative influence of enclosed gas bubbles on the shear strength of fine-grained soils. The constitutive model by Sultan and Garziglia (2014) is applied to simulate the gas-influenced undrained shear strength of a marine soil from a sediment starved margin on the Balearic Promontory and a range of Finite Element Limit Analyses (FELA) are conducted to determine the resulting loss of stability of different slope geometries. Within the scope of these calculations, the first set of Modified Cam Clay model parameters for a soil from the western Mediterranean is introduced. Based on the simulations, it can be concluded that marine gas occurrence decreases the overall stability of a slope compared to the saturated state. However, it also becomes obvious that the prevailing slope geometries with low inclinations are stable with a substantial capacity reserve for all simulated scenarios. Conclusively, gassy soil can be designated as a preconditioning factor decreasing the slope stability, as the state of failure can only be reached if the slope was in a precarious state before, or in combination with other impacts.Deutsche Forschungsgemeinschaft (DFG

Decline in Slope Stability as a Consequence of Gassy Soil in Submarine Slopes on the Balearic Promontory

Submarine landslides can entail a substantial hazard for offshore infrastructure as they are capable of triggering tsunamis and may develop into highly mobile turbidity currents capable of breaking seabed cables. Despite considerable research activity, the trigger mechanisms for such landslide events cannot be clearly defined. Recently, marine gas occurrence has been investigated as a possible trigger mechanism. The behaviour of a fine-grained gassy soil is influenced by a variety of micromechanical processes; amongst destructuring due to fracture formation or gas bubble expansion, and bubble flooding with subsequent cavity collapse. Capturing and modeling these processes in order to assess the destructive potential of enclosed gas bubbles in submarine slopes is to date a considerable scientific challenge. With the help of a large number of Finite Element Limit Analyses (FELA), which are based on laboratory tests on a gravity core from the western Mediterranean Sea, submarine slope stability in the respective region was evaluated. Based on these analyses, gassy soil can be defined as a preconditioning factor but not as a capable trigger mechanism for submarine landsliding