"Double tsunami" proves double trouble.

The destructive tsunami generated by the March 2011 Tohoku-Oki earthquake was a long-hypothesized "merging tsunami" that doubled in intensity over rugged ocean ridges, amplifying its destructive power before reaching shore, researchers have discovered.

Satellites captured not just one wave front that day, but at least two, which merged to form a single double-high wave far out at sea--one capable of traveling long distances without losing its power. Ocean ridges and undersea mountain chains pushed the waves together, but only along certain directions from the tsunami's origin.

The discovery helps explain how tsunamis can cross ocean basins to cause massive destruction at some locations while leaving others unscathed, and raises hope that scientists may be able to improve tsunami forecasts.

"It was a one-in-ten-million chance that we were able to observe this double wave with satellites," explains Tony Song of NASA's Jet Propulsion Laboratory, Pasadena, Calif., the study's principal investigator. "Researchers have suspected for decades that such 'merging tsunamis' might have been responsible for the 1960 Chilean tsunami that killed many in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now.

"It was like looking for a ghost. A NASA/French Space Agency satellite altimeter happened to be in the right place at the right time to capture the double wave and verify its existence."

The sea floor topography nudges tsunami waves in varying directions and can make a tsunami's destruction appear random. For that reason, hazard maps that try to predict where tsunamis will strike rely on subsea topography. Previously, these maps only considered topography near a particular shoreline. This study suggests scientists may be able to create maps that take into account all undersea topography, even subsea ridges and mountains far from shore.

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