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Sediment gravity flows
Sediment gravity flows are geologically common processes in marine environments 1. They occur in different settings and at a range of scales 1. Sediment gravity flows entail different failure styles (e.g. debris flows, turbidity currents) 2, 3. The latter is controlled by the trigger (e.g. slope failures 1, hyperpycnal flows from river discharge 4, 5, storm surges and volcanic eruptions 6, 7, 8), sediment type and concentration 3, and the slope gradient, among others. Gravity flows are some of the most important processes controlling global sediment and carbon fluxes between continental margins and abyssal plains 1, 9.
Gravity flows also influence the development of sediment basins and control sediment facies distribution 1, 5, 7. They can be highly erosive and actively incise furrows and canyons into the underlying substrate 6, 7, 9. Turbidity currents are also known to be capable of breaking submarine telecommunication cables, as in the 1929 Grand Banks event, where flow velocities reached up to 19 m s-1 2, 10.
The behaviour, time-scale and erosive potential of submarine gravity flows remain poorly understood 8, 9, 11. Submarine gravity flows may occur infrequently, in remote environments and without warning, making them difficult to observe directly 7, 9, 11,. As a result, properties such as flow velocity, which is needed to reconstruct sediment concentration and the geomorphic impact of the flow, are poorly quantified 11. A potential impact of gravity flows on tsunami generation has been theorised, but needs yet to be tested 12, 13.
Sketch outlining the generation and movement of a turbidity current and its deposition. Source: NOAA.
Debris flow in the Illgraben catchment area of the alps, Switzerland. Source: AGU blog, image extracted from youtube video of Pierre-Emmanuel Zufferey.
Studying sediment gravity flows
Different methods have been used to investigate submarine gravity flows in the past 1.
Geophysical data (e.g., sub-bottom profiler (SBP) and multi-channel seismic reflection data (MSC)) can provide information on source areas, erosional and depositional characteristics, and the spatial extent of gravity flow deposits 2, 5, 9. A limitation of these data is their vertical resolution (>30 cm) and lack of age information 1, 2.
Sedimentological analysis of sediment cores allows reconstruction of the gravity flow characteristics (e.g. velocity, concentration) and estimation of their emplacement age 14. The upper sediment column, however, can be distorted or absent as a result of core sampling and correlation of multiple sediment cores is needed for a full stratigraphic information 15.
Numerical models are used to reconstruct flow dynamics, but need sedimentological and geophysical data for calibration 12, 13.
Analog experiments are another approach to reconstruct flow dynamics, but their application is restricted due to scaling issues 16.
Direct observations of gravity flows have been attempted at a number of sites (e.g., Monterey Canyon) 8. The cost associated with developing and deploying these instruments, however, is high and their application on open slopes, where the route of a gravity flow is not always predictable, is difficult 5, 8.
Recent tests have shown that fibre optic cables can be used to detect earthquakes, whereas monitoring of fault displacement will be tested soon[1]17, 18. We hypothesise that fibre optic cables can also be used to detect submarine gravity flows.
Schemata showing different geophysical data acquisition techniques used to investigate the seafloor and sub-bottom structures. Source: USGS.
Top: Gravity corer used during a research cruise to collect sediment from the seafloor. Bottom: Sediment gravity core filled with sediment from the deep sea.
