The tidal wave that hit Japan in March was formed by two separate tsunamis, in a phenomenon that had been theorized but never before observed.
NASA observations, along with data from European radar satellites, show at least two wave fronts, which merged to form a single, double-height wave far out at sea. As this crossed ocean ridges and undersea mountain chains, the waves were pushed together, increasing its destructive power.
"It was a one in 10 million chance that we were able to observe this double wave with satellites," says NASA research scientist Y Tony Song.
A NASA-French Space Agency satellite altimeter happened to be in the right place at the right time to capture the double wave.
Along with the NASA-European Jason-2 and the European Space Agency's EnviSAT, it passed over the tsunami on March 11. All three carry radar altimeters, which measure sea level changes to an accuracy of a few centimeters. Each satellite crossed the tsunami at a different location, measuring the wave fronts as they occurred.
"Researchers have suspected for decades that such 'merging tsunamis' might have been responsible for the 1960 Chilean tsunami that killed about 200 people in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now,” says Song. "It was like looking for a ghost."
Tsunami hazard maps have up to now considered only the topography near a particular shoreline. But undersea ridges and mountain chains can nudges tsunami waves in different directions, making its destruction appear unpredictable.
This study suggests scientists may be able to create maps that take into account all undersea topography, for better tsunami prediction.
"Tools based on this research could help officials forecast the potential for tsunami jets to merge," says Song. "This, in turn, could lead to more accurate coastal tsunami hazard maps to protect communities and critical infrastructure."