I’ve studied tsunamis for 25 years – here are the deadly warning signs one is on its way

One of the most powerful earthquakes ever recorded triggered huge tsunami waves across the Pacific Ocean.

Some as high as four metres were observed after the 8.8-magnitude tremor struck off the city of Petropavlovsk, on Russia’s remote Kamchatka Peninsula on Wednesday.

Safety alerts have been downgraded but still remain in parts of Japan, the Pacific and over 3,000 miles (5,000 kilometres) away in Chile.

Professor David Tappin, a marine geologist at the British Geological Survey, woke up to over 50 emails from global tsunami warning systems after the quake.

The expert, who has studied tsunamis for over 25 years, explained to Metro how tsunamis form from the seabed and how they travel so far across the globe.

What causes a tsunami?

Tsunami’s are caused by large scale disturbances on the seafloor.

Most commonly, and in the case of yesterday’s tsunami warnings, it is earthquakes that trigger the massive waves.

‘Earthquakes are the most hazardous tsunami mechanism,’ Prof Tappin explained.

Violent earthquakes occur due to a build-up of pressure between the tectonic plates that make up the Earth’s crust.

The crust ‘plunges down into the mantle’

Prof Tappin laid out what happened yesterday: ‘Off Kamchatka, the Pacific plate is moving westwards towards Russia and it’s plunging down into the mantle

‘Due to the pressure, there was a massive movement yesterday evening that caused an earthquake.’

The tsunami expert said that when the seabed moves during an earthquake, that same movement happens to the ocean above.

So, for example, if the sea bed ruptured by 10 meters, then the waves generated on the sea surface would be 10 metres high.

Prof Tappin said: ‘The sea surface reflects that vertical movement of the sea bed.

‘So during an earthquake, the sea surface collapses and that generates a tsunami.’

What causes the tide to recede?

One of the most infamous warning signs of a deadly tsunami is when the tide pulls away from the shoreline.

Beachgoers may be tempted to investigate, or pick up dead fish, but in reality a massive tsunami wave could be just minutes away.

But the tide does not always recede before the giant waves strike, as Prof Tappin explained.

When the tectonic plates move, the seabed either moves up or down.

When there is ‘downward movement’ that is when a ‘trough’ can be created in the sea.

‘As the trough approaches the coastline, that’s what causes the wave to recede,’ Prof Tappin said.

But the opposite can also happen, where the seabed moves upwards due to the build-up of pressure, triggering a ‘positive wave’.

This leads to an initial, particularly high, wave that hits the coastline.

What happens to tsunami waves as as they travel across the ocean?

It took just hours for tsunami waves as high as 4 metres to strike Japan after the earthquake off Kamchatka.

Yet tsunami waves then smashed into the coastlines of North and South America hours later, with many coastal communities told to stay cautious over the next 24 hours.

However as these devastating waves travel over the ocean, they are hardly noticeable.

Prof Tappin explains that is because the distance between each wave, called the wavelength, is actually hundreds of kilometres before they reach the coast.

At this stage, the ocean is nearly two miles in depth and there is hardly any friction stopping the waves’ movement.

This lack of friction is why the tsunami that hit the US overnight were still powerful.

The Galapagos Islands and parts of the US saw 4ft waves, as tall as those faced by many in Japan.

Those waves, which appear ‘benign’ in the deep ocean, only become tall and frightening when it gets close to land.

Prof Tappin said: ‘When it approaches the opposite shore, the water depths approach and there is a frictional effects which leads to the build up of the tsunami.

‘The wavelength reduces but the height of the wave increases.’

What warning systems are in place for tsunamis?

The world’s first tsunami warning system was set up in 1946 after a devastating disaster that year.

Headquartered in Honolulu, Prof Tappin explained how it works: ‘It is composed of around 100 seismometers, which measure earthquakes.

‘They are working 24/7, and if there is an earthquake larger than magnitude seven, then the warning centre identifies this.’

All eyes then turn to tide gauges, which measure the height of an ocean, based around the Pacific

‘If there is an abnormality in the wave height after a magnitude seven quake then the warning bell rings.

‘The center will then send out a warning to local organizations who are responsible for deciding when and how to evacuate people.’

After the  Kamchatka Peninsula earthquake, warnings or alerts about tsunami waves were issued in more than a dozen countries, including in Russia, Japan, Taiwan, the Philippines, Mexico and California.

The difference a warning system can make can be huge.

The tsunami in Japan in 2011 saw 15 metre waves, but because of the pacific warning system only 18,000 people died.

Whereas in the Indian Ocean tsunami in 2004 there was no warning system and 230,000 people lost their lives.

After 2004 a warning system was also established for the Indian Ocean and Mediterranean.

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