Sediment gravity flows in the western Ionian Basin (central Mediterranean Sea)
The central Mediterranean is one of the most dangerous regions in the European Union in terms of submarine geohazards 4, 19. The continental margin offshore eastern Sicily, in particular, is characterised by geological and structural complexity, and intense seismic and volcanic activity 4, 20, 21. Earthquakes in the region (e.g., 1693, M=8.3, Catania; 1908, M=7.3, Messina; 1990, M=5.6, Augusta) are often associated with destructive tsunamis 19. The sources of many tsunamis that impacted the eastern Sicilian coast have not been constrained 19. This is particularly the case of the 1908 Messina earthquake and tsunami, which resulted in >60,0000 casualties and complete destruction of the towns of Messina and Reggio Calabria 19, 21, 22. A definite rupture fault for this earthquake and a source for the tsunami have yet to be identified 19, 21, 23. It is also still debated whether the tsunami was a consequence of fault rupture, sediment failure or a combination of both 21, 22, 23. Breakage of the Malta and Zante telecommunication cable and analysis of sediment cores showed that this event was accompanied by a turbidity current 14, 24. The associated turbidite unit is thought to be a result of widespread sediment failure with different source regions 14. A detailed assessment of the 1908 turbidity current flow evolution, erosional and depositional processes, and the total volume of sediment displaced, or spatial distribution have not been carried out since the seminal study of Ryan and Heezen in 1965 24. The data used in this study were limited in terms of spatial coverage and resolution.
The geohazard implications of gravity flows in the western Ionian Basin is poorly constrained 4. This knowledge gap needs to be addressed urgently, for a number of reasons. At the local scale, the eastern coast of Sicily is characterised by a high population density and hosts touristic and industrial infrastructure that play an important role in the economy of the island 25. At the regional scale, and because of the small size and semi-closed nature of the Mediterranean Sea 4, marine geohazards pose a high risk to a coastal population of >200 million in >15 countries 26,. Recent estimates have put the probability of a 1 m-high tsunami in the Mediterranean Sea in the next 30 years close to 100% 26. The central Mediterranean also hosts a dense seafloor infrastructure (e.g. submarine cables, pipelines, drilling platforms).
Image showing the destruction due to the 1908 Messina earthquake and tsunami in Italy. Source: Britannica.
Newspaper article from 1908 reporting about the destruction from the 1908 Messina event in Italy and Malta. Source: Times Malta.
Literature
Literature
[1] Mosher, D. C., Moscardelli, L., Shipp, C., Chaytor, J., Baxter, C., Lee, H., and Urgeles, R., 2010. Submarine Mass Movements and Their Consequences. In: Mosher, D. C. et al. (eds.), Submarine Mass Movements and Their Consequences, Advances in Natural and Technological Hazards Research, 28, 1-10, Springer, Dordrecht.
[2] Piper, D. J. W., Cochonat, P., and Morrison, M. L., 1999. The sequence of events around the epicentre of the 1929 Grand Banks earthquake: initiation of debris flows and turbidity current inferred from sidescan sonar. Sedimentology, 46, 79-97.
[3] Talling, P. J., Wynn, R. B., Masson, D. G., et al., 2007. Onset of submarine debris flow deposition far from original giant landslide. Nature letters, 450, 541-544.
[4] Goswami, R., Mitchell, N. C., Argnani, A. and Brockleburst, S., 2014. Geomorphology of the western Ionian Sea between Sicily and Calabria, Italy. Geo-Marine Letters, 34, 419-433.
[5] Goswami, R., Mitchell, N. C., Brocklehurst, S. H. and Argnani, A., 2016. Linking subaerial erosion with submarine geomorphology in the western Ionian Sea (south of the Messina Strait), Italy. Basin Research, 1-18.
[6] Micallef, A., Camerlenghi, A., Georgiopoulou, A., et al., 2019. Geomorphic evolution of the Malta Escarpment and implications for the Messinian evaporative drawdown in the eastern Mediterranean Sea. Geomorphology, 327, 264-283.
[7] Meiburg, E. and Kneller, B., 2010. Turbidity Currents and Their Deposits. Annual Review of Fluid Mechanics, 42, 35–56.
[8] Paull, C. K., Ussler III, W., Greene H. G., Keaten, R., Mitts, P. and Barry, J., 2003. Caught in the act: the 20 December 2001 gravity flow event in Monterey Canyon. Geo-Marine Letters, 22, 227–232.
[9] Mountjoy, J. J., Howarth, J. D., Orpin, A. R., et al., 2018. Earthquakes drive large-scale submarine canyon development and sediment supply to deep-ocean basins. Science Advance, 4, eaar3748.
