Although not a direct threat, the tsunami released by last month’s volcanic eruption in Tonga should serve as a warning for Australia to be prepared, according to a leading disaster expert.
Beaches across Australia’s east coast were closed and national emergency notifications issued on January 16, following the massive Hunga Tonga-Hunga Ha’apai underwater blast.
While local evacuations weren’t triggered, residents were sternly advised to retreat to safe places at least 10 metres above sea level. Outlying Lord Howe, Norfolk and Macquarie islands were also placed on notice.
Although devastating for Tonga, there was no comparison between the event and the Indian Ocean Tsunami which claimed more than 220,000 lives in 14 countries on Boxing Day 2004.
However Australians should be under no illusions that tsunamis pose a serious threat, says natural hazards specialist Andrew Gissing.
They also occur more frequently than realised.
Research shows more than 50 small ones have reached the NSW coast alone since European settlement.
There are about 8000 kilometres of active tectonic plate boundaries, called subduction zones, around Australian waters, Mr Gissing says.
“Any of them are capable of generating a tsunami that could hit our coastlines in about two to four hours.”
The Macquarie University research fellow has worked on tsunami emergency plans for the NSW, Victorian and Australian governments.
He is also general manager of risk frontiers at the Melbourne-based national Bushfire and Natural Hazards Co-operative Research Centre.
“In the very rare event of a large tsunami inundating vulnerable low-lying areas of the Australian coast, it would be very challenging to evacuate large populations in a short time frame,” Mr Gissing said.
NSW’s plan for such a disaster notes that a large event affecting the entire state coastline could directly threaten between 250,000 and 1.5 million people, depending on its magnitude, the time of day and season.
Like they did in the wake of Hunga Tonga-Hunga Ha’apai, widespread alerts would kick in, with emergency services evacuating residents to areas a kilometre inland.
Automated warning texts would also be issued to everyone at risk via the national mobile phone warning system Emergency Alert.
“Large tsunamis are very rare,” Mr Gissing said.
“We are talking about a worst-case scenario unlikely to happen more than once in a thousand or 2000 years on average but because the impact would be so huge, having a plan is critical.”
Subduction zones mark potential tsunami sources where one tectonic plate can slip under another.
They include the Java and Timor trenches to Australia’s northwest, the New Hebrides trench to the east and Puysegur trench to the southeast.
Of the smaller events along the NSW coast since 1788, Mr Gissing says the biggest was in 1960 when a 9.5-magnitude earthquake off Chile tore local boats from moorings and damaged oyster leases.