THE Asian tsunami of 26 December last year sent waves ricocheting around the world. But the pattern of the waves was puzzling. Beaches in Brazil, on the other side of the Earth from the underwater earthquake, were hit by larger waves than the shores of the Cocos Islands just 1700 kilometres away. Now there is an explanation: underwater structures such as mid-ocean ridges and continental shelves funnelled the waves across massive distances.
Vasily Titov of the US National Atmospheric and Oceanic Administration鈥檚 laboratory in Seattle and his colleagues collected data from tide gauges around the world and satellite data of sea-level changes that day. They then used computer models to analyse how the tsunami spread around the globe.
They found that two main factors affected the reach of the tsunami: the direct effect of the earthquake鈥檚 force and how the topography of the sea floor guided the waves. Close to the epicentre the waves were controlled mostly by the shape of the fault and the long, thin body of water the earthquake displaced. 鈥淭he Cocos Islands lie to one side, so they didn鈥檛 receive much direct energy,鈥� says Titov. Tide gauges on the islands recorded waves about 50 centimetres high, while gauges in India and Sri Lanka at a similar distance from the epicentre recorded waves nearly 10 times that.
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Waves further afield were influenced by the sea floor (see Map). 鈥淭he energy shot along mid-ocean ridges and continental shelves, to reach locations like Peru and Mexico,鈥� says Titov. Some beaches there recorded waves more than 50 centimetres high (Science, DOI: 10.1126/science.1114576).
The computer model also explains why some nearby islands, such as Nias, suffered little initially, but were hit by a large wave hours later. 鈥淣ias was close to the source, but it lay to the side of the main energy beam. It received its largest wave around 4 to 6 hours later, reflected back from the shores of Sri Lanka,鈥� says Titov.
Now the researchers plan to use their model to predict the impact of potential tsunamis originating elsewhere, such as along the large underwater fault running up the US west coast.
