Japan was hit by 'double tsunami'

Scientists recreate how multiple waves from undersea quake merged into single front that devastated north-eastern Japan

The tsunami that devastated the north-east coast of Japan on 11 March was created by at least two wave fronts that merged to form a far more destructive "double tsunami", scientists in the US have said.

Waves created when a magnitude-9.0 earthquake struck off the coast came together to create a "merging tsunami" captured by satellites for the first time, according to Nasa and researchers at Ohio State University. Peaks and troughs on the ocean floor helped channel the waves into one huge wave, amplifying its destructive force, they said.

The tsunami swept across a long stretch of coastline, swallowing up entire towns and villages, and leaving almost 20,000 people dead or missing.

Nasa said two of its satellites and a European satellite happened to be passing over the tsunami on the day of the disaster. They were equipped with instruments capable of measuring changes in sea levels to an accuracy of a few centimetres.

"Nobody had definitively observed a merging tsunami until now," said Y Tony Song, a research scientist at Nasa's jet propulsion laboratory in California. "It was a one in 10 million chance that we were able to observe this double wave with satellites."

Song said the same phenomenon could have caused the Chilean tsunami in 1960, in which 200 people in Japan and Hawaii were killed. He described previous attempts to acquire images of similar waves as they travelled towards land as "like looking for a ghost".

The satellite images show how two wave fronts merged to form a single, bigger wave far out at sea. It was then pushed in a certain direction by underwater ridges and mountain chains, sustaining its force as it roared towards the shore.

Graphics of the Japan tsunami produced by Nasa showing how two waves merged into one
One of the Nasa satellite radar reconstructions showing how the tsunami fanned out across the Pacific carrying the energy of several merging tsunami 'jets'. Photograph: Nasa/JPL-Caltech/Ohio State University

Scientists say the data will improve their understanding of how tsunami move across oceans, and may help them improve tsunami forecasts.

"Tools based on this research could help officials forecast the potential for tsunami jets to merge," Song said. "This in turn could lead to more accurate coastal tsunami hazard maps to protect communities and critical infrastructure."

The researchers said they had verified their findings using independent data, including GPS readings from Japan and buoy data provided by US authorities.

Japan has the most sophisticated tsunami warning system in the world but was taken by surprise by the sheer force and height of the waves that ruined coastal communities in three prefectures in March.