Monday, January 10, 2005

How do tsunamis differ from other water waves?

How do tsunamis differ from other water waves?:

"Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour."

I had not realized that tsunami's have so long a wave length. The article emphasizes the speed and conservation of energy of such a wave crossing the ocean. But think also that a tsunami with a wave length of 100 km, would have higher than average magnitude for 50 km. If you think of a ten-meter high wave, say a thousand km long, that is a lot of water.

700 km per hour, and 50 km long swell means that water would keep piling up on the land for several munutes, then fall away for several minutes, with another several minutes of a second tidal wave following.

Looking at the wind-driven waves at the beach is in no way preparatory for thinking of a tidal wave!

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