DEFINITION OF MOVEMENT OF TSUNAMI

Tsunami are waves with exceptionally large distances between

individual crests, and they move like other waves across the

ocean. We define wavelength as the distance between crests,

wave-height as the vertical distance from the crest to the bottom

of the trough, and the amplitude as one-half of the wave

height. Most ocean waves have wavelengths of 300 feet (100

m) or less; tsunami are exceptional in that they have wavelengths

that can be 120 miles (200 km) or greater. When tsunami

are traveling across deep ocean water, their amplitudes are

typically less than three feet (1 m). You would probably not

even notice even the largest of tsunami if you were on a boat in

the deep ocean. Circular or elliptical paths that decrease in size

with depth describe the motion of water in waves. All motion

from the waves stops at a depth equal to one-half the distance

of the wavelength. Tsunami therefore are felt at much greater

depths than ordinary waves, and this effect may be used with

deep ocean bottom tsunami detectors to help warn coastal

communities when tsunami are approaching.

Waves with long wavelengths travel faster than waves

with short wavelengths. Since the longer the wavelength the

faster the wave in deep open water, tsunami travel extremely

fast across the ocean. Normal ocean waves travel at less than

55 miles per hour (90 km/hr), whereas many tsunami travel

at 500 to 600 miles per hour (800 to 950 km/hr), faster than

most commercial airliners!

When waves encounter shallow water the friction of the

seafloor along the base of the wave causes them to slow down

dramatically, and the waves effectively pile up on themselves as

successive waves move into shore. This causes the wave height

or amplitude to increase dramatically, sometimes 15 to 150

feet (4.5–45 m) above the normal still water line for tsunami.

When tsunami strike the coastal environment, the first

effect is sometimes a significant retreat or drawdown of the

water level, whereas in other cases the water just starts to rise

quickly. Since tsunami have long wavelengths, it typically takes

several minutes for the water to rise to its full height. Also,

since there is no trough right behind the crest of the wave, on

account of the very long wavelength of tsunami, the water

does not recede for a considerable time after the initial crest

rises onto land. The rate of rise of the water in a tsunami

depends in part on the shape of the seafloor and coastline. If

the seafloor rises slowly, the tsunami may crest slowly, giving

people time to outrun the rising water. In other cases, especially

where the seafloor rises steeply, or the shape of the bay causes

the wave to be amplified, tsunami may come crashing in

huge walls of water with breaking waves that pummel the

coast with a thundering roar and wreaking utmost destruction.

Because tsunami are waves, they travel in successive

crests and troughs. Many deaths in tsunami events are related

to people going to the shoreline to investigate the effects of

the first wave, or to rescue those injured or killed in the initial

crest, only to be drowned or swept away in a succeeding

crest. Tsunami have long wavelengths, so successive waves

have a long lag time between individual crests. The period of

a wave is the time between the passage of individual crests,

and for tsunami the period can be an hour or more. Thus, a

tsunami may devastate a shoreline area, retreat, and then

another crest may strike an hour later, then another, and

another in sequence.

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