Amid Alert For Japan, How Earthquakes Cause Tsunami Explained

Last Updated:April 20, 2026, 17:22 IST

The biggest waves were forecast to hit Iwate and Aomori prefectures at the top of Japan's main Honshu island, and the northern island of Hokkaido. News18 explains

A magnitude 7.5 earthquake struck off the northeastern coast of Japan on Monday, as authorities urged residents to stay away from coastal areas where tsunami waves of up to 3 metres (9.84 ft) were expected.

How does an earthquake cause a tsunami? News18 explains.

What happened in Japan?

The biggest waves were forecast to hit Iwate and Aomori prefectures at the top of Japan’s main Honshu island, and the northern island of Hokkaido, authorities said.

In the hour following the earthquake, which struck at 4:52 p.m. (0752 GMT), tsunami waves as high as 80 cm had been detected, while warnings remained for waves as high as 3 metres.

Several port towns including Otsuchi and Kamaishi, both hard-hit by a massive earthquake and tsunami in 2011, issued evacuation orders for thousands of residents, according to public broadcaster NHK.

Prime Minister Sanae Takaichi said the government had set up an emergency task force and urged citizens in the affected areas to evacuate to safety.

What causes a tsunami?

A tsunami is caused by an earthquake when the seafloor suddenly shifts vertically, displacing a massive volume of water above it. This movement creates a series of waves that radiate outward in all directions, similar to the ripples formed by throwing a rock into a pond.

Most earthquake-generated tsunamis occur at subduction zones, where one tectonic plate is forced beneath another. As the subducting plate slides down, the two plates often become “stuck" due to friction, causing the overriding plate to bend and store energy like a compressed spring. When the stored energy exceeds the friction, the overriding plate snaps back into its original position, thrusting the seafloor and the water column above it upward.

This sudden “shove" creates a large dome or bulge of water on the surface. Gravity then pulls this water back down, sending waves racing across the ocean in all directions.

Sometimes, the shaking from an earthquake triggers underwater landslides, which also displace enough water to start a tsunami.

Can every earthquake cause a tsunami?

Not every earthquake causes a tsunami. According to the International Tsunami Information Center and NOAA, several conditions must be met:

The earthquake must occur under the ocean or very close to the coast.

Usually, a magnitude of 7.5 or higher is required to produce a destructive tsunami.

It must be a shallow event (typically less than 70 km deep) to ensure enough energy reaches the seafloor to move it.

There must be significant vertical displacement. Earthquakes with horizontal movement (like those on strike-slip faults like the San Andreas) rarely cause major tsunamis.

How a tsunami progresses

Once triggered, a tsunami moves through several distinct phases:

Initiation: The water is displaced at the earthquake’s epicenter.

Propagation: In the deep ocean, waves can travel at speeds over 800 km/h (matching a commercial jet). These waves are often only a few feet high and have very long wavelengths, making them nearly invisible to ships.

Amplification (Shoaling): As the wave enters shallow coastal waters, friction with the seafloor slows it down. The water behind it “piles up," causing the wave height to increase dramatically—sometimes to over 100 feet.

Drawback: Often, the sea will recede significantly (a “drawback") just before the first wave arrives, exposing the seafloor for hundreds of meters.

Inundation: The wave strikes the shore as a fast-rising flood or a turbulent “wall of water" rather than a typical breaking surf wave.

KEY FAQs

How does an earthquake trigger a tsunami?

Most tsunamis start with an undersea earthquake along a subduction zone. When tectonic plates suddenly slip, the seafloor is pushed up or down, displacing a massive column of water. That displacement radiates outward as long-wavelength waves—this is the essence of a Tsunami generation.

Why don’t all earthquakes cause tsunamis?

Three things matter: location, depth, and motion. The quake must be under (or very near) the ocean, be shallow, and involve significant vertical movement of the seabed. Many quakes are too deep, too small, or mostly sideways (strike-slip), so they don’t move enough water to generate a tsunami.

Why are tsunamis so dangerous near coasts?

In deep ocean, waves are fast but low. As they approach shore, the water shallows and the waves slow, compress, and grow taller—a process known as Wave shoaling. This turns a barely noticeable swell offshore into powerful, fast-moving surges that can flood coastal areas.

With agency inputs

News explainers Amid Alert For Japan, How Earthquakes Cause Tsunami Explained

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