28m high: Scientists model NZ’s risk from tsunami-making quakes

A pioneering earthquake analysis has shed new light on the potential for large tsunamis - including one 28 metres tall - sweeping into our coastlines.

While they’re relatively rare events - of 68 recorded since 1840, fewer than a dozen have measured higher than 5m - tsunamis nonetheless pose a major natural hazard, with a worst-case, one-in-500-year tsunami potentially causing up to about 33,000 casualties.

Now, scientists have gained a much deeper insight into that risk, using a 30,000-year catalogue of some 2585 “synthetic” earthquakes measuring greater than magnitude 7.0.

“We can use the model to investigate how individual tsunami events might impact the coast, and how far inland they travel,” said study author Laura Hughes, of Victoria University.

Preliminary results showed the tallest tsunami wave height at the coast – caused by a quake measuring just over magnitude 9.1 - reaching a towering 28m, or the equivalent of a nine-storey building.

About 73 per cent of the modelled events produced waves at least one metre high, while 15 per cent, 3 per cent, and 2 per cent of the events resulted in waves of at least five, 10, and 15 metres respectively.

On average, the researchers estimated a tsunami with a five-metre wave height occurred every 77 years - while a tsunami 15-metres high might occur around once every 580 years.

Elsewhere in the study, the team mapped where the biggest waves tended to hit along our coastline, with northern and eastern areas found to be most at risk.

The largest hazard was posed by the Tonga-Kermadec and Hikurangi subduction zones, in which the latter scientists recently estimated a one-in-four chance of a large quake striking within 50 years.

They also calculated the hazard for different “return periods”, finding some areas of the North Island’s coastline had relatively high probability of experiencing waves taller than five metres within 2500-year stretches.

The new modelling technique has since been used as the basis for estimating tsunami impacts and losses around the country.

“We now know we can successfully use our synthetic earthquake catalogues to estimate risk from earthquake-induced perils such as tsunamis, ground-shaking, landslides and liquefaction,” said study author Dr Bill Fry, of GNS Science.

“This opens the door to next-generation probabilistic risk modelling for many of our natural hazards.”

The study, just published in the Journal of Geophysical Research: Solid Earth, also involved researchers from Niwa, the University of Canterbury and Lamont Doherty Earth Observatory in the US.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.