The Science of Tsunamis
Tsunami – The Concept
The word “Tsunami” comes from Japanese, meaning “Harbour Wave.”
Now, notice something interesting:
- It is not just one wave, but rather a series of very long-wavelength waves.
- These waves form in large water bodies like seas and oceans (and in rare cases, large lakes).
- The reason? A major disturbance — it could be above the water surface or below it — but the key factor is that it displaces a large volume of water.
Sometimes, people call it a “tidal wave.” But this is misleading because tides are caused by the gravitational pull of the Moon and the Sun, whereas tsunamis have nothing to do with tides.
Causes of Tsunamis
Think of it this way: wherever a massive displacement of water occurs suddenly, the possibility of a tsunami arises. The main causes are:
- Earthquakes – Example: the 2004 Indian Ocean Tsunami, triggered by a powerful earthquake near Sumatra.
- Volcanic eruptions – Example: the 1883 eruption of Krakatoa which produced gigantic waves.
- Landslides under the sea – Example: the 2018 collapse of Anak Krakatoa caused deadly tsunamis in Indonesia.
- Underwater explosions.
- Meteorite impacts – Rare, but capable of displacing enormous water masses.
👉 Globally, subduction zones are especially prone to tsunamis — like those off Chile, Nicaragua, Mexico, and Indonesia.
👉 Among all oceans, the Pacific Ocean has witnessed the maximum number — over 790 tsunamis since 1990!
Mechanism of Tsunami Waves
1. Disturbance Phase
- In case of a megathrust earthquake, the ocean floor itself gets displaced.
- For example, in 2004, the seabed off Sumatra shifted suddenly, lifting a large column of water.
- This happens because:
- The subducting plate (heavier) moves beneath a lighter plate.
- Stress builds up until the “locked” zone suddenly slips.
- This abrupt movement uplifts parts of the oceanic crust, pushing water upward.
- Similarly, a marine volcanic eruption or a submarine landslide disturbs the equilibrium of the sea level.
- Even an asteroid impact can generate tsunamis, though very rare.
2. Propagation Phase
Now, once the water is displaced, the laws of physics take over:
- Gravity tries to restore the sea surface to its original level.
- But in the process, ripples of energy spread outward at very high speeds — this is the tsunami.
Now, an important transformation occurs:
- In deep water, tsunamis move very fast (hundreds of km per hour), but their amplitude (height) is very small, often unnoticed by ships.
- As they enter shallow waters, their speed decreases.
- Since the total energy remains constant, the height of the waves increases.
- This increase in height near the shore is called the shoaling effect.
- So, a wave hardly noticeable in deep ocean may rise to 10 metres or more near the coast.
3. Arrival at the Shore
This is the most dangerous stage:
- Often, before the giant wave comes, the sea water withdraws unusually far — almost as if the sea is “taking a breath.”
- This is followed by a sudden, massive incoming wave.
- And remember, a tsunami is not just one wave — it usually comes in multiple surges, separated by a few minutes.
- The first wave is not always the biggest — sometimes, the deadliest wave comes later.
Properties of tsunami waves
Basics of Wave Terminology
Before we jump into tsunamis, we need to understand the general language of waves:
- Crest → The highest point of a wave.
- Trough → The lowest point of a wave.
- Wave Height → Vertical distance from the trough bottom to the crest top.
- Amplitude → Half of the wave height.
- Wave Period → Time taken between two successive crests (or troughs) passing a fixed point.
- Wavelength → The horizontal distance between two crests.
- Wave Frequency → Number of waves crossing a point in one second.
👉 These terms are universal — they apply to both normal waves and tsunamis.
Normal Ocean Waves
Now, let us first understand how regular waves behave:
- Ocean water shows two types of motion:
- Horizontal motion → seen in ocean currents and waves.
- Vertical motion → seen in tides.
- Normal wind-generated waves:
- The water particles don’t really travel forward. Instead, they move in circular orbits.
- This means as a wave passes, objects move up and forward (with the crest), and down and backward (with the trough).
- Near the shore, when water depth is less than half the wavelength, the circular motion can’t be sustained, so the wave breaks (dies).
👉 Key point: Normal waves affect only surface water. The deep ocean remains undisturbed.
Tsunami Waves vs. Normal Waves
This is where things get very interesting. Let’s compare:
(i) Wavelength and Period
- Normal waves: Wavelength = a few meters; Period = 5–20 seconds.
- Tsunamis: Wavelength = 500+ km; Period = 10 minutes to 2 hours.
👉 That’s why tsunamis are called long waves — their sheer scale is extraordinary.
(ii) Energy Loss
- Waves lose energy faster if their wavelength is small.
- Normal waves: Short wavelength → energy dissipates quickly, especially when reaching the coast.
- Tsunamis: Very long wavelength → they lose very little energy, allowing them to travel across entire oceans with deadly force.
(For example: The 2004 Indian Ocean tsunami that originated near Sumatra affected East Africa, thousands of km away.)
(iii) Wave Speed
- Normal waves: Rarely exceed 60 kmph.
- Tsunamis:
- At 1000 m depth → 350 kmph.
- At 6000 m depth → 850 kmph.
- Equivalent to the speed of a jet aircraft in deep waters!
👉 This is why ships in the open ocean don’t even notice tsunamis — the wave is so long and low that it passes like a gentle swell, but at tremendous speed.
(iv) Shoaling Effect
- In the deep ocean, tsunami amplitude is tiny compared to its wavelength, so it goes unnoticed.
- As it enters shallow waters:
- Speed decreases (due to friction with seabed).
- To conserve energy, wave height increases dramatically.
- In narrow inlets or harbours, the funnelling effect can amplify waves to 20–30 metres high!
👉 This is why coastal settlements and harbours are at maximum risk.
