A2 Tsunamis and Background Disturbances

Topic

Tsunamis are long-period gravity waves—lasting between five and sixty minutes—that originate when the seafloor undergoes a vertical disturbance; this can be caused by an earthquake involving a vertical component of tectonic plate movement, an underwater landslide, a submarine volcanic eruption, or the collapse of a volcanic caldera. Unlike wind waves, which disturb only the uppermost layers of the sea, tsunamis move the entire water column at once—from the seafloor to the surface—and it is precisely this movement of the whole water column that allows a tsunami to carry vastly more energy than a wind wave of the same height.

In the open ocean, a tsunami goes almost unnoticed: its amplitude is barely between one-tenth of a meter and one meter, even though its wavelength spans between one hundred and one thousand kilometers, and it travels at speeds ranging from two hundred to seven hundred kilometers per hour. As it approaches the coast, however, the shallow-water effect causes the wave to slow down; this deceleration leads to a massive amplification of the amplitude—which was so small in the open ocean—before the wave reaches land.

This coastal amplification follows a relationship known as Green's amplification: the tsunami's amplitude increases as the water depth decreases, in inverse proportion to the fourth root of that depth. Consequently, a reduction in depth from four thousand meters in the open ocean to just ten meters near the coast can amplify the wave up to forty times. However, the final height the water reaches on the coast depends not only on Green's amplification but also on the nature of the seafloor and the coastal geometry—factors that can cause the same tsunami to flood one stretch of coastline far more extensively than another, even if both receive the identical wave from the open ocean. Not all tsunamis originate on the ocean floor; meteotsunamis are generated when a fast-moving atmospheric disturbance—such as a storm, a front, or a waterspout—creates resonance on the ocean surface. This resonance—the same Proudman resonance that amplifies the storm surge of a tropical cyclone—occurs when the speed of the atmospheric pressure disturbance matches the propagation speed of a long wave in the water. When this alignment happens, the ocean surface rises by five to twenty centimeters—an amplification that can reach one to two meters along particularly resonant coastlines.

Detecting a tsunami in time relies on warning systems that monitor both seismic activity and sea levels in real time. The time it takes for a tsunami to travel from its source to distant shores is measured in hours—for instance, the 2004 Indo-Pacific tsunami took seven to eight hours to reach the African coast—and it is precisely this travel time that allows for warnings to be issued between five minutes and two hours before the tsunami strikes a given coastline.

Consequently, when a tsunami finally reaches the shore, it does not arrive as a single wave; instead, it comes in successive wave trains over the course of hours. Often, it is the second or third train—not the first—that reaches the greatest height. From the initial vertical displacement of the seafloor to Green's amplification (which magnifies the wave as it nears the coast) and the warning systems racing against that travel time, every aspect of a tsunami ensures that the true threat arrives not with the first visible wave, but with those that follow.

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