EVALUATION OF TSUNAMI FAR-FIELD EFFECTS FROM POSTULATED COLLAPSES OF STRATOVOLCANOES
Dr. George Pararas-Carayannis - Paper published in Science of Tsunami Hazards, Vol 20, No.5, pages 251-277, 2002.
…Thus, these models forecast incorrectly tsunami far field effects. Shallow water effects, which are due to the nonlinear nature of the tsunami, are treated as linear and overestimated. Only waves of much longer wavelength can propagate effectively across ocean basins. Even though local destructive tsunami waves can result from the postulated mechanisms, waves of such short periods will rapidly decay away from the source region with considerable height attenuation….
Subsequent modeling by Mader (2001) confirms this and provides realistic estimates of tsunami far-field effects for the same hypothetical La Palma slide. Using the wave profile output obtained from a high speed (110 meters/second), pneumatic landslide generator of the Swiss Federal Institute of Technology at Zurich, Switzerland (Fritz, 2001), and based on a "worst case" scenario for La Palma (650 meter high, 20 kilometer radius water wave after 30 kilometers of travel), Mader's numerical model treats the resulting tsunami as an intermediate wave of short wavelength and period - taking into account both dispersion and geometric spreading effects. Specifically, the shorter period and wave amplitudes in his model, result in significant wave height attenuation with distance - to less than one-third of the shallow water amplitudes. The upper limit of his modeling study shows that the east coast of the U.S. and the Caribbean would receive waves less than 3 meters high. The European and African coasts would have waves less than 10 meters high. However, full Navier-Stokes modeling of the same La Palma failure, brings the maximum expected tsunami wave amplitude off the U.S. east coast to about one meter.
Mega-tsunami to devastate US coastline
A tsunami wave higher than any in recorded history threatens to ravage the US coastline in the aftermath of a volcanic eruption in the Canary Islands, UK and US scientists will report today. Locations on both African and European Atlantic coastlines - including Britain - are also thought to be at risk.
The new research, a collaboration between Dr. Simon Day of the Benfield Greig Hazard Research Centre at UCL and Dr. Steven Ward of the University of California, reveals the extent and size of the mega-tsunami, the consequence of a giant landslide that may be triggered by a future eruption of the Cumbre Vieja volcano.
Previous research by Simon Day and colleagues predicted that a future eruption would be likely to cause a landslide on the western flank of Cumbre Vieja. A block of rock approximately twice the volume of the Isle of Man would break off, travelling into the sea at a speed of up to 350 kilometres per hour. The disintegration of the rock, this earlier study predicted, would produce a debris avalanche deposit extending 60 kilometres from the island. The energy released by the collapse would be equal to the electricity consumption of the entire United States in half a year.
The new model - which provides further insights into the consequences of the collapse - predicts that the landslide would create an exceptionally large tsunami with the capability to travel great distances and reaching speeds of up to 800 kilometres per hour. Immediately after Cumbre Vieja's collapse a dome of water 900 metres high and tens of kilometres wide will form only to collapse and rebound. As the landslide continues to move underwater a series of wave crests and troughs are produced which soon develop into a tsumani 'wave train' which fuels the waves progress. After only 10 minutes, the model predicts, the tsunami will have moved a distance of almost 250 kilometres.
The greatest effects are predicted to occur north, west and south of the Canaries. On the West Saharan shore waves are expected to reach heights of 100 metres from crest to trough and on the north coast of Brazil waves over 40 metres high are anticipated.
Florida and the Caribbean, the final destinations in the North Atlantic to be affected by the tsunami, will have to brace themselves for receiving 50 kilometre high waves - higher than Nelson's column in London, some 8 to 9 hours after the landslide. Towards Europe waves heights will be smaller, but substantial tsunami waves will hit the Atlantic coasts of Britain, Spain Portugal and France.
For tsunamis striking flat-lying coastline regions such as Florida, calculating the inundation distance - the extent to which water penetrates inland taking the form of fast moving floods after waves break - is crucial to assessing potential damage. Dr. Day and his colleagues estimate inundation distances in the region of several kilometres from the coast. Accurate estimates of the scale of economic loss are yet to be made but are thought to be in the multi-trillion USD range.