‘Springy’ Bedrock Sediment Allows for More Destructive Tsunami

THE devastating Indian Ocean tsunami of 2004 may have been made worse by springy sediment on the seabed.

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: if a piece of bedrock slips downwards, for example, 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,” says Denys Dutykh of the École Normale Supérieure 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 Frédéric Dias (www.arxiv.org/abs/0806.2929). The Indonesian earthquake was close to that worst-case ratio, with about 3 kilometres of sediment and a fault 25 kilometres deep. Another possible danger zone is the eastern Mediterranean, but the risk there is still uncertain, says Dutykh, as estimates of sediment depth vary wildly.

Asian Tsunami Disaster – Learn more about the greatest natural disaster in living memory in our comprehensive special report.

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