What would happen if a tsunami hit Sydney?
Sitting in the centre of the Indo-Australia plate, Australia is a geologically stable country. The seismic ructions that wreak havoc on neighbours such as New Zealand, Indonesia, and Pacific nations are absent in Australia, and our distance from any plate boundaries mean tsunami are a much reduced threat.
However, tsunami can travel vast distances, so while the danger is reduced it's still a possibility. Recently, researchers at the University of Newcastle have been modelling the effects of a tsunami striking Sydney Harbour.
You might think the tsunami risk to Australia is fairly remote, but you may be surprised to learn they have hit our shores before, and it could happen again.
Whirlpools, flooding, and ocean currents strong enough to sink boats are just some of the possible impacts if a tsunami were to hit Australia's largest city, Sydney, according to research.
The chance of a major tsunami affecting New South Wales is very low, but University of Newcastle researchers say the tsunami risk to Australia is under-researched and needs to be taken seriously.
In 1960 after a quake in Chile, a tsunami hit the east coast and sank boats in Sydney and Newcastle.
Working with the Bureau of Meteorology, the researchers have mapped out several different scenarios of what could happen, including a very unlikely one-in-5,000-year event, to other, more likely, "it could happen in your lifetime" events.
"It's absolutely something we need to plan for, even if this is something we consider to be an infrequent event," lead author Kaya Wilson said. "The costs of being unprepared are so great."
"It's important to remember that a tsunami hitting Sydney Harbour is unlikely to be that kind of Hollywood style wall of water coming towards you. It would be experienced as a sequence of waves that may last for hours or even days."
"The worst-case scenario that we modelled was a one-in-5000-year event. We're lucky in Australia, that it would be caused quite far away, and the Australian Tsunami Warning Centre would issue a warning that hopefully we'd receive at least two hours before the tsunami were to hit."
"It would affect the whole east coast, but within Sydney Harbour we would expect extremely large waves."
"The largest we recorded for this worst-case event was 10 metres from peak to trough — we'd expect land to be inundated across Manly Corso, and we'd expect the waves to come from both the ocean and harbour side, eventually isolating North Head."
Sydney current speeds with largest modelled event.
In that worst-case scenario, water would move incredibly fast at up to eight knots, and bays along the south side of the harbour such as Double Bay, Rose Bay and Rushcutters Bay would be inundated.
A one-in-20-year event, more likely in our lifetime, might go generally unnoticed.
"In terms of events that are more serious, we're talking a one-in-110-year — on average — event upwards," Mr Wilson said.
"So, with those more serious events, we could expect the sort of currents that would really disrupt the harbour.
"It would be dangerous for anyone in the water or to people on ships and ferries — you wouldn't want to be in the water.
"We know ships will probably be torn from their moorings, we know that currents will be very unpredictable, and the safest thing would be to be out of the water and to stay away from the shore."
While 110 years or more may seem a remote chance, Mr Wilson says it's not that far-fetched.
"For a tsunami of that size, that could occur once on average every 200 years," he said.
"That statistic can be translated into a 30 per cent chance of occurring over an average lifetime."
Warning systems are good
It's not like tsunamis haven't struck Australia before.
The Bureau of Meteorology has a complete list of all the tsunamis that have affected Australia over the years.
Many probably went largely unnoticed.
The worst one, off the mainland on Norfolk Island in 1805, damaged homes and swept one away.
But in 1960, after a quake in Chile, a tsunami hit the east coast and sank boats in Sydney and Newcastle.
Damage at Clontarf after the 1960 Chile tsunami
"We had quite a lot of damage around the Spit Bridge, lots of yachts were torn from their moorings, we had a timber punt down by Anzac Bridge — I think it was capsized," Mr Wilson said.
"There was damage in Newcastle and lots of damage widely reported."
"That is a sort of tsunami that we would expect to happen within our lifetimes again. We could expect a similar size in the harbour once every 50 to 100 years."
Mr Wilson and his team also work closely with the New South Wales State Emergency Service, which oversees tsunami responses and coordinates evacuations.
"Our research is ultimately about providing more data for our national institutions to make better decisions and keep people safe," he said.
New South Wales SES planning coordinator Nicholas Kuster said they were implementing data from the new study into their own tsunami plans.
"To get this kind of information, it's gold for us emergency services," Mr Kuster said.
Earthquake science explained
The seeds of an earthquake lie in the tectonic plates that make up the Earth's surface and on which the continents sit.
"It helps us refine our evacuation areas in time and helps us prioritise vulnerable areas in the event of a real-life, land-threat tsunami — but also marine-threat tsunamis as well.
"[The research] is expensive, time-consuming, and tsunami planning hasn't really been a large priority for Australia before the 2004 tsunami."
The researchers say a tsunami originating from one of the nearest source zones, the Puysegur Trench, south of New Zealand, could reach Sydney in about two hours.
But Australia has good systems in place, so a warning would go out well before then.
Mr Kuster said it was crucial to always heed the warnings.
"We have a saying up here, 'Up, up and away': Up 10 metres, up the estuary 10 kilometres, and away from the coast one kilometre."
The work is published in the journal Scientific Reports, with the researchers next planning to model potential tsunami impacts in their home city, Newcastle.
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