Tsunamis are often confused with many other types of waves. Here are descriptions of the different kinds:
tidal wave - These waves are caused by the moon’s gravitational attraction. They are generally only a few feet high, although higher waves are found during the new and full moons. They can rise 5 to 6 feet above normal when the new and full moon occurs at the same time as the moon’s perigee (the time when the moon is closest to the earth). Tidal waves are harmless unless accompanied by storm conditions or on unusual coastlines, where the topography creates daily tides as high as 50 feet.
tidal bore - These are quickly advancing front waves of incoming tides and are found in shallow estuaries. It is usually a foaming water wall that signals approaching tides. The height can vary from a few inches to a few feet, depending on the tide’s strength, the attraction of the moon, and the geography of the estuary. In the Tsientang River in China, bores have been recorded at 25 feet traveling 13 knots.
internal waves/underwater waves - These are the strong, vertical motions that a current causes. They usually occur when the current is passing through a narrow passage between a pair of islands, diving into a deep ocean trench, or when two underwater currents of differing density merge together. One can detect internal waves from the surface by noting patch of strong turbulence, which can extend as long as 125 miles. Beneath the ocean surface, the wave, or water of different density, can measure as high as 300 feet. Some experts think that these are responsible for the disappearance of some submarines, causing them to drop below safe traveling depths.
storm waves/sea surges - These are wind-driven waves created on top of normal tides, and are often caused by hurricanes and cyclones. They reach 30 to 40 feet in height, producing a constant pounding motion as opposed to the tsunami’s characteristic single huge wave.
seiche - This is the rhythmic vibration of water in an enclosed water body. Water moves slowly back and froth from shore to shore in waves no higher than 5 feet. They are created by either seismic action or storms.