When an offshore earthquake near Japan resulted in a destructive tsunami in March 2011, I, along with sailors everywhere, held my breath. My eyes were glued to the Internet. I watched videos of massive volumes of water rushing ashore, displacing families, killing people and destroying villages. It was a harsh reminder of the destructive power of the ocean. Many of my friends and family were grateful I wasn’t sailing on the waters the tsunami traversed. But what if I had been? A tsunami is a huge wave when it rushes onshore, but at sea, even the most perceptive sailor might not notice it.
Most waves on the ocean are caused by wind blowing across the water’s surface, but a tsunami is caused by an earthquake on the seafloor of the deep ocean. As the seafloor shifts and is pushed up or down, a large volume of the overlying water is displaced vertically, sending out waves in all directions. These waves move quickly, up to 500 mph, and have wavelengths that are dozens or even hundreds of miles long, compared to the 30- or 40-yard wavelengths of normal ocean waves.
It is this extreme wavelength and the way it interacts with the ocean floor when it reaches shallow water that differentiates a tsunami. Specifically, there are two main types of ocean waves: shallow-water waves, where the depth is less than half the wavelength; and deep-water waves, where depth is more than half the wavelength. Because tsunamis have very long wavelengths, they are always shallow-water waves, even at their point of origin in the deep ocean.
Although we observe waves as being on the surface, there is also energy and motion extending down into the water. The longer the wavelength, the deeper that energy extends, meaning that tsunamis include motion far beneath them.
As a tsunami approaches land and the seafloor rises below it, all this energy has to expand upward and the wave increase in height. At the same time, friction with the seafloor slows the wave, compressing the wavelength and pushing the crest even higher. Consequently, a gentle tsunami wave at sea, in which the water rises and falls only a few feet over the course of a number of minutes, becomes an immense sledgehammer of water rushing onshore, and can grow to over 100 feet.
When the tsunami reaches shore it can appear as a series of large waves, or as a rising wall of water. It can also appear as a fast-rising tide, which is why they have been called “tidal waves” in the past.
Tsunami is Japanese for “harbor wave.” When fishermen of old went to sea and saw only calm waters, they were shocked when they returned to land to learn their villages had been damaged by a large wave. Because the tsunami seemed to appear only in the harbor they named it, “harbor wave.” It is often said that “a ship is safest at sea.” In the case of a tsunami, that is most certainly true.
Teresa Carey is a U.S. Coast Guard licensed captain and oceanography educator. She currently is co-producing a film about sailing and adventure.
For more, visit teresacarey.com