Exploring Our Fluid Earth

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Fig. 5.30. Tsunamis are caused by large displacements of water in the ocean.

Image copyright and source

Image by Byron Inouye

A tsunami (pronounced "tsoo-nah'-mee") is a series of destructive ocean waves generated by the displacement of a large volume of water. Anything that disturbs a large amount of water has the potential to generate tsunami waves. Potential events might include earthquakes, volcanic eruptions, or landslides (Fig. 5.30), as seen in the following list of actual tsunamis.

Fig. 5.31. A large ferryboat sits on land in Miyako, Iwate, Japan destroyed by the 2011 tsunami.

Image courtesy of US Marine Corps by Lance Cpl. Garry Welch

• Earthquake: Significant loss of life and environmental destruction resulted from the 2011 Japan tsunami that was caused by a magnitude 9.0 (on the 10-pt Richter scale) earthquake 70 km off the coast of eastern Japan. The waves it generated reached as high as 40 m and traveled as much as 10 km inland (Fig. 3.31).
  
   
• Volcanic Eruption: The eruption of Krakatoa in Indonesia in 1883 created a tsunami that killed over 120,000 people.
  
   
• Landslide: A giant landslide produced a massive tsunami in Lituya Bay, Alaska, in 1958. Eyewitness accounts say the tsunami was about 30 m high, it ran up the sides of the bay to heights of over 520 m. Similarly, a tsunami can be produced when parts of a glacier break off into the water.

The large amount of damage that tsunamis can cause comes not only from the direct force of the waves, but also from the indirect effects like fire and flooding.

Most tsunamis are generated by earthquakes. Earthquakes caused by the movements of earth’s crust push up a large volume of water over a wide area. Fortunately, not all earthquakes lead to tsunamis. Geologists estimate that, on average, two tsunamis occur each year; only about one tsunami in every fifteen years causes major devastation.

The term tsunami comes from a Japanese word that means “harbor wave” because of the devastating effects these waves had on low-lying coastal communities. Some people refer to tsunamis as tidal waves, but this word is a misleading description because, unlike true tides, tsunamis are not formed by the gravitational pull and rotation of celestial bodies. Tsunamis may have been misnamed tidal waves because they rarely look like a breaking wave. Instead, tsunami may initially resemble a very forceful and rapidly rising high tide. There are three main differences between tsunamis and tidal waves:

1. Water level changes occur much faster from tsunami waves, often within 10 to 15 minutes. In a true tidal wave, the water level changes gradually over 6 and 12 hr periods.
   
    
2. Tsunami waves travel much faster than the water in tides. The fastest tidal currents in the world move at about 37 km/hr; tsunami waves can travel in deep water at speeds over 900 km/hr.
   
    
3. The incoming waves in a tsunami can become much larger than those in a true tidal wave. The Bay of Fundy has one of the world’s largest tidal ranges, high tide can be as much as 16 m higher than low tide (see Weird Science: Extreme Tidal Ranges). The largest tsunami can be over 40 m high, striking coastlines with much lower tidal ranges.

Tsunamis form wave sets that radiate from their point of origin, like the ripples caused by throwing a rock into a pond. As the tsunami waves travel across the deep open ocean, they are usually no more than 30 cm high, which means ships cannot detect tsunamis passing beneath them. Tsunamis have very long wavelengths ranging from 120 m to 720 km. Because of their wavelengths, the energy of tsunami waves can extend thousands of meters down to the abyssal plain, the bottom floor of the ocean basins. Tsunamis can therefore be classified as shallow-water waves (D < 1/20 L, see the topic Wave Energy and Wave Changes with Depth in the unit Waves). Tsunami wave energy often touches the ocean floor, which means the tsunami’s travel is slowed by friction.

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Fig. 5.32. Tsunami formation

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Image by Byron Inouye

When a tsunami approaches shore its speed decreases and its height, or amplitude, increases (Fig. 5.32). As the tsunami waves reach shallow water and are slowed by contact with the bottom, faster-moving water near the surface piles up, creating a wall of water. Although the wavelength decreases, it still has a long period, so it may take a long time to reach its full height. A tsunami trough, which may precede the first tsunami crest, looks like a very rapidly receding tide. Typically, there are six to eight waves in a tsunami wave set. The first wave may not be the largest. The time between successive wave crests can vary between 5 and 90 minutes.

The 2004 Indonesian Christmas tsunami was the most fatal tsunami in recorded history. This tsunami, generated by an earthquake in Sumatra, inundated low-lying areas throughout the Indian ocean basin and killed over 226,000 people. It was devastating in part because the tsunami was generated relatively nearby and struck low-lying areas, which allowed the tsunami to travel far inshore. It was also devastating because no adequate tsunami warning system was in place in the region. After this tsunami, scientists and political leaders worked to get better warning systems in place for developing nations like Indonesia. There is now an internationally coordinated Tsunami Warning System.

Fig. 5.33. Estimates of tsunami travel time across the Pacific ocean basin for a tsunami originating in Hawai‘i. The concentric dotted circles represent the travel time in hours from Honolulu.

Image by Byron Inouye

Tsunami Warning Centers use earthquake information, tide gauges, and tsunami detection buoys. A tsunami watch is issued when a potential tsunami-inducing earthquake occurs. This message indicates where the earthquake occurred and estimates the time of arrival of the tsunamis at various locations. For example, if the average depth of the ocean is 5 km and the maximum speed of tsunamis is about 800 km/hr, a tsunami generated by an earthquake near Honolulu, Hawai‘i, will take eight hours to reach Tokyo, Japan (Fig. 5.33). Tsunami Warning Centers are able to take into account much more information and generate much more precise estimates.

Generally, areas that may be in danger from tsunamis of distant origin are those less than 15 m above sea level and within 1 km of the coast. For tsunamis of local origin, danger areas are those less than 30 m above sea level and within 1 km of the coast. If an area is very close to the epicenter of the seismic event, residents may only have a short amount of time to evacuate. The tsunamis following the 1964 Good Friday earthquake in Alaska struck Kodiak Island minutes after the quake, but did not reach the Hawaiian Islands until hours later.

Once there is evidence that a tsunami may have formed based on data from tide gauges and buoys, the tsunami watch may be upgraded to a tsunami advisory or warning. An advisory indicates strong currents or dangerous waves may occur; beaches may be closed and ships repositioned or secured. A warning indicates a tsunami of potentially devastating force is expected. Coastal residents are evacuated and other emergency actions are taken.

Residents in low-lying areas should always be prepared for destructive waves because it is difficult for oceanographers to predict the height of the tsunami waves with the same amount of accuracy as they can predict the time the tsunami will arrive. Like other waves, a tsunami striking shore has a height and shape determined by its direction and the shape of the bottom. Thus, the height of a tsunami may vary considerably along a coastline. One coastal community may experience a very destructive tsunami while another close by may be comparatively unharmed. Some of the highest waves occur when the waves enter a funnel-shaped bay or strike a cliff or headland.