Cambridge University seismologists have developed a model that may explain why some tsunamis are larger than predicted – such as the Japanese March 2011 tsunami that baffled earthquake experts.
The research published in the journal Earth and Planetary Science Letters, describes a previously overlooked factor that may have made the Japanese tsunami so severe: a huge collapse of soft material on the sea bed, resulting in a much greater movement of water than would have solely been caused by the earthquake.
A tsunami occurs when an earthquake rapidly changes the shape of the sea bed and displaces the water above it.
Professor of Earth Sciences, James Jackson said: “As the [tectonic] plates move against each other, the rocks on their boundaries slowly bend under the pressure, until they eventually crack and slide on faults. When they do, there is an upwards and outwards movement that takes just a few seconds: a movement of 10 metres is a large earthquake and out at sea this causes a tsunami.”
The Japanese government provided a huge investment to allow scientists world-wide access to the collected after the tsunami. The data showed a plate movement of more than 60 metres. However, rocks cannot bend to this extent, suggesting that something else was responsible for increasing the movements.
The researchers interpreted that that the squeezing together of two plates in the earth’s crust not only caused a fracture, but also resulted in a massive collapse of the debris that had built up on the sea bed.
The research throws light on other unusually large tsunamis such as Sumatra in 2004 and Java in 2006. The data from these events has been compared to that of the Japanese tsunami and reveals similarities that strongly suggest that these disasters also resulted partly from the collapse of debris.
“We hope that our research represents a step forward in understanding how large tsunamis occur and in what circumstances they are likely to happen,” said Professor Jackson.