Tsunami: terror from the sea

By Emma Young December 19, 2010
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Often unexpected, a tsunami can strike with deadly force, obliterating entire towns. Could yours be next?

IT WAS BOXING DAY 2004 and Mrs Rinaldiana was walking near her home in Banda Aceh, Sumatra. “Suddenly, I saw the buildings move, shaken hard,” she said. People running from the shore shouted that the sea was rising.

Mrs Rinaldiana and her family sought shelter in a crowded two-storey house, but it was soon destroyed. “My daughter was hit by the collapsed house, and then…she was…gone,” she told Professor Walter Dudley of the University of Hawaii. Mrs Rinaldiana was knocked unconscious. When she came to, the wave was carrying her and she was surrounded by debris. “I thought this was hell. I thought it was the end of the world,” she said.

The South-East Asian earthquake and tsunami killed more than 250,000 people, says Dale Dominey-Howes, co-director of the Australian Tsunami Research Centre at the University of New South Wales in Sydney. Australia escaped that time, but we’re still at risk of a potentially devastating tsunami.

Tsunamis a present threat to Australia

MOST TSUNAMIS ARE triggered by earthquakes, but they can also be caused by landslides, meteorite strikes and volcanic eruptions. Tsunamis begin with a deep-sea movement of water and can travel at a staggering 1000 km/h. In shallower water, the tsunami slows down. But the column of water behind it moves slightly faster and as it catches up, it piles on top and the wave grows in height.

Often, a tsunami isn’t a single wave, but a series, separated by anything from five to 90 minutes. This can give people a false sense of reassurance. “If a small wave passes, people might think, ‘Oh, that wasn’t much’, but tsunamis are wave trains and they can go on for hours or even days – and frequently the biggest is the second or third wave,” says Geoff Crane of the Joint Australian Tsunami Warning Centre (JATWC).

Dale has studied geological and archival records dating back to 1788 and found that about 40 tsunamis have hit Australia in this time. The biggest was triggered by a quake just off Java in 2007. The resulting tsunami hit north-western Australia with a height of up to 10 m, flooding 500 m inland. Fortunately, the area is sparsely populated, so no-one was killed.

If something like this hit Sydney or Melbourne, it would be devastating. “If it occurred without warning on a Saturday afternoon in summer the impacts would be catastrophic,” Dale says. So far, he thinks Australia has been lucky. “I suspect it’s only a matter of time before we are affected by something damaging.”

The biggest quake threat to Australia is the subduction zone around Java and Sumatra, which could unleash a big tsunami that would strike the north-west. Another site south of New Zealand has the potential for a tsunami that would reach the east coast in just two hours, but it’s unlikely it would be higher than 2 m.

Six big Tsunamis have hit Sydney in past 10,000 years – and could again

DR TED BRYANT, A tsunami expert who recently retired from the University of Wollongong, has gathered evidence that monumental tsunamis have pummelled the east coast throughout history – and could do so again. He believes that, based on studies of sediment layers and rock erosion, six big tsunamis caused by meteorite strikes or marine landslides have hit Sydney during the past 10,000 years. The most recent probably occurred in 1491, Ted says, and produced a wave that washed over the harbour’s headlands, 60 m above sea level. That tsunami didn’t travel far inshore, but others did.

Ted argues that a tsunami that hit the Shoalhaven delta near Nowra, between 5000 and 4000 years ago, ran 10 km inland, while deposits near the Blue Mountains hint that another tsunami ran over cliffs more than 60 m high, and then rushed inland. A similar event today would have the potential for widespread destruction. And we wouldn’t necessarily pick up an incoming comet or meteor, whose impact into the sea could trigger a tsunami. “An object a few hundred metres wide, more than likely, would just come in with no warning,” says Ted.

What to do in a tsunami alert

If a tsunami warning is issued, it’s vital that people heed it, Geoff stresses. The 8.8 magnitude quake off the coast of Chile in February unleashed a tsunami that took half a day to reach us, giving time to get warnings out. Despite beach closures on the east coast, many people still swam. Though the waves that hit in February were just 40 cm high, they were still dangerous, because they created huge, invisible turbulence under the sea.  “It sounds bugger-all,” says Dale. “But we still had very, very strong currents and choppy water on the coast.”

If a tsunami alert is issued, or if you feel an earthquake, it’s imperative to get to high ground, says Dale, who led a United Nations survey team in Samoa, following a quake in September 2009 that generated a 16 m tsunami. The team found that the people who survived were those who didn’t wait for an official warning, but immediately dropped everything and ran for high ground.

Mrs Rinaldiana agrees. “My advice is… if a major earthquake happens, search for a hill.” After regaining consciousness, she grabbed branches and was washed over houses, finally being dumped on top of a building 8 km from home. Rescued, she was taken to a hospital where she found the surviving members of her family, lucky to be alive in the wake of a tsunami that killed about a quarter of a million people.

Source: Australian Geographic Jul – Sep 2010