What are Tsunamis, and how are they formed?

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A wave that is are barely noticeable to ships in the deep ocean can transform into a Tsunami waves as it nears the shore. Surprisingly, the wave that initially seemed harmless suddenly gains a huge amplitude - a devastating Tsunami. How does this terrifying transformation happen? Let's explore this.

The slow-moving ocean beds sometimes interact when one tectonic plate slides under another. Did you notice the bulging on the upper layer shown in the image? How long will this motion continue?

Experiment 1 : Understanding the sudden energy release

To understand how this sudden shift happens, let’s look at a simple mechanical demonstration using two tightly clamped wooden pieces. If we keep tightening the clamp to force them past each other, tension builds up until they suddenly slip as shown. Similar to this experiment, ocean beds release centuries of accumulated strain energy in a split second, triggering a Tsunami.

Mechanical demonstration of wooden pieces

Why do earthquakes occur along tectonic plate boundaries?

If you could magically drain the ocean, you would see sections of Earth’s surface called tectonic plates. The surface is divided into several plates that move relative to one another, including seven major tectonic plates.

The molten lava in the Earth’s core circulates continuously due to convective heat transfer. The tectonic plates float on the molten core like ice floats on water.

Due to the continuous motion of lava, the tectonic plates are constantly moving. When two tectonic plates diverge, a rift valley is formed. The East African Rift Valley is a great example of this kind of motion.

Here the tectonic plates converge, producing mountains, trenches, and sometimes active volcanoes, as seen in the Cascade Range of the northwestern USA. The movement of tectonic plates is very slow, just a few centimeters per year.

If we mark dots on all the earthquake-prone areas of the globe, we will find that earthquakes occur where two tectonic plates meet. Their relative motion and the released energy at plate boundaries cause earthquakes.

When an earthquake occurs deep under the ocean, it can trigger a Tsunami. This happens at a subduction zone, where an oceanic plate, denser than the continental crust, sinks beneath the continental plate. In this movement, strain energy accumulates in the top tectonic plate over time. Strain accumulates in the overriding plate, which bulges upward while the oceanic plate descends, and the continental crust bends inward, forming large trenches along the plate boundary.

Look at the deep trenches on the Pacific Ocean floor. How long does this energy buildup last? In some subduction zones it accumulates for centuries; in others the strain is released gradually in slow-slip events that don’t cause Tsunamis. But in places like the Japan Trench and the Chile-Peru Trench, energy can be released in a fraction of a second, producing Tsunamis. Those waves travel over 200 km/h but have low amplitudes typically under half a meter while their wavelengths span hundreds of kilometers. Near the shore, however, decreasing depth changes everything. What effect does shallower water have on the wave?

Experiment 2 : How decreasing water depth reduces wave speed

We can prove the physics of this slowdown using a controlled wave tank experiment. When we generate a wave in a full-depth tank, it takes 1.9 seconds to travel from one end to the other. However, when we cut the water depth in half, the exact same wave takes roughly 2.3 seconds. This demonstrates that decreasing water depth directly reduces wave speed.

Wave shoaling : The physics behind a Tsunami's growth

As the wave nears the shore, its speed drops while its frequency stays constant. This means the wave has to decrease its wavelength in the shallow region. With energy conserved, the wave’s amplitude must increase; this is known as wave shoaling. Consequently, a Tsunami grows exponentially larger near the shore before crashing and causing widespread destruction.

In the open ocean, water particles primarily oscillate in place rather than traveling forward. However, because a Tsunami moves the entire water column from the seafloor to the surface, it carries an immense amount of momentum that pushes a massive volume of water onto the shore.

The earthquake that shook the Indian Ocean

The 2004 Indian Ocean Tsunami was the most devastating Tsunami mankind has ever witnessed. It rose more than 30 meters and killed over 230,000 people. The Tsunami was caused by a massive undersea earthquake near Sumatra, Indonesia. This earthquake was extremely strong, measuring about 9.1 to 9.3 on the Richter scale.

In Sumatra alone, over 100,000 people died. The earthquake occurred when the Indian plate was forced beneath the Burma plate, causing the seafloor to suddenly shift and uplift. It lasted nearly ten minutes, making it one of the longest recorded. The energy released was equivalent to 23,000 Hiroshima atomic bombs. That sudden movement displaced vast amounts of water, generating enormous waves up to 30 meters high that traveled as fast as 800 km/h. Coastal areas near the epicenter, such as Indonesia, were hit within minutes; more distant regions, like India and Africa, were struck hours later.

Beyond Earthquakes : The other ways Tsunamis can form

1. Volcanic Eruptions

Underwater earthquakes are the primary cause of Tsunamis, but three other causes exist. Volcanic eruptions are another major cause. When an underwater volcano erupts, it can blow apart or collapse, pushing water outward and forming large waves eventually creating a Tsunami.

2. Underwater Landslides

Underwater or coastal landslides can also trigger Tsunamis. When large amounts of rock, mud, or ice suddenly fall into the sea, they displace water and generate powerful waves. These waves can be devastating if they reach land.

3. Meteorite Impacts

Although very rare, when a large mass falls into the ocean, it can also cause a Tsunami—such as a meteorite impact. The Tsunami in the Vajont Dam was formed in a similar way, a huge mass from a landslide hit the water body. When a big mass hits the water, it creates a splash that can turn into huge waves. Luckily, this kind of Tsunami doesn't happen often.

Can Tsunamis be prevented?

In 2004, the Indian Ocean had no Tsunami warning system. The 2004 tragedy was a wake up call for all nations. They launched buoys to detect deep ocean changes as early as possible. Seafloor pressure sensors on the ocean floor detect any change in water level height. This pressure data is sent to the buoy, which is fitted with an antenna, and then relayed to a satellite.

Remember, in the 2004 Tsunami, the wave took approximately 20 minutes to reach Sumatra, 1–2 hours to reach Thailand, and 2–3 hours to reach Sri Lanka and India. Because DART buoys transmit data faster than a Tsunami wave travels, authorities are able to evacuate people in Tsunami-prone areas.

Can Tsunamis ever be prevented? Japan believes so. After the catastrophic 2011 earthquake and Tsunami, they began building a 400-km seawall up to 15 meters high to lessen future impacts. Before 2011, wall heights ranged from 5 to 10 meters, and the 15-meter Tsunami easily overtopped them, destroying many seawalls along the northeastern coast, including the double barriers in Taro. Japan’s early warning initially estimated a 3-meter wave, so people assumed it wouldn’t cross the seawall. If a technologically advanced country can misjudge a Tsunami, what confidence can we have in warning systems elsewhere?