What is a Tsunami?

A tsunami is a series of waves caused by a sudden disturbance of a large body of water such as a sea, ocean, or lake. The origin of the word comes from Japanese, where “tsu” means “harbor” and “nami” means “wave.” Tsunamis are relatively common in Japan.

Tsunamis generally consist of a series of waves, with periods ranging from minutes to hours, arriving in multiple waves, called a wave train. Large events can generate wave heights of tens of meters.

Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous, and they can affect entire ocean basins. The 2004 Indian Ocean tsunami caused tidal fluctuations globally, including here in Trinidad and Tobago.

Tsunamis are not tidal waves

Tsunamis are sometimes (and mistakenly) referred to as tidal waves. Tidal waves result from changes in gravity on Earth from the Sun and Moon. In extreme circumstances, tidal bores can occur.

Tidal waves have a period (meaning the time between peaks) of approximately 12 hours with a steady displacement of water. Tidal impacts generally remain localized to the coast (bays, beaches, etc.)

Tsunamis and tides both produce waves that can progress inland, but tsunamis travel much further inland and cause significant damage.

How does a tsunami differ from a usual wave?

Tsunamis move onshore more like a surge than just a wave. (NOAA)
Tsunamis move onshore more like a surge than just a wave. (NOAA)

Normal ocean waves are caused by the wind, weather, tides, and currents, whereas tsunamis are powered by a geological force.

Tsunami waves are surface gravity waves formed as the displaced water mass moves under the influence of gravity and radiates across the ocean like ripples on a pond. Regular wind waves only involve the motion of the uppermost layer of the water. Still, tsunami waves involve the movement of the entire water column from the surface to the seafloor.

Characteristics of waves at the shore
Characteristics of waves at the shore

Both common ocean waves and tsunami waves have a crest and a trough and can be described by their period (time between two successive waves), wavelength (horizontal distance between waves), speed, and amplitude (wave height).

In the open ocean, tsunamis have extremely long wavelengths (the distance from one wave crest to the next wave crest), which can be between 100 and 300 km, whereas normal ocean waves have wavelengths of only 30 or 40 meters. Because a tsunami has such a large wavelength, its energy mobilizes the entire water column down to the sea bed.

Propagation of ocean swells with wave characteristics
Propagation of ocean swells with wave characteristics

Deepwater ocean surface waves cause water motion to a depth equal to half their wavelength. Ocean surface wave motion only reaches a depth of a few hundred meters or less. In contrast, tsunami waves behave like “shallow water waves” in the deep ocean. Their period is also very long, mostly an hour in deep water, whereas the period of common waves ranges from 1 to 30 seconds.

While traveling in the deep ocean, tsunamis have small amplitudes (wave height) of less than 3 ft (1 m) and negligible wave steepness, which is why they are not frequently noticed by people in ships, causing nothing more than a slight swell usually about 300 mm above the normal sea surface (that is a gentle rise and fall for most vessels).

Swells versus Tsunami features summary by wave type
Swells versus Tsunami features summary by wave type

Tsunami waves are much faster than wind-generated waves. Because a tsunami behaves like a “shallow water wave,” the speed of a tsunami wave is based on the water’s depth. Typically a tsunami wave travels across a deep ocean at an average speed of 400 to 500 MPH (800 KM/H) or more, whereas normal ocean waves travel at speeds of 5-60 MPH (8-100 KM/H), but the tsunami waves slow down dramatically as it approaches land and the sea shallows.

What happens when the tsunami reaches land?

The tsunami wave shoaling effect - as the tsunami wave reaches the shoreline, the wave slows down and builds in height.
The tsunami wave shoaling effect – as the tsunami wave reaches the shoreline, the wave slows down and builds in height.

Tsunami waves only become dangerous once they reach the shallow waters near the coast in a wave shoaling process. In coastal areas where water levels gradually become shallower, the wave will slow down dramatically, become compressed, and grow steeper due to the decreasing water depth.

The process of steepening of the wave’s surface is like a whip action. As the wave descends the whip from handle to tip, the energy is released into a smaller and smaller mass. This energy is transferred like the cracking of a whip in a violent crescendo.

When the wave enters shallow water, it slows down, and its height (amplitude) increases. When the tsunami wave enters shallow water, its length shortens, and its height rises. Thus the wave is forced upward into a towering wall of water, sometimes as much as 100 ft (30 m) high. This is because of the mass and energy of a tsunami wave.

In wind-generated waves, only the upper layers are moved, but with a tsunami wave, the complete mass of water is being propelled into motion from the seabed to the surface. When these giant waves break, they often destroy piers, buildings, and beaches and take human life.

Not just one wave!

Tsunami wave train approaching the Japanese coastline after the 2011 Tohoku earthquake, note tsunami has already made landfall (left; Photo by Douglas Sprott is licensed under CC BY-NC 2.0). Gauge data showing the wave train from the 1960 Chilean earthquake tsunami along the Japanese coast (Onagawa city) with tide data superimposed (right, Modified from Atwater et al. 1999)
Tsunami wave train approaching the Japanese coastline after the 2011 Tohoku earthquake (Top Photo by Douglas Sprott). Gauge data showing the wave train from the 1960 Chilean earthquake-tsunami along the Japanese coast (Onagawa city) with tide data superimposed (Bottom: Modified from Atwater et al. 1999)

A wave train is a series of waves created by a single tsunami event. These waves can be of varying heights. After the initial wave, several waves will follow. In some cases, four or five smaller waves may immediately follow the first wave crashing into the land like a speeding vehicle.

Tsunami waves reach the shore as a series of successive “crests” (high water levels) and “troughs” (low water levels). These successive crests and troughs can occur anywhere, and the period (the time between the crests of a tsunami’s waves) generally varies from 5 to about 90 minutes. They usually occur 10 to 45 minutes apart. After a powerful tsunami event, multiple waves may arrive over a prolonged time period. Long after the initial event, smaller tsunamis may continue over several days.

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