London, July 2 : The devastating effect of the Indian Ocean tsunami of 2004 may have been amplified by springy sediments on the seabed.
According to a report in New Scientist, sediment is more elastic than the hard bedrock of the Andaman Sea, where the quake occurred. As a result, it can act like a spring during an earthquake.
For example, if a piece of bedrock slips downwards, the sediment is briefly stretched out vertically before collapsing and compressing.
The effect is to amplify the movement of bedrock, generating a larger wave than would otherwise occur.
This could explain why the 2004 tsunami was far stronger than predicted by computer models of the quake that produced it.
"If you take into account the sediment, a much smaller slip along the fault will give you the same wave size," said Denys Dutykh of the Ecole Normale Superieure in Cachan, France.
This effect is strongest if the thickness of the sediment layer is about 12 per cent of the depth of the fault, according to calculations by Dutykh and colleague Frederic Dias.
The Indonesian earthquake was close to that worst-case ratio, with about 3 kilometres of sediment and a fault 25 kilometres deep.
According to Dutykh, another possible danger zone is the eastern Mediterranean, but the risk there is still uncertain, as estimates of sediment depth vary wildly.