MAKING WAVES

Tsunamis – what causes them?

When the earth moves the whole world knows about it. Screens and newspapers all over the world are peppered with pictures of wrecked buildings and terrestrial mayhem.

But in fact more than 80 per cent of seismic action takes place on the sea floor – and the only indication that anything is amiss are the gigantic waves that can sweep seafronts in their wake.

These waves are called tsunamis from the Japanese tsu (harbour) and nami (wave) and are caused by the sudden violent displacement of water caused by earthquakes, volcanoes or underwater explosions. The pressure wave of displaced seawater is powered laterally at 90 degrees to the source.

And the world’s foremost seafloor tectonic hot spot is the Tonga Trench between New Zealand and the Friendly Islands.

Normal ocean waves are created by wind, tides or the gravitational influence of the moon, but tsunamis have a much longer wavelength and it can take several minutes or even hours between successive wave peaks. They also affect the whole water column – from sea floor to surface.

Sometimes they are mistakenly referred to as ‘tidal waves’ but, as the name suggests, these are wave forms resulting from tidal irregularities.

There’s nothing new about tsunamis – the Greek historian Thucydides recorded giant waves in the 5th century and scientists have concluded that, about 65 million years ago, tsunamis triggered by an asteroid plunging into the sea near Mexico’s Yucatan Peninsula may have exterminated the dinosaurs – and

almost every other living thing on the planet. The asteroid created an undersea crater over 1.6km deep and water flow, into and outwards, from this huge crater made the giant waves.

It’s difficult to wrap your head around the fact that tsunamis are not just huge quantities of water travelling at speed from A to B. In fact the water is just a medium for vast amounts of energy to travel through. The water molecules hardly move much at all.

Which is why tsunamis at sea are relatively small wave trains moving at very high speed – up to about 900 km/h. Mariners are advised to put to sea as soon as possible after hearing a warning – the devastating walls of water that annihilate everything before them on a seashore, manifest themselves as a slight swell at sea. It would be a hard call to make though – to leave the ‘safety’ of a port, however illusory, to steam for the open ocean.

It depends on how much warning one has. The US National Oceanic and Atmospheric Administration (NOAA) deploys a

number of DART buoys worldwide. Seafloor transponders register earth movements and transmit wave heights to a surface buoy which sends a warning via satellite to the Pacific Tsunami Warning Centre in Hawaii. The impending tsunami can then be tracked to its likely landfall.

And according to NOAA, people need to be at least 33m above sea level when a tsunami strikes – but ‘the higher the better’.

Waves from a magnitude 8.1 earthquake which struck 100nm (185km) from Samoa on 29 September, 2009 set off alarms at the tsunami warning centre about 5,550km away and made landfall 15 minutes later. To do this they would have been travelling at 400 knots (741 km/h). A series of four waves about 6m high smashed into the islands, sweeping 1.5km inland and killing 189 people.

In March, 2011 the tsunami that hit the Japanese coast packed a wave front that was an estimated 40m high and killed 18,000 people.

I was on a yacht in Port Taranaki when the Harbourmaster’s office rang with the tsunami warning. Within 30 minutes the turbulence hit, swirling around the man-made harbour like water in a washing machine. The assembled Solo Tasman Yacht Race fleet joggled round and tugged sharply at mooring lines. The water depth fluctuated by about a metre but the commercial shipping had already fled to sea.

But the big daddy of all tsunamis was a different beast altogether. The waves created by the eruption at the Indonesian island of Krakatoa in 1883 were called ‘impact tsunamis’ and, like the asteroid strike, are caused by forces from above. Krakatoa’s waves came from falling magma and swept six times round the world, annihilating thousands of people and other life forms en route.

By comparison the waves that struck the heavily populated shorelines of the Indian Ocean in 2004 was responsible for 230,000 people dead or missing from the 14 affected countries.

A cubic metre of seawater weighs 1.025 tonnes and, travelling at speed, creates a force that cannot be resisted. The Indian Ocean tsunamis exerted a force of about 200 million tonnes per kilometre on the coastline – though yachts anchored offshore just experienced a steep, rapid swell and fluctuations in depth.

It’s easy to say – but it pays to get well away from land when the tsunami warning sounds. B

“Tsunamis move at up to 900km/ h – outrunning one isn’t easy.”