In a research from the University of Liverpool the 1929 Grand Banks underwater avalanche was reconstructed to better understand these common geohazards.
The avalanche was triggered by the Mw 7.2 Grand Banks earthquake in 1929. The shock was centered in the Atlantic Ocean off the south coast of Newfoundland in the Laurentian Slope Seismic Zone. The Grand Banks underwater avalanche was huge, generating a tsunami that killed 28 people and burying an area the size of the UK in half a meter of sand and mud. It was highly destructive and broke seafloor telecommunications cables along its path. Deposits of the 1929 event are recorded in the tops of sediment cores located in 4000–5000 m water depth across the Eastern Valley channel network.
The research team mapped the bathymetry of the seafloor where the 1929 avalanche passed through and collected core samples of deposits that it left behind. They then combined this forensic evidence with the historic measurements of flow speed from the old cable breaks to reconstruct the properties of the avalanche. Dr. Stevenson, who was chief sedimentologist on the research cruise, said: "It is awe inspiring when you piece together how big and powerful this avalanche was: 230 m thick, which is about the height of Canary Wharf in London, moving at 40 mph, and highly concentrated with fist-sized boulders, gravel, sand and mud. It would not have been a good place to be at the time".
Dr. Stevenson states that the underwater avalanches still remain a mystery due to the difficulties of measuring directly. ''What tends to happen is that avalanches destroy the measuring equipment you place in their path. The Grand Banks avalanche was the first, and remains the only, giant underwater avalanche that has been directly measured. At the time, it transformed how scientists viewed the seafloor and it's taken almost 90 years for us to revisit the area and confidently piece together its fundamental properties. This research cruise has enabled us to reconstruct the fundamental properties of this underwater avalanche which has implications for seafloor infrastructure. It can help provide engineers and modellers with the information they need to design expensive seafloor installations to withstand similar flows around the world, or build them out of harm's way''. Concluding, he points out that the Grand Banks avalanche is the one real example from which researchers can validate their theories.
Sources: University of Liverpool, Nature.com
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