Why does tsunami is considered as harbor wave?

Answer 1

A tsunami is a series of waves generated when a body of water, such as a lake or ocean is rapidly displaced on a massive scale. Earthquakes, landslides, volcanic eruptions and large meteorite impacts all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating.

The word "tsunami" is Japanese for "harbor wave", because tsunamis cause little or no visible effect in deep sea, and often Japanese fishermen would be out at sea fishing in deep sea when a tsunami came, and in the evening they came home and found their home village devastated by the tsunami, and thus they theorized that tsunamis only happen in harbors and elsewhere close inshore.

Although in Japanese tsunami is used for both the singular and plural, in English tsunamis is well-established as the plural. The term was created by fishermen although they had not been aware of any wave in the open water. A tsunami is not a sub-surface event in the deep ocean; it simply has a much smaller amplitude (wave heights) offshore, and a very long wavelength (often hundreds of kilometres long), which is why they generally pass unnoticed at sea, forming only a passing "hump" in the ocean.