Walls of Water: the Science and History of Tsunamis

The word tsunami comes from the Japanese: tsu, meaning harbor, and nami, meaning wave. It is a fitting origin. Japan sits in one of the most seismically active regions on Earth, and its people have lived with tsunamis for as long as history has been recorded. The word captures something important: tsunamis are not open-ocean phenomena. They are born in the deep sea and realize their full destructive potential only when they reach the shore.

A tsunami is not a single wave. It is a series of waves, sometimes dozens, generated when a massive displacement of water sets the ocean in motion. The first wave is not always the largest. The sea may appear to pull back dramatically before the waves arrive, exposing the seafloor in a phenomenon that has lured the curious to their deaths throughout history. And the waves can travel for hours and thousands of miles, crossing entire ocean basins without losing their power.

How Tsunamis Form

Tsunamis are fundamentally different from the surface waves generated by wind. Wind waves involve only the surface layer of the ocean. Tsunamis involve the entire water column from surface to seafloor. This is why they carry so much more energy and why they behave so differently from ordinary waves.

A tsunami begins when something displaces a large volume of water suddenly and vertically. The most common cause is an undersea earthquake. When two tectonic plates are locked together at a subduction zone, stress builds over centuries. When the fault suddenly ruptures, one plate lurches upward, sometimes by several meters along a fault zone hundreds of kilometers long. The seafloor displaces upward, and the column of water above it is pushed up as well. Gravity immediately works to flatten this displacement, and the energy radiates outward as waves in all directions.

Not all undersea earthquakes generate tsunamis. The earthquake must generally be large, typically magnitude 7.5 or greater, shallow enough to displace the seafloor, and involve primarily vertical rather than horizontal motion.

Submarine landslides can generate locally devastating tsunamis even without an earthquake trigger. When an unstable mass of sediment on the continental slope suddenly fails, it can displace enormous volumes of water. The 1958 Lituya Bay event in Alaska, triggered by a landslide, sent a wave 524 meters up the opposite hillside, the tallest wave surge ever recorded.