Local Tsunamigenic Sources In Greece, Identified By Pattern Recognition

This study is an attempt to determine potential tsunamigenic morphostructural nodes in mainland Greece using pattern recognition algorithms. The earthquakes that have produced local tsunamis in the region were confined to morphostructural nodes whose locations were found by morphostructural zoning. The recognition problem consisted in separating all nodes in the region into the tsunamigenic class and the non-tsunamigenic class based mainly on the geomorphologic parameters of the nodes. The data on tsunamigenic earthquakes in Greece for training the Cora-3 algorithm were taken from the GHTD global historical catalog of tsunamigenic events (http://www.ngdc.noaa.gov/hazard/tsu_db.shtml). The recognition procedure resulted in determining 27 tsunamigenic nodes, with most of these being situated in the southern tip of the Peloponnese Peninsula, as well as in the gulfs of Corinth and Patras. Three tsunamigenic nodes were identified in the area of the Malian Gulf on the Aegean coast of Greece. According to the relevant literature, most local tsunamis in Greece were initiated by submarine slides and slumps due to earthquakes. According to the characteristic geomorphologic features derived in this study, the tsunamigenic nodes are situated in settings of contrasting relief characterized by steep slopes. This favors submarine landslides when subjected to earthquake excitation. The results reported in this paper form a basis for developing a methodology to be used in long-term tsunami hazard assessment, supplying information on local potential tsunamigenic sources required for tsunami regionalization of coastal areas in Greece.

Dating submarine landslides using the transient response of gas hydrate stability

Submarine landslides are prevalent on the modern-day seafloor, yet an elusive problem is constraining the timing of slope failure. Herein, we present a novel technique for constraining the age of submarine landslides without sediment core dating. Underneath a submarine landslide in the Orca Basin, Gulf of Mexico, in 3D seismic data we map an irregular bottom simulating reflection (BSR), which mimics the geometry of the pre-slide seafloor rather than the modern bathymetry. Based on the observed BSR, we suggest that the gas hydrate stability zone (GHSZ) is currently adjusting to the post-slide sediment temperature perturbations. We apply transient conductive heat flow modeling to constrain the response of the GHSZ to the slope failure, which yields a most likely age of ~8 ka demonstrating that gas hydrate systems can respond to slope failures even on the millennia timescales. We also provide an analytical approach to rapidly determine the age of submarine slides at any location.

Evaluation of horizontal submarine slide impact force on pipeline via a modified hybrid geotechnical-fluid dynamics framework

There are situations in offshore energy development where potential impact forces between submarine slides and pipelines need to be estimated. The horizontal slide-pipeline impact force, parallel to the main travel direction of the sliding mass and normal to the pipeline axis, is generally dominant compared to other force components, and hence of particular concern. In practice, pipelines may be suspended at varying distances above the seabed (gap) and existing methods do not consider how this will affect the horizontal slide-pipeline forces. This paper investigates the effects of pipeline-seabed gap and pipeline diameter on the horizontal slide-pipeline impact force via 181 computational fluid dynamics (CFD) simulations at Reynolds numbers of 0.36 - 287. Results show that variation in the pipeline-seabed gap and pipeline diameter alters the slide mass flow behavior as it flows past the pipeline and hence the impact force when the pipeline-seabed gap is below a critical value. A modified hybrid geotechnical-fluid dynamics framework for estimating the horizontal impact force is proposed by considering the effects of the pipeline-seabed gap and pipeline diameter, which is validated with existing experimental datasets.

Numerical simulation of submarine landslide and generated tsunamis : Application to the Mayotte seismo-volcanic crisis

<p>Since May 2018, Mayotte island has experienced an important seismic activity linked to the on-going sismo-volcanic crisis. The epicenters of the seismic swarms are located between 5 and 15 km east of Petite Terre for the main swarm, and 25 km east of Petite Terre for the secondary swarm. Although variations in the number of earthquakes and their distribution have been observed since the start of the eruption in early July 2018 [Lemoine A.(2020), Cesca et al.(2020)], a continuous seismicity persists and could generate several earthquakes of magnitudes close to M4 widely felt by the population. This recurrent seismicity could weaken the steep submarine slopes of Mayotte, as highlighted by the high resolution bathymetry data collected during the MAYOBS cruise in May 2019 (Feuillet et al.,submitted) and trigger submarine landslides with associated tsunamis.</p><p>To address the hazards associated with such events, we analyzed morphological data to define 8 scenarios of potential submarine slides with volumes ranging from 11,25.10<sup>6</sup> to 800.10<sup>6</sup> m<sup>3</sup> and we simulate the landslide dynamics and generated waves. We use two complementary numerical models: (i) the code HYSEA to simulate the dynamic of the submarine granular flows and the water wave generation, and (ii) the Boussinesq FUNWAVE- TVD model simulate the waves propagation and the inundation on Mayotte. The effect of the time at which the models are coupled is investigated.</p><p>The most impacting submarine slide scenarios are located close to Petite Terre at a shallow depth. They can locally generate a sea surface elevation more than a meter in local areas especially at Petite Terre. The various simulations show that parts of the island are particularly sensitive to the risk of tsunamis. Indeed, some scenarios that does not cause significant coastal flooding still seems to cause significant hazards in these exposed areas. The barrier reef around Mayotte has a prominent role in controlling the wave propagation towards the island and therefore reducing the impact on land. It should be noted that the arrival of tsunamis on the coastline is not necessarily preceded by a retreat from the sea and the waves can reach the coasts of Mayotte very quicky (few minutes).</p><p> </p><p>Cesca, S., Letort, J., Razafindrakoto, H.N.T. et al. Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation. Nat. Geosci. <strong>13, </strong>87–93 (2020). https://doi.org/10.1038/s41561-019-0505-5</p><p>Feuillet, N, Jorry, S. J., Crawford, W, Deplus, C. Thinon, I, Jacques, E. Saurel, J.M., Lemoine, A., Paquet, F., Daniel, R., Gaillot, A., Satriano, C., Peltier, A., Aiken, C., Foix, O., Kowalski, P., Laurent, A., Beauducel, F., Grandin, R., Ballu, V., Bernard, P., Donval, J.P., Geli, L., Gomez, J. Guyader, V., Pelleau, P., Rinnert, E., Bertil, D., Lemarchand, A., Van der Woerd, J.et al. (in rev). Birth of a large volcano offshore Mayotte through lithosphere-scale rifting, Nature.</p><p>Anne Lemoine, Pierre Briole, Didier Bertil, Agathe Roullé, Michael Foumelis, Isabelle Thinon, Daniel Raucoules, Marcello de Michele, Pierre Valty, Roser Hoste Colomer, The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption, Geophysical Journal International, Volume 223, Issue 1, October 2020, Pages 22–44, https://doi.org/10.1093/gji/ggaa273</p>

A review of the gas hydrate distribution offshore Chilean margin

In last decades, the Chilean margin has been extensively investigated to better characterize the complex geological setting through the acquisition of geophysical data and, in particular, seismic lines. The analysis of seismic lines allowed identifying the occurrence of gas hydrates and free gas in many places along the margin. Clearly, the gas hydrate reservoir could be a strategic energy reserve for Chile, but, on the other hand, the dissociated of gas hydrate due to climate change could be an issue to face. Moreover, this region is characterized by large and mega-scale earthquakes that may contribute to gas hydrate dissociation and consequent submarine slides triggering. In this context, Chilean margin should be considered a natural laboratory to study the hydrate system evolution.