Joseph Pauzolis, age 11, of Staten Island, N.Y., for his question:
WHAT CAUSES TIDAL WAVES?
Krakatoa is a name that many historians link with one of the worst tidal wave disasters in the memory of man. Krakatoa is an island located in the Sunda Strait, between Java and Sumatra. In August 1883, a volcano erupted on the island, blowing much of the land to bits. A gigantic tidal wave followed which washed over the shores of nearby islands, killing about 36,000 persons.
Although the term tidal wave is properly applied to the crest of the tide as it moves around the earth, in popular usage it is applied to a destructive wave or to higher water not related to tidal phenomena at all.
There are two types of tidal waves, seismic sea waves and storm or wind waves.
Either a volcanic explosion or a submarine earthquake will cause a seismic sea wave. The quake or eruption displaces water, which results in surface waves that usually radiate from the epicenter of the disturbance.
A famous example of this type of tidal wave happened in April of 1946. An underwater landslide occurred off the Aleutian Islands which caused a great wave that hit the Hawaiian Islands. Havoc ensued in the town of Hilo. The wave moved on and was felt in the Japanese islands and even Chile.
Tidal waves caused by wind or storms usually occur in areas subject to tropical storms and bordering on shallow waters.
In 1900, a destructive tidal wave caused by a storm inundated Galveston, Texas, with winds blowing over 100 miles per hour. The storm was combined with a very low barometric pressure which brought high water more than 15 feet above the mean level.
Scientists using seismographs can predict almost exactly when a tidal wave will hit following an earthquake. It will take about the same number of hours for a tidal wave to hit the same location as the number of minutes it took for the tremor to reach that spot.
The reason scientists can make this calculation is that earthquake tremors travel at speeds of about 350 miles per minute while the tidal wave travels 400 to 500 miles per hour. The exact speed will depend on the depth of the water.
A seiche is a long wave in the relatively shallow water of a lake, bay or harbor. You'll find them on Lake Geneva in Switzerland and on Lake Superior and San Francisco Bay. They are usually caused by high winds or atmospheric changes.
A major seiche struck 25 miles of the Lake Michigan shoreline from Whitting, Ind., to Waukegan, I11., on June 26, 1954.