HOW ARE TSUNAMIS FORMED: Physical Characteristics of Tsunamis

Physical Characteristics of Tsunamis

🌊 Physical Characteristics of Tsunamis

Tsunamis are unique among ocean waves because of their immense energy, long wavelengths, and ability to travel great distances across entire ocean basins. Understanding their physical characteristics helps explain why they are so destructive when they reach coastal areas.

1. 🌐 Wavelength

A wavelength is the distance between two successive wave crests (the highest points of the waves).
Tsunamis have extremely long wavelengths, often ranging from 100 to 500 kilometers (about 60 to 300 miles).
This is much longer than ordinary wind-driven ocean waves, which usually have wavelengths of only 100–200 meters.
Because of their long wavelength, tsunamis behave as shallow-water waves, meaning their movement is influenced by the depth of the ocean floor rather than by surface winds.

2. ⚡ Wave Period

The wave period is the time between the arrival of two successive crests at a specific location.
For tsunamis, this period can range from 5 minutes to over an hour.
This long period means that a tsunami is not a single giant wave, but rather a series of waves (called a wave train) that can strike the coast repeatedly.
Often, the first wave is not the largest — the most destructive waves may come later.

3. 🌊 Wave Height (Amplitude)

In deep ocean waters, a tsunami’s wave height is usually very small — often less than 1 meter (3 feet) — making it almost invisible to ships at sea.
However, as the tsunami approaches shallow coastal waters, the wave slows down and its height increases dramatically due to the process of wave shoaling.
Near shore, the waves can reach heights of 10 to 30 meters (33 to 100 feet) or even higher, depending on the coastal shape and ocean floor topography.
In some narrow bays or inlets, the waves can become much taller due to funneling effects.

4. 🌀 Wave Speed

The speed of a tsunami depends on the depth of the water through which it travels.
The deeper the water, the faster the wave moves.
The formula for tsunami speed is:
$v = \sqrt{g \times d}$
where:
- $v$ = wave speed,
- $g$ = acceleration due to gravity (9.8 m/s²),
- $d$ = water depth.
In deep ocean waters (around 4,000–5,000 meters deep), tsunamis can travel at speeds of 700–800 km/h (450–500 mph) — nearly as fast as a jet airplane.
As they move into shallower water, their speed decreases to around 30–50 km/h (20–30 mph), while their height increases.

5. 🌍 Wave Energy

Tsunamis carry enormous amounts of energy because of their long wavelengths and large volumes of moving water.
Unlike normal waves, whose energy is confined to the surface, tsunami energy extends throughout the entire depth of the ocean.
This energy allows a tsunami to travel thousands of kilometers with very little loss of strength.
For example, a tsunami generated off the coast of Chile can reach Japan or Hawaii with destructive power still intact.

6. 🏖️ Behavior Near the Shore

As a tsunami approaches land, several physical changes occur:
1. Wave speed decreases because the water becomes shallower.
2. Wave height increases dramatically due to compression of energy.
3. Wavelength shortens, causing waves to pile up closer together.
4. The sea level may recede suddenly before the first wave arrives, exposing the ocean floor — a natural warning sign of an incoming tsunami.
Once the wave hits the coast, it can inundate low-lying areas, flowing inland for several kilometers and carrying massive debris with it.

7. 🔁 Wave Reflection and Refraction

Tsunami waves can bend (refract) around islands and reflect off coastlines or underwater ridges, altering their direction and concentrating their energy.
This explains why some regions far from the earthquake’s epicenter can still experience significant destruction, while others nearby may have only minor effects.

8. 🌅 Tsunami Wave Trains

A tsunami is not a single wave, but a series of waves arriving over a period of several hours.
The time between waves can vary from 10 minutes to an hour.
The second or third wave is often the largest and most destructive.
Because of this, it’s dangerous to return to low-lying areas too soon after the first wave recedes.

⚙️ Summary of Tsunami Physical Features

Characteristic	Typical Value or Range	Description
Wavelength	100–500 km	Extremely long, compared to normal waves
Wave Height (Deep Ocean)	< 1 m	Hardly noticeable at sea
Wave Height (Near Shore)	10–30 m or more	Very destructive
Wave Speed (Deep Ocean)	700–800 km/h	As fast as a jet plane
Wave Period	5–60 minutes	Long intervals between waves
Energy Distribution	Entire water column	Not just at the surface
Wave Behavior	Refracts, reflects, and amplifies near coastlines	Leads to uneven destruction patterns

🌐 Conclusion

Tsunamis are powerful natural phenomena characterized by their long wavelengths, high speeds, low amplitudes in deep water, and dramatic height increase near shore. Their physical features make them capable of traveling across entire oceans with immense destructive power. Understanding these characteristics is vital for improving early warning systems, designing coastal defenses, and educating communities to respond effectively when tsunamis strike.

OTHER SOURCES

All types of waves, including tsunamis, have a wavelength, a wave height, an amplitude, a frequency or period, and a velocity.

Wavelength is defined as the distance between two identical points on a wave (i.e. between wave crests or wave troughs). Normal ocean waves have wavelengths of about 100 meters. Tsunamis have much longer wavelengths, usually measured in kilometers and up to 200 kilometers

* Wave height refers to the distance between the trough of the wave and the crest or peak of the wave.
* Wave amplitude- refers to the height of the wave above the still water line, usually this is equal to 1/2 the wave height. Tsunamis can have variable wave height and amplitude that depends on water depth as we shall see in a moment
* Wave frequency or period - is the amount of time it takes for one full wavelength to pass a stationary point.
* Wave velocity is the speed of the wave. Velocities of normal ocean waves are about 90 km/hr while tsunamis have velocities up to 950 km/hr (about as fast as jet airplanes), and thus move much more rapidly across ocean basins. The velocity of any wave is equal to the wavelength divided by the wave period.

V = l/P

Tsunamis are characterized as shallow-water waves.

These are different from the waves most of us have observed on a the beach, which are caused by the wind blowing across the ocean's surface.

Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength of 100 to 200 meters.

A tsunami can have a period in the range of ten minutes to two hours and wavelengths greater than 500 km. A wave is characterized as a shallow-water wave when the ratio of the water depth and wavelength is very small.

The velocity of a shallow-water wave is also equal to the square root of the product of the
acceleration of gravity, g, (980cm/sec/sec) and the depth of the water, d.

V=Ög * d

The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wavelength, it will lose little energy as it propagates.

Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy.

For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in less than one day.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation.

Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of total energy of the tsunami, however, remains constant

Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase.

Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more.

If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably.

Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises (called run-up ).
Run-up is usually expressed in meters above normal high tide.

Run-ups from the same tsunami can be variable because of the influence of the shapes of coastlines. One coastal area may see no damaging wave activity while in another area
destructive waves can be large and violent.

The flooding of an area can extend inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat.

Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters.

A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter high wave because the wavelengths and velocities of tsunamis are so large, the period of such waves is also large, and larger than normal ocean waves.

Thus it may take several hours for successive crests to reach the shore. (For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave period is about 16 minutes).

Thus people are not safe after the passage of the first large wave, but must wait several hours for all waves to pass. The first wave may not be the largest in the series of waves.

For example, in several different recent tsunamis the first, third, and fifth waves were the largest.
How Tsunamis Are Formed.
how-tsunamis-are-formed.

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HOW ARE TSUNAMIS FORMED