Judson Clark, age 16, of Lynn, Mass., for his question:

WHAT CAUSES A TIDAL WAVE?

A tidal wave is a destructive wave that sweeps in from the oceanlike a huge tide. Tidal waves, however, are not related to true tides.

Destructive tidal waves are caused by undersea earthquakes, called "seaquakes," or by hurricanes far out in the ocean.

Scientists use seismographs to predict almost exactly where a tidal wave will arrive at a given seacoast. For example, they know that an earthquake off the Aleutian Islands may cause a tidal wave that will hit the coast of Hawaii.

If a seismograph in Hawaii shows that an earthquake has occured off the Aleutians, the forecaster calculates the number of minutes that it took for the earth tremor to reach Hawaii. It will then take about the same number of hours for the tidal wave to hit Hawaii.

The reason for this is that earthquake tremors travel at speeds of 350 miles per minute or more, while the tidal wave travels 400 to 500 miles per hour. The exact speed depends on the depth of the water.

In 1970, a cyclone and tidal wave struck East Pakistan, now Bangladesh, killing about 200,000 persons. In 1946, a tidal wave severely damaged Hilo, Hawaii, while in 1900 a tidal wave caused by hurricanes at sea brought great damage to Galveston, Texas.