Tsunami
Danger, danger, danger - Get to high ground. . . .
In its most basic meaning, a tsunami ( pronounced sue-nahm-ee ) is a series of water waves (a tsunami wave train) caused by the displacement of a large volume of water.
Figure 1. Mt. Fuji off Kanagawa
Hokusai (1760-1849) created "Mt. Fuji Off Kanagawa" (popularly known in the West as "The Wave"), between 1826 and 1833.
Water can be displaced by earthquakes, volcanic eruptions, landslides, meteorite impacts, detonations of nuclear devices at sea, or any disturbance above or below water that can push aside or move a large amount of water.
Such waves have a basic pattern to them, and scientists study their height and length. For example, in physics, the wavelength of a wave is the distance over which the shape of the wave repeats itself.
Here's an illustration:
Figure 2.
Here we have the basic vertical (y) and horizontal (x) orientation. The upside down y, λ , represents the length of the wave. So,
- Wavelength is the distance between consecutive points, such as crests, troughs, or when the wave crosses the horizontal ( x ) axis.
- The wave height is measured from the top of the crest to the bottom of the trough.
Tsunamis in deep water can have a wavelength greater than 300 miles (500 kilometers) and a period of about an hour.
This is very different from a normal California tube, which generally has a wavelength of about 300 feet (100 meters) and a period of about ten seconds.
Note This underscores that although tsunami is a Japanese word meaning "harbor wave," they are not harbor waves, nor are they "tidal waves" – because tsunamis are not caused by the tides, which are caused by the gravitational force of the moon on the sea, nor are tsunamis regular waves, which are caused by the wind.
Let's look at the “cause” of these types of waves.
Generation
Tsunamis are seismic sea waves, and as we first mentioned, they are caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes. However, they are most commonly caused by major earthquakes.
Such earthquakes occur along subduction zones. And as a result, they pose a potential danger to coastal communities and islands that dot the Pacific.
Here's an illustration that shows exactly where such earthquakes occur.
Figure 3.
As the ocean plate dives, or subducts, beneath another plate, such movement creates earthquakes. This process is one of move, stop - until when the pressure builds up - and it moves again, releasing the pressure and triggering an earthquake.
Here's how the sea water is moved.
Figure 4.
As you can see, with the release of pressure, the plate punches the water up.
After this huge volume of water has moved, the resulting wave is very long (the distance from crest to crest can be hundred of miles long) but not very tall (roughly 3 feet tall). The wave spreads across the sea in all directions and can travel great distances from the source at tremendous speeds.
Speed
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entire ocean. In comparison, a regular wave (generated by the wind) travels at up to about 90 kph (56 mph). Here's an illustration of a simulated tsunami generated by a large subduction earthquake (8.5) in the Alaska-Aleutian Subduction Zone - and where the waves would be after two hours of travel.
Figure 5.
As you can see, tsunami waves are huge and move very fast.
When it Hits Land
As a tsunami wave approaches the coast (where the sea becomes shallow), the trough (bottom) of the wave hits the beach floor, causing the wave to slow down, to increase in height, and to decrease in wavelength (the distance from crest to crest).
As a result, at landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce higher tsunami waves.
In addition to the tsunami crashing on shore, its waves push a large amount of water up the shore - above the regular sea level. This process is called Run-up, and can cause tremendous damage inland. In fact, runup is more common than huge tsunami waves.
Besides run-up - a vertical measurement - a tsunami can travel inland for a great distance. This is called Inundation, and is a horizontal measurement.
Here's an illustration of these terms.
Figure 6.
When the tsunami reaches the coast and moves inland, the water level can rise many meters (run-up). In extreme cases, water level has risen to more than 15 m (50 ft) for tsunamis of distant origin and over 30 m (100 ft) for tsunami waves generated near the earthquake's epicenter.
The first wave may not be the largest in the series of waves. As a result, the coastal community may see no damaging wave activity while another nearby community may see large, violent, destructive waves.
The flooding (Inundation) can extend inland by 300 m (1000 ft) or more, covering large expanses of land with water and debris.
Here's two photos: A before and an after of a coastal community ( Meulaboh) in Indonesia.
Figure 7. © DigitalGlobe.
Figure 8. © DigitalGlobe.
As you can see, tsunamis can ravage a community.
So, what has been done to give coastal communities warning about a possible tsunami?
Warning Systems
Because of the small wave height a tsunami has during much of its journey through deep water, they are almost impossible to see.
Thus, a reliable early detection system for tsunamis needs to be developed.
Such a system would consists of two equally important components:
The Pacific Tsunami Warning Center (PTWC) in Hawaii is the regional operational center for tsunami information in the Pacific.
Here's an illustration of its system of DART buoys (Deep-ocean Assessment and Reporting of Tsunamis System).
Figure 9.
The DART® system uses ocean going buoys. Here's an illustration how they are part of an early warning system.
Figure 10.
As you can see by the map, this system is focused primarily in the Pacific ocean, but a series of international agreements are underway to expand the system into all of the major oceans prone to tsunami events.
Coupled with this, is the world-wide earthquake monitoring systems in use in most countries, so when an earthquake registers on seismic equipment all of the major national warning organizations are notified.
And while this system development and international integration is not fully functional, it is an on-going project.
Until complete integration, luckily, as dangerous as tsunamis are, they do not happen very often.
Links
Here's a link to pages created by Tony Demark, using images © DigitalGlobe. Tony has created a dramatic series of before and after images of Tsunami destruction. This is extremely helpful in understanding the destruction of tsunamis.
http://homepage.mac.com/demark/tsunami/
Here's a link to the NOAA Center for Tsunami Research:
http://nctr.pmel.noaa.gov/tsunami-forecast.html
Here what to do to get ready for a tsunami:
http://www.noaa.gov/features/tsunami/index.html
Here's animated site showing how the DART system works:
http://nctr.pmel.noaa.gov/Dart/Jpg/DART-II_05x.swf
Figures & Acknowledgments
We want to thank all of the wonderful sites which helped illustrate this discussion, and we wish that all of you visit them as part of your reading.
Figures
Figure 1. ifaq.wap.org
Figure 2. en.wikipedia.org
Figure 3. ioc3.unesco.org
Figure 4. ioc3.unesco.org
Figure 5. nctr.pmel.noaa.gov
Figure 6. ioc3.unesco.org
Figure 7. http://homepage.mac.com/demark/tsunami/16a.jpg
Figure 8. homepage.mac.com
Figure 9. www.ndbc.noaa.gov
Figure 10. www.civildefence.govt.nz
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