By Angus Dalton
When the earthquake struck, the world rattled; a 10-millimetre shudder on a planetary scale. Earth wobbled on its axis and its rotation quickened, shortening days by 2.68 microseconds. The North Pole jolted off kilter by the length of a paper clip.
Closer to the cataclysm, off the coast of Sumatra, the numbers were starker. The Indo-Australian tectonic plate, which pushes into and below the Burma microplate, had lurched forward by 15 metres, relenting violently to two centuries of geologic pressure.
A man looks over the remains of Aceh after the tsunami.Credit: Jason South
That shoved parts of the seafloor up by several metres along a 1200-kilometre stretch that ruptured at hypersonic speed with the force of 23,000 Hiroshima bombs. The convulsion displaced an unfathomable volume of water, sending waves that would reach heights of 30 metres surging for the Indonesian coast.
The most horrific of the numbers: 275,000 people killed by that extraordinary tsunami, unleashed by the third most powerful earthquake ever measured. It was Boxing Day, 2004.
Andrew Gissing was watching the Test cricket at home in Wagga. As the news took over, with camcorder footage of people in board shorts sprinting from the torrent, he raced to gather clips of the first tsunami of that scale ever captured on film for analysis.
Back then Gissing, now the chief executive of Natural Hazards Research Australia, was a planning officer with the NSW State Emergency Service. He’d just been tasked with developing the state’s first-ever tsunami plan – an undertaking made all the more urgent by the Boxing Day disaster.
“There wasn’t a lot done for tsunami in Australia before 2004. There’s obviously been a lot done since,” Gissing said. “There was no warning system in the Indian Ocean and there was no dedicated warning system for Australia.
“It really woke up the entire world to the risk of tsunami.”
What’s changed since 2004
Following the devastation, the government poured $68.8 million into building a tsunami warning system for Australia and contributing to a detection network in the Indian Ocean that could alert more vulnerable neighbouring countries.
Sea level gauges and tsunameters proliferated in the years following the Boxing Day disaster.Credit: Geoscience Australia
By 2014, the Bureau of Meteorology operated 44 sea-level gauges and six deep-ocean tsunameter buoys. Geoscience Australia established a 24/7 operations centre and now draws data from 300 seismic stations.
If a similar quake struck now, Australia, Thailand, India and Sri Lanka would have several hours of warning.
Geoscience Australia estimates the majority of 61,000 people killed in regions further afield from the Boxing Day earthquake’s epicentre may have been saved by the monitoring systems now in place.
But evacuating people close to a tsunami’s source remains, as the agency puts it, a hideously difficult problem. More than 70 per cent of the Boxing Day deaths were caused by the tsunami waves that swamped Sumatra within minutes of the quake.
Tsunami detection buoys are used by the Bureau of Meteorology to monitor sea levels. Credit: Bureau of Meteorology
“In places like Aceh, where you had the earthquake immediately offshore and waves hitting within 10 minutes, it’s so very, very hard to get any form of early warning,” Gissing said.
That’s because the precise timing of the events that unleash tsunamis – earthquakes, volcanic eruptions, underwater landslides and oceanic meteor strikes – cannot be predicted.
Between 75 and 90 per cent of tsunamis are caused by earthquakes, which we can at least detect well. The Joint Australian Tsunami Warning Centre can detect a tremor within 10 minutes and, five minutes later, forecast the potential tsunami threat and release warnings.
Australia’s east coast looks out into the “Ring of Fire”.Credit: SMH
Western Australia is the most vulnerable stretch of our coastline to tsunami threat, from earthquakes originating in the Sunda trench that produced the Boxing Day tsunami. (WA avoided damage and deaths in 2004, but 30 swimmers had to be rescued from chaotic tides.)
Australia’s east coast, meanwhile, looks out into the “Ring of Fire” network of grinding fault lines and 127 stewing volcanoes around the Pacific Ocean, exposing Queensland, NSW, Victoria and Tasmania coastlines to tsunami risk. A powerful earthquake in the Puysegur Trench off New Zealand could produce a tsunami that reaches Sydney within two hours, Gissing said.
“There’s not a lot of time, still, even when the warning system exists. But it at least gives us a chance.”
A tsunami hazard assessment map released by Geoscience Australia in 2018.Credit: Geoscience Australia
How often do tsunamis strike?
Worldwide, tsunamis that kill people or damage buildings close to their source occur twice a year on average. Tsunamis strong enough to cause death and destruction more than 1000 kilometres away strike about twice per decade.
Australia has experienced at least 50 small tsunamis since colonisation. The most dramatic swamped the east coast after the largest earthquake ever recorded struck the Chilean coast in 1960.
Waves that had travelled 11,000 kilometres sunk boats in Newcastle, inundated Bateman’s Bay, damaged wharves and stole the beach from Clontarf.
Analysis by the University of Newcastle found Manly Corso is the most vulnerable place in Sydney to tsunami. It gave a 12 per cent likelihood of a tsunami powerful enough to flood Manly occurring in our lifetime.
The impact on Clontarf Beach at Middle Head in Sydney Harbour from the 1960 earthquake 11,000km away.
Another factor requiring more research is the risk of tsunamis produced by underwater landslides, Gissing said.
“In theory, if you have a large undersea landslide that occurs quickly, that displaces water and can cause an initial tsunami, which will essentially occur without warning because you wouldn’t get an earthquake signature beforehand.
“You’d basically find that tsunami wave then arriving within minutes of highly populated beaches along our coastline.”
Gissing said the chance of a catastrophic tsunami affecting Australia is possible but extremely low.
“I’m not saying that it’s going to be something that people should be expecting to occur every few years – this is something that could occur every couple of hundred years – but communities need to be aware.
“We’re much, much better prepared than what we were 20 years ago. Advances in technology are only going to help speed up warning processes in the future.
“But that can only do so much because ultimately, you can’t stop the wave.”
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