Melissa Palmer, 12, of Lilburn, Ga., for her question:

WHAT IS A TIDAL WAVE?

Tidal waves are large destructive waves that sweep in from the ocean like a giant tide. They are not related to true tides since they usually are caused by undersea earthquakes or by hurricanes far out in the ocean.

When seismographs show that an earthquake occurs in the ocean off the Aleutians, scientists can calculate exactly when the tidal wave will hit the coast of Hawaii. Earthquake tremors travel at speeds of 350 miles per minute or more, while tidal waves move at speeds between 400 to 500 miles per hour. The exact speed depends on the depth of the water.

A cyclone caused tidal wave hit East Pakistan (now Bangladesh) in 1970, killing about 200,000 persons.

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