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News Navigator: How can a tsunami get from Chile to Japan?
The Mainichi answers common questions readers may have about tsunami, what causes them and how far they can reach.
Question: Chile was recently hit by a massive earthquake, and the Meteorological Agency issued a tsunami warning for Japan. How can a tsunami get all the way to Japan from Chile?
Answer: First of all, tsunami are caused by changes in the topography of the sea bed -- earthquakes or undersea volcanic activity resulting in the surface of the sea suddenly rising or sinking. If the seismic event is a major one, it causes the tsunami's wavelength to lengthen. The longer the wavelength, the more energy the wave can preserve as it travels and the farther it can go.
Q: Is that the only reason?
A: The location of Japan in relation to Chile plays a part in the strength of the wave when it gets here. Japan and Chile are about 17,000 kilometers apart -- or about on the opposite sides of the planet. Just as lines of longitude grow further apart and then join together again as they are drawn from the North Pole to the South Pole, the tsunami caused by the earthquake in Chile spread out around the equator but built back up as it headed north toward Japan. Furthermore, there are almost no obstacles between Chile and Japan.
Q: How fast is a tsunami?
A: A tsunami's speed is proportional to the depth of the water. For example, in the open Pacific Ocean, which reaches a depth of around 4,000 meters, a tidal wave will move at about 720 kilometers per hour -- the same speed as a jet aircraft. In 80 meters of water, however, the wave's speed will drop to around 100 kilometers per hour.
Q: What's the difference between a tidal wave and a tidal surge?
A: A tidal surge is caused by severe winds, like those of a typhoon, over an area of ocean, and has a wavelength of between a few meters to several hundred meters. The power of each wave is quite small, and the surge is broken up when it strikes the coast. A tsunami has a wavelength of between a few to several hundred kilometers, and is essentially a wall of water carrying tremendous force that can cause equally tremendous destruction.
Q: Is there anything we should keep in mind that might warn us of an oncoming tidal wave?
A: Large tsunami sometimes strike without warning, while there is also a tendency for succeeding waves to be more powerful than the first one that hits. In fact, the tsunami that hit Japan after a major Chilean earthquake in 1960 was just such a wave. People should not be complacent if the first wave of a tsunami is small, but rather keep careful track of Meteorological Agency warnings while taking refuge on high ground until the all clear. (Answers by Toshiyasu Kawachi, Science and Environment News Department)
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(Mainichi Japan) March 1, 2010