What Is a Tsunami?

Japan Struggles To Deal With Nuclear Crisis And Tsunami Aftermath
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Definition

The word tsunami is a Japanese word meaning "harbor wave," but in modern usage, it refers to an ocean wave caused by water displacement, as compared to a normal ocean wave, which is caused by winds or normal gravitation influence of the sun and moon. Undersea earthquakes, volcanic eruptions, landslides or even underwater explosions can displace water to create a wave or series of waves--the phenomenon known as a tsunami.

Tsunamis are often called tidal waves, but this is not an accurate description because tides have little effect on giant tsunami waves. Scientists often use the term "seismic sea waves" as a more accurate title for what we commonly call a tsunami, or tidal wave. In most cases, a tsunami is not a single wave, but a series of waves. 

How a Tsunami Begins

The strength and behavior of a tsunami are difficult to predict. Any earthquake or undersea event will alert authorities to be on the lookout, but most undersea earthquakes or other seismic events do not create tsunamis, which is in part why they are so difficult to predict. A fairly large earthquake may cause no tsunami at all, while a small earthquake may trigger a very large, destructive one. Scientists believe that is not so much the strength of an earthquake, but its type, that may trigger tsunamis. An earthquake in which tectonic plates abruptly shift vertically is more likely to cause a tsunami than lateral movement of the earth.

Far out in the ocean, tsunami waves do not get very high, but they move very fast. In fact, the National Oceanic and Atmospheric Administration (NOAA) reports that some tsunami waves can travel hundreds of miles per hour--as fast as a jet plane. Far out as sea where the water depth is great, the wave can be almost imperceptible, but as the tsunami gets closer to land and the ocean depth decreases, the speed of the tsunami wave slows down and the height of the tsunami wave increases dramatically—along with its potential for destruction.

As the Tsunami Approaches the Coast

A strong earthquake in a coastal region puts authorities on alert that a tsunami may have been triggered, leaving a few precious minutes for coastal residents to flee. In regions where the danger of tsunami is a way of life, civil authorities may have a system of sirens or broadcast civil defense warnings, as well as established plans for the evacuation of low-lying areas. Once a tsunami makes landfall, the waves can last from five to 15 minutes, and they do not follow a set pattern. NOAA warns that the first wave may not be the largest.

One signal that a tsunami is imminent is when the water retreats far from shore very rapidly, but by this time you very little time to react. Unlike the depiction of tsunamis in movies, the most dangerous tsunamis are not those that hit shore as towering tall waves, but those with long surges that contain a huge volume of water that can flow inward over land for many miles before dissipating. In scientific terms, the most damaging waves are those that arrive at the shore with a long wavelength, not necessarily a large amplitude. On average, a tsunami lasts about 12 minutes--six minutes of "run up" during which the water may flow inland for a considerable distance, followed by six minutes of drawback as the water recedes.

However, it is not uncommon for several tsunamis to hit over a period of several hours.

Tsunamis In History

  • The first historically recorded tsunami was in 426 BCE, documented by the Greek historian Thucydides in the book History of the Peloponnesian War, in which he postulated that ocean earthquakes were the cause of such waves.  
  • An earthquake-caused tsunami in 365 CE devastated Alexandria, in Northern Africa. 
  • The 1908 Messina earthquake and tsunami killed more than 123,000 lives in Sicily and Calabria.
  • On the day after Christmas 2004, a violent earthquake struck in the Indian Ocean, off Indonesia. The energy released by the quake set off a tsunami that hit the coasts of Indonesia, Sri Lanka, India, and Thailand. More than 200,000 people perished.
  • In March 2011, a 9.0 magnitude earthquake rocked Japan, sending a massive set of waves on its coastline. Over 18,000 people died; buildings, roads, seaports, and railroads were destroyed; a nuclear power plant was severely damaged.

    Environmental Consequences of Recent Tsunamis

    The death toll and human suffering caused by a tsunami understandably preempt environmental concerns, but when a large tsunami scours everything down to bare earth, the resulting marine pollution is also devastating and can be observed from great distances. When waters are receding from flooded lands, they take with them a large amount of debris: trees, building materials, vehicles, containers, ships, and pollutants like oil or chemicals.

    Several weeks after the 2011 Japan tsunami, empty boats and pieces of docks were found floating off the Canadian and U.S. coast, thousands of miles away. However, much of the pollution from the tsunami was not so visible: tons of floating plastic, chemicals, and even radioactive material continue to swirl in the Pacific Ocean. Radioactive particles released during the Fukushima nuclear power meltdown worked their way up the marine food chains. Months later, bluefin tuna, which migrate long distances, were found with elevated levels of radioactive cesium off the coast of California.