The tsunami that tore through northern Japan on March 11 was catastrophically strong. The waves—triggered by a 9.0 earthquake—swamped coastal towns, destroyed homes and offices and led to the deaths of nearly 16,000 people. (For a chilling look at the devastation, check out these photos by the Japanese photographer Kishin Shinoyama.) The tsunami also led to a meltdown at the Fukushima Dai-ichi nuclear plant, resulting in the worst atomic accident since Chernobyl—with long-term effects that are still up for debate.
Obviously the sheer power of the quake helps explain why the tsunami was so devastating—but that’s not the only reason. Scientists from NASA and Ohio State University now say that the waves were actually a “merged tsunami,” and that the phenomenon doubled the power of the disaster.
Satellites from NASA and European agencies show at least two wave fronts created by the quake—not just one as you might expect from a single quake. Those wave fronts merged to form a single, double-high wave out to sea. As it traveled towards land, ocean ridges and undersea mountains pushed the wave fronts together, keeping the tsunami stable even as it hurtled towards the coast.
Said NASA research scientist Y. Tony Song at the American Geophysical Union meeting in San Francisco:
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. 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.
As scientific studies go, this one was pure luck—three satellites with the right kind of equipment all happened to be passing over the tsunami as it happened. The information should be able to help scientists better predict where, when and how severely tsunami waves will hit, enabling them to improve predictions and speed warnings when needed. That’s less of an issue for Japan, which already has by far the best tsunami response system in the world, but poorer nations need all the help they can get.
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Bryan Walsh is a senior writer at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME