Myth: Any big surge of water from the oceans is called a tidal wave; the terms “Tsunami” and “Tidal Waves” mean the same and are interchangeable.
Tsunamis are mistakenly called tidal waves because, when approaching land, they look as a tide which suddenly rushes away and crashes back in a form of a huge wave. However, there is a significant difference between tsunamis and tidal waves based on their origins and characteristics.
A tidal wave is quite a predictable event impacted by the atmosphere. It is a result of the daily tides caused by the imbalanced gravitational influences of the Moon, Sun, and planets. Tidal waves are most pronounced in narrow bays or in rivers along the coast. Due to this fact, water levels may raise by several feet in a matter of hours. It is also possible that a tidal wave will burn out before it reaches the coast. As a rule, tidal waves follow the currents and are unlikely to cause a landfall in areas of temperate climates or northern countries.
A tsunami, on the other hand, is an extraordinary event. It is a series of waves caused by a rapid, massive displacement of the seafloor or disruption of standing water. The ocean floor may be displaced by an earthquake; landslides moving into oceans, bays, or lakes; volcanic eruptions; a crashing asteroid; or underwater explosions, by which the water column is uplifted. But the most common cause for a tsunami is an undersea earthquake. The water above such an event is disturbed so significantly by this uplifting that it creates a surface wave travelling at around 500 mph (on average). Because of the diverse causes, a tsunami has the potential to develop anywhere, unlike a tidal wave.
There is also difference in wavelengths of tsunamis and a tidal waves. While a tsunami differs from 5 minutes to an hour, the wavelengths of a tidal wave differ from 12 to 24 hours.
So, as tsunamis are not related to tides, it is incorrect to consider them a type of tidal wave. Although, the impact of a tsunami could be influenced by the tidal level at the time it strikes.
Bonus Science Stuff
The highest tidal waves are found in the Bay of Fundy, in the Canadian province of New Brunswick, where the water level can rise with the tide by 50 feet.
The word ‘tsunami’ comes from Japanese, meaning “harbor wave”; the origin of the word refers to fishermen who usually didn’t notice tsunami waves in the open sea and only saw the damage on the coast. In German, ‘tsunami’ is ‘Flutwelle’ which means ‘flood wave’ and seems to be more relevant to what it actually is. In the scientific community, tsunamis are often referred to as “seismic sea waves”, even though not all tsunamis are caused by seismic activity.
A tsunami wave isn’t much different in height compared to other waves in the ocean. But, due to its wavelength, it “piles up” when it approaches land. That is why a tsunami generally goes unnoticed in the open ocean. In addition, tsunamis move throughout the depth of the ocean and not just its surface. That is why tsunamis contain such destructive energy and move at great speed for unbelievable distances, still remaining powerful enough to cause heavy damage along coastlines. Tsunamis can also travel up rivers and streams that lead to the ocean.
The Indian Ocean Earthquake on December 26, 2004 had a magnitude of 9.15 and triggered a series of tsunamis that killed approximately 230,000 people across a several countries. It was the deadliest tsunami in recorded history.
There is a natural tsunami warning: approaching tsunamis are usually heralded by noticeable rise or fall of coastal waters.
[Reposted (with editing) from Misconception Junction]