Travis Griggs, age 8, of Gastonia, N.C., for his question:
WHAT CAUSES TIDAL WAVES?
Krakatoa is the name of a volcanic island located halfway between Sumatra and Java in the Sunda Strait. It erupted in August, 1883, and caused one of the world's worst disasters. Much of the island was blown to bits and a huge tidal wave developed that washed over the shores of nearby islands killing about 36,000 people. Volcanic dust floated about the region for more than a year.
A tidal wave is a destructive wave that comes sweeping in from the ocean. While a tidal wave looks very much like a huge tide, it is not related to true tides.
Scientists call a tidal wave tsunami, the Japanese word for storm wave.
A tidal wave can be caused by undersea earthquakes, which are called seaquakes, or by hurricanes far out in the ocean. Using seismographs, scientists are able to predict almost exactly at what time and at what seacoast a tidal wave will arrive.
An earthquake that centers off the Aleutians near Alaska may cause a tidal wave that will hit the coast of Hawaii. On a seismograph in Hawaii, a scientist can see exactly where the quake centered, and then a forecaster can calculate the number of minutes it took for the tremor to reach Hawaii. It will then take about the same number of hours for the water wave to hit Hawaii.
An earthquake tremor will 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.
A tidal wave 50 feet high wrecked Lisbon, Portugal, in 1755. In 1900 Galveston, Texas, was hit by a tidal wave caused by hurricanes at sea. In 1946 severe damage occurred in Hilo, Hawaii, as a result of a tidal wave. In 1970 a cyclone and tidal wave hit East Pakistan, which is now Bangladesh. More than 200,000 people were killed in this disaster.
A seiche is a long wave that can be found in the relatively shallow water of a lake, bay or harbor. The wave may be anywhere from two inches to 10 feet in height. High winds small earthquakes or even atmospheric changes can cause a seiche, which is related in many ways to the tidal wave. Minor seiches occur almost daily on Lake Geneva in Switzerland and on Lake Superior in Canada and the United States.
Small seiches occurred in San Francisco in 1883 and after the disastrous volcanic eruption on the island of Krakatoa.
On June 26, 1954, a seiche struck 25 miles of the Lake Michigan shoreline from Whiting, Ind., to Waukegan, I11. Eight persons were drowned by the wave which was 10 feet high. High winds and a sudden rise in air pressure over the lake apparently formed the seiche.