In the past century, 58 tsunami incidents have claimed more than 260,000 lives, surpassing any other natural hazard, per the UN. The Indian Ocean tsunami on December 26, 2004 recorded the highest death toll of the century, with approximately 2,27,000 fatalities in 14 countries, with Indonesia, Sri Lanka, India and Thailand hardest-hit.

To understand why tsunamis occur and what protective measures can be taken in the event of a tsunami, FactChecker spoke with Manasa Ranjan Behera, Associate Professor, Department of Civil Engineering at the Indian Institute of Technology, Bombay.

There are various natural stressors that make a coast vulnerable, and tsunamis are major stressors, said Behera. Others include cyclones, storm surge flooding (a sudden rise in water levels that can cause extreme flooding in coastal areas), coastal erosion, and so on. Human-made stressors include poorly planned infrastructure built on or near the coast.

What is a tsunami and why does it occur?

A tsunami is a series of long waves that is caused by a sudden displacement in the ocean floor or crack in the ocean bed. Displacements are most commonly caused by earthquakes. "When certain portions on the ocean floor crack and get displaced upwards, downwards or sideways, the disturbance creates waves that radiate a lot of energy in all directions from the ocean floor, causing the sea water to rise. A huge movement in the earth's crust displaces a lot of water in different directions in the sea," explained Behera.

To illustrate, if we throw a stone in a still pond, we'll see ripples on the surface around the spot where the stone entered the water. Even a small disturbance like that will displace water. Earthquakes, which affect large areas of the ocean floor, cause disturbances on a very large scale, such as a tsunami.

When a tsunami is first generated in the depths of the ocean, the height of the surface wave is only around a metre. But as it travels toward the coast, the wave can rise as high as 15 metres – between 4 to 5 building storeys high – depending on location, explained Behera. This is because the strong force of the energised tsunami wave, which gains momentum as it rises upward and outward from the deep sea, gets blocked by the approaching shore, forcing the wave upward as it nears land. The varying height of tsunami waves is because the depth of the sea is not the same everywhere, said Behera. The average depth of the sea is about 3,688 metres (3.7 km), whereas in deep seas it is as much as 4,000 to 5,000 metres (4-5 km).

"Once a tsunami forms, its speed depends on the depth of the ocean. In the deep ocean, a tsunami can move as fast as a jet plane, over 500 mph, and its wavelength – the distance from crest to crest – may be hundreds of miles. Mariners at sea will not normally notice a tsunami as it passes beneath them; in deep water, the top of the wave rarely reaches more than three feet higher than the ocean swell," according to the National Oceanic and Atmospheric Administration, an American regulatory agency within the United States Department of Commerce.

Besides earthquakes, volcanic eruptions, landslide collapse, submarine eruptions and meteoric falls can also cause tsunamis, said Behera.

Several countries like India that are vulnerable to tsunamis have early warning systems. The Indian National Centre for Ocean Information Services (INCOIS), a Ministry of Earth Sciences body, is the nodal organisation responsible for sending out tsunami alerts. The government has placed around 14 sensors, called tsunami warning buoys, in the deep seas around the Bay of Bengal and Arabian Sea, to help measure waves in the open ocean. The system is designed to detect tsunami waves, as opposed to normal waves. The signals are analysed by INCOIS, which obtains further information about when a tsunami could reach the shore, and issues warnings accordingly.

What are the early warning signs of a tsunami?

The main signs before a potential tsunami, according to Behera, are:

1. An earthquake near the coast that lasts for more than 20 seconds;

2. An unusual withdrawal or recession of water levels from the shoreline (up to 50 metres; beyond the usual low tide recession). However, not all water withdrawals amount to a tsunami scare. "In absence of warnings from INCOIS, people should not panic. INCOIS will be the first to know if there will be a tsunami," said Behera; and

3. Lastly, when coastal water makes unusual noises (like an approaching train, overhead plane, or whistling).

What measures should be taken before and during a tsunami?

"First, to escape a tsunami, people living in coastal areas are advised to get to higher ground, ideally to a spot at least 100 feet above sea level, or two miles inland away from the shoreline," said Behera.

"Secondly, when water from the sea-shore recedes unusually, people should not go to observe this or stand near the shore. They must go inland instead, approximately 2-3 miles away from the shoreline," said Behera.

Third, fisherfolk in boats at sea are advised not to return to the port area because tsunamis can cause swift changes in water levels and unpredictable currents in harbour areas, according to the central National Disaster Management Authority.

It's also important to know the difference between a 'tsunami warning' and a 'tsunami watch'. The former means a tsunami may have been generated in the ocean and could be close to the coastal area, whereas a tsunami watch means that a tsunami has not yet been verified by the government authorities, but could be an hour away.