In the early afternoon of 13 September 1999, the headmistress of the tiny school at Omoa on Fatu Hiva, in the Pacific, glanced out of her window to see water draining from the nearby beaches. She acted without hesitation. Her pupils were ordered to run to the back of their school, climb out of the windows and head for high ground.
A few minutes later, with the last of her children crawling out of windows, a tsunami smashed through a concrete wall on the school’s seaward side, and poured into the classroom. The wave destroyed the building – but without casualties. Every pupil was saved, thanks to their headmistress.
‘The crucial point is that this teacher knew about tsunamis,’ says marine geologist Dr David Long of the British Geological Survey in Edinburgh. ‘People in the Pacific region are often taught about them and know when one is about to strike. On beaches in California, Japan and Chile, there are notices warning people to head inland if an earth tremor begins or if the sea starts to recede.’
Such awareness contrasts with most people’s actions last Sunday. Tourists and locals wandered over newly exposed seabed to peer at flapping fish or crabs. Only then did they notice a white crest swelling in the distance. This behaviour helps explain why last week’s death toll was so appallingly high – and suggests measures that might urgently be taken to prevent future disaster.
The omens certainly look grim. Tsunamis are typically triggered by huge underwater earthquakes, and Professor Kerry Sieh, a geologist at the California Institute of Technology and an expert on Sumatran seismology, fears a repeat may be on the cards. Minor earthquakes occur on a regular basis, he points out, but truly huge ones, like last Sunday’s, appear to arrive in clusters. Between 1952 and 1965, the world experienced six earthquakes of 8.5 or more on the moment magnitude scales (the most commonly used successor to the Richter scale). There had not been another – until last week.
More to the point, the section of the Sumatran subduction zone that Sieh studies has not moved – unlike its neighbour, which jumped so catastrophically last Sunday. It remains a potential earthquake hazard. ‘I worry about my segment,’ Sieh said last week. ‘It is still locked.’
Understanding tsunamis has, therefore, never been more important, though they remain difficult phenomena to study. In the deep waters of an ocean, they move as barely detectable ripples at hundreds of miles an hour. Near land, however, in shallow water, they slow down. One wave after another piles up on top of its predecessor, creating a single wave that slowly surges towards shore, usually at a few dozen miles an hour.
As this giant wave approaches, sediment and sand is sucked up, causing the seabed to drop and water to pour away. ‘Once this starts to happens, you still have time before the main wave arrives – usually between 10 and 30 minutes,’ explains Long.
Such knowledge could have saved thousands last Sunday. For this reason, tsunami awareness is now considered to be of overriding importance for protecting life, particularly in a world becoming ever more vulnerable to rising sea levels. A tsunami detection system like the one installed around the Pacific, can help, but the real need is for good education.
‘An early warning system will be helpful,’ says Long. ‘But you still have to pass on that warning to crowded towns and beaches. How do you get people to evacuate bars, restaurants, hotels and beaches? On the other hand, if people are trained to spot tsunami symptoms and know how to react, you will be in a much stronger position.’
Nor are technological fixes foolproof. Tsunamis can be triggered in several different ways – by earthquakes, by asteroids crashing into the sea (an event that killed off the dinosaurs), by underwater landslides and by volcanoes – which makes prediction extremely difficult.
In 1998, seismologists detected a 6.8 moment magnitude earthquake off the coast of Papua New Guinea. This was rated as a low-risk event: it usually takes a seabed earthquake of 7.5 or more to set off a tsunami. But this quake triggered an underwater landslide and set off a tidal wave. Thousands of people died.
‘The alternative is to send out warnings at the slightest danger signal, though you would soon face the danger of shouting ”wolf” too many times,’ says geophysicist Bill McGuire, author of A Guide to the End of the World. ‘This has already proved a problem when tsunami warnings have been issued and the waves have turned out to be tiddlers only a few centimetres high.’
Yet McGuire is convinced of the need for detection systems. He has identified a giant chunk of rock that could fall into the sea when the Cumbre Vieja volcano in La Palma next erupts. This could send a tsunami up to 100 metres high spreading across the Atlantic. New York would be devastated; London merely inundated.
‘Britain is relatively well placed in terms of tsunamis,’ says Dr Ed Hill of the Proudman Oceanographic Laboratory in Liverpool. ‘We sit inside a large continental shelf, and its shallow waters protect us.’
Last week’s tsunami was still 70cm high when it reached Antarctica. Later, off California, a ripple of 8cm was recorded. But no effect was noted in UK waters. Europe’s continental shelf soaked up the last of its energy.
This does not make us invulnerable, however. Scientists have discovered that, 7 000 years ago, a landslide on Norway’s coast set off a tsunami that pummelled eastern Britain. Its effect is hard to gauge, however, for the country was then populated by only a few tribes of hunter-gatherers.
Not so today. Millions live along our coasts and all are now at risk of any event that perturbs the planet’s rising oceans. And of these, none is more terrible than a tsunami. – Guardian Unlimited Â