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A seiche can be described as an oscillation in a body of water. Any enclosed or partially enclosed body of water, such as a lake, bay or harbor, has a number of natural frequencies at which it will resonate, depending on the dimensions of the body. The water can be set into a wave motion at one of these frequencies — for example, by wind or by seismic activity — resulting in standing waves that move vertically, but not horizontally. There will also be one or more nodes, located halfway between the crests of the waves, at which there is no vertical motion. The word "seiche" — which is pronounced "saysh" or "sigh-shh," according to some sources — has apparently been in use in Switzerland as a term for oscillatory waves in lakes since early times and was first used in a scientific context by F.A. Forel, a Swiss seismologist, in 1890.
While the stimulus that created the seiche is present, the amplitude of the waves tends to increase until they cause water to overflow. The seiche may continue for some time after the original stimulus has gone. In large lakes, this phenomenon is very common, but usually very small scale and seldom noticed. Occasionally, however, very large seiches several yards in height can develop, causing flooding and posing a serious danger to people in the lake and on the shore.
Although both can be caused by seismic activity, tsunamis and seiches are quite distinct from one another. A tsunami is a direct result of movement on the ocean floor caused by earthquakes, whereas a seismic seiche results from the seismic waves from an earthquake coinciding in frequency with one of the natural frequencies of a body of water. Seiches and tsunamis can, however, occur together, and in some cases seiche waves can add to the damage caused.
Earthquakes can cause seiche waves in areas very far from the epicenter. The first recorded instance of this phenomenon took place in 1755, when a large earthquake in Lisbon, Portugal produced visible standing waves in several Scottish lochs. There is even an instance of an earthquake in Alaska in 1964 producing standing waves in Australia.
Seiches can also be caused by weather conditions. Water can be set oscillating by the action of wind or by sudden changes in atmospheric pressure. Sometimes high winds will cause water to pile up towards the downwind shoreline, resulting in an oscillation that generates large standing waves — around the Great Lakes of America these are sometimes known as “sloshes.” Some particularly severe sloshes have caused fatalities and damage to boats and shoreline structures.