December 12, 2013
Historically, Volcanic Island Landslides Do Not Cause Massive Tsunamis
April Flowers for redOrbit.com - Your Universe Online
Researchers at the National Oceanography Center say that the risk posed by tsunami waves generated by Canary Island landslides may need re-evaluation. Their findings, published in the journal Geochemistry, Geophysics, Geosystems, suggest that these landslides result in smaller tsunami waves than previously thought by some authors, because of the processes involved.
The researchers hope that their results will 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. Additionally, the team found that volcanic activity could be a pre-condition to major landslide events in the region.
According to the study, the main factor that influences the amplitude of a landslide-generated tsunami is the volume of material released into the ocean. Prior studies, which have assessed landslide volumes, made the assumption that the entire landslide volume enters the ocean as a single block. These studies, and the media coverage they generated, suggested that an event could generate a "megatsunami" so large that it would travel across the Atlantic Ocean. Such an event would devastate the east coast of the US, as well as part of southern England.
The validity of this theory is in doubt because of the current findings, however. Instead of a single block failure, as previously thought, the landslides of the past have occurred in multiple stages. This reduces the volume of material entering the sea, producing smaller tsunami waves.
Dr James Hunt explains, "If you drop a block of soap into a bath full of water, it makes a relatively big splash. But if you break it up into smaller pieces and drop it in bit by bit, the ripples in the bath water are smaller."
Using deposits of underwater sediment flows — or turbidity currents — which form as the landslide mixes with surrounding seawater, the research team was able to identify the mechanism.
The deposits, called 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. The turbidite samples used in this study were collected from a region of the seafloor hundreds of miles away from the islands.
This was not enough evidence, however, to allow the researchers to assume the 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.
"If you drop the smaller pieces of soap in one by one but in very quick succession, you can still generate a large wave," says Dr Hunt.
The team found mud between the layers of sand deposited by the landslides, providing evidence that the stages of failure occurred some time apart. Mud particles are so fine that they possibly take weeks to settle out in the ocean, according to the researchers. This mud would take even longer to form a layer that would be resistant enough to withstand a layer of sand moving over the top of it.
The findings suggest that the tsunamis are not as large as originally thought, however, the researchers still caution that tsunamis that threaten the UK should be taken seriously.
As part of the Arctic Research Program, the Natural Environment Research Council (NERC) is investing in a major program looking at the risk of tsunamis from Arctic landslides. The European Union also just funded a $98 million FP7 project called ASTARTE. ASTARTE will assess tsunami risk and resilience on the European North Atlantic and Mediterranean coasts.