AlbertPacino
Skilled
Beaches 20,000 km away from the devastating tsunami received waves three times larger than some islands much closer to the quake – but why?
The tsunami that ricocheted around the world following the Indian Ocean earthquake on 26 December 2004 left a puzzling pattern of waves in its wake.
Beaches in Peru and Mexico, nearly 20,000 kilometres from the earthquake, received waves that were three times larger than those hitting the shores of the Cocos Islands, just 1700 km away. Now it turns out that the waves were funnelled along underwater structures, such as mid-ocean ridges and continental shelves.
Vasily Titov, from the US National Atmospheric and Oceanic Administration in Seattle, and colleagues used numerical model simulations together with tide gauge and satellite data to study the way the wave propagated around the world.
They found that there were two main factors affecting the manner in which the tsunami wave spread – focusing from the source, and guidance from the topography of the sea floor. Close to the epicentre the waves were controlled mostly by the shape of the earthquake fault and the long-thin rectangle of water it violently displaced.
Reflected energy
“Cocos Island lies to one side, so it didn’t receive much direct energy,†explains Titov. Meanwhile, waves further afield were shaped more by sea floor topography. “The energy shot along mid-ocean ridges and continental shelves, to reach far-field locations like Peru and Mexico,†says Titov.
The simulation also explains why some nearby islands, like Nias, did not suffer much initially, but were hit by a large wave many hours later. “Although Nias was close to the source, 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,†Titov explains.
Now the researchers plan to use their simulation to model the impact of tsunamis originating all over the world, including other large underwater earthquake faults, such as the one running up the west coast of the United States. “The Indian Ocean data has enabled us to verify our model and now we hope to be able to use it as a real-time tsunami warning centre,†says Titov.
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