Deep Ocean Floor Can Focus Tsunami Waves

As the waves of a tsunami approach a coastline, the topography of the seafloor near the coast plays a major role in determining how large those waves become and what places get hit harder than others.

For example, when the waves of the massive tsunami generated by last year’s magnitude 9.0 Japan earthquake crossed the Pacific Ocean and reached the U.S. West Coast, they hit Crescent City, Calif., particularly hard because of two features of the seafloor off the coast: a piece of the ocean floor raised by tectonic activity that runs directly toward the city and the position and shape of the city’s harbor.

Scientists had suspected that the same phenomenon might also take place in the deep ocean, where underwater mountains, called seamounts, chasms and even islands could deflect tsunami waves in some places and amplify them in others.

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