[10] Heezen, B. C., and Ewing, M., 1952. Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake. American Journal of Science, 250, 849-878.
[11] Stevenson, C. J., Feldens, P., Georgiopoulou, A., et al., 2018. Reconstructing the sediment concentration of a giant submarine gravity flow. Nature Communications, 9, 2616.
[12] Assier-Rzadkieaicz, S., Heinrich, P., Sabatier, P. et al., 2000. Numerical Modelling of a Landslide-generated Tsunami: The 1979 Nice Event . Pure appl. Geophys., 157, 1707–1727.
[13] Fine, I. V., Rabinovich, A. B., Bornhold, B. D., Thomson, R. E., and Kulikov, E. A., 2005. The Grand Banks landslide-generated tsunami of November 18, 1929: preliminary analysis and numerical modeling. Marine Geology, 215, 45-57.
[14] Polonia, A., Nelson, C. H., Romano, S., Valani, S. C., Colizza, E., Gasparotto, G. and Gasperini, L., 2017. A depositional model for seismo-turbidites in confined basins based on Ionian Sea deposits. Marine Geology, 384, 177-198.
[15] Skinner, L. C., and McCave, I. N., 2003. Analysis and modelling of gravity- and piston coring based on soil mechanics. Marine Geology, 199, 181-204.
[16] Amy, L A., Peakall, J. and Talling, P. J., 2005. Density- and viscosity-stratified gravity currents: Insight from laboratory experiments and implications for submarine flow deposits. Sedimentary Geology, 179, 5-29.
[17] Marra, G., Clivati, C., Luckett, R., Tampellini, A., Kronjäger, J., Wright, L., Mura, A., Levi, F., Robinson, S., Xuereb, A., Baptie, B., and Calonico, D., 2018. Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science, 361, 486-490.
[18] Gutscher, M.-A., Royer, J.-Y., Graindorge, D., et al., 2019. Fiber Optic monitoring of active faults at the seafloor: the FOCUS project. Photonic Techniques and Technologies, Special EOS Issue 3, 32-36.
[19] Tinti, S. and Armigliato, A., 2003. The use of scenarios to evaluate the tsunami impact in southern Italy. Marine Geology, 199, 221-243.
[20] Gutscher, M.-A., Kopp, H., Krastel, S., et al., 2017. Active tectonics of the Calabrian subduction revealed by new multi-beam bathymetric data and high-resolution seismic profiles in the Ionian Sea (Central Mediterranean). Earth and Planetary Science Letters, 461, 61-72.
[21] Polonia, A., Torelli, L., Gasperini, L. and Mussoni, P., 2012. Active faults and historical earthquakes in the Messina Straits area (Ionian Sea). Natural Hazards and Earth System Science, 12, 2311–2328.
[22] Billi, A., Funiciello, R., Minelli, L., Faccenna, C., Neri, G., Orecchio, B. and Presti, D., 2008. On the cause of the 1908 Messina tsunami, southern Italy. Geophysical Research Letters, 35, L06301.
[23] Favalli, M., Boschi, E., Mazzarini, F. and Pareschi, M. T., 2009. Seismic and landslide source of the 1908 Straits of Messina tsunami (Sicily, Italy). Geophysical Research Letters, 36, L16304.
[24] Ryan, W. B. and Heezen, B. C., 1965. Ionian Sea Submarine Canyons and the 1908 Messina Turbidity Current. Geological Society of America Bulletin, 76, 915-932.
[25] Armigliato, A., Tinti, S., Pagnoni, G., Zaniboni, F. and Paparo, M. A., 2015. Worst-Case Scenario Tsunami Hazard Assessment in Two Historically and Economically Important Districts in Eastern Sicily (Italy). American Geophysical Union, Fall Meeting 2015, abstract id. NH23C-1891.
[26] Sørensen, M. B., Spada, M., Babeyko, A., Wiemer, S. and Grünthal, G., 2012. Probabilistic tsunami hazard in the Mediterranean Sea. Journal of Geophysical Research, 117, B01305.
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