However, the scientists could not assume that multistage failure necessarily results in less devastating tsunamis - the stages need to occur with enough time in between so that the resulting waves do not compound each other. Turbidites provide a record of landslide history because they form from the material that slides down the island slopes into the ocean, breaks up and eventually settles on this flatter, deeper part of the seafloor. Their deposits, known as 'turbidites', were collected from an area of the seafloor hundreds of miles away from the islands. The scientists were able to identify this mechanism from the deposits of underwater sediment flows called turbidity currents, which form as the landslide mixes with surrounding seawater. But if you break it up into smaller pieces and drop it in bit by bit, the ripples in the bath water are smaller." Lead author Dr James Hunt explains: "If you drop a block of soap into a bath full of water, it makes a relatively big splash. Instead of a single block failure, the landslides in the past have occurred in multiple stages, reducing the volumes entering the sea, and thereby producing smaller tsunami waves. The recent findings shed doubt on this theory. Such studies - and subsequent media coverage - have suggested an event could generate a 'megatsunami' so big that it would travel across the Atlantic Ocean and devastate the east coast of the US, as well as parts of southern England. Previous efforts, which have assessed landslide volumes, have assumed that the entire landslide volume breaks away and enters the ocean as a single block. The main factor influencing the amplitude of a landslide-generated tsunami is the volume of material sliding into the ocean. The study also concluded that volcanic activity could be a pre-condition to major landslide events in the region. The findings - reported recently in the scientific journal Geochemistry Geophysics Geosystems - can be used to inform risk assessment from landslide-generated tsunamis in the area, as well as mitigation strategies to defend human populations and infrastructure against these natural hazards. They found that each large-scale landslide event released material into the ocean in stages, rather than simultaneously as previously thought. The researchers used the geological record from deep marine sediment cores to build a history of regional landslide activity over the last 1.5 million years.
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