A sessa is a standing wave in an enclosed body of water caused by natural frequencies and can be triggered by seismic activity or weather conditions. They can cause flooding and danger to people. Sesse and tsunamis are distinct but can occur together. Earthquakes can also cause dry waves far from the epicenter.
A sessa can be described as a swing 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 to which it will resonate, depending on the size of the body. Water can be moved in a wave motion at any of these frequencies – for example, by wind or seismic activity – resulting in standing waves that move vertically, but not horizontally. There will also be one or more nodes, located midway between the crests of the waves, at which there is no vertical motion. The word “seiche” – pronounced “saysh” or “sigh-shh”, according to some sources – has apparently been used in Switzerland as a term for oscillating waves in lakes since ancient times and was first used in a scientific background by FA Forel, a Swiss seismologist, in 1890.
While the stimulus that created the sessa is present, the amplitude of the waves tends to increase until the water overflows. Sex may continue for some time after the original stimulus has passed. In large lakes, this phenomenon is very common, but usually on a very small scale and rarely noticed. Occasionally, however, very large sesse several meters high can develop, causing flooding and posing a serious danger to people in the lake and on the shore.
While both can be caused by seismic activity, tsunamis and seismic events are quite distinct from each other. A tsunami is a direct result of movement on the ocean floor caused by earthquakes, while a seismic sessa results from seismic waves from an earthquake that coincide in frequency with one of the natural frequencies of a body of water. However, sesse and tsunami can occur together and in some cases sesse waves can increase the damage caused.
Earthquakes can cause dry waves in areas far away from the epicenter. The first documented case of this phenomenon took place in 1755, when a large earthquake in Lisbon, Portugal produced standing waves visible in several Scottish lochs. There is also a case of an earthquake in Alaska in 1964 producing standing waves in Australia.
Sesses can also be caused by weather conditions. The water can be swayed by the action of the wind or by sudden changes in atmospheric pressure. Sometimes strong winds cause water to accumulate towards the leeward shore, causing a sway that generates large standing waves – around the Great Lakes of America these are sometimes known as ‘sloshes’. Some particularly severe snowfalls have caused casualties and damage to boats and coastal structures.
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