REPORT: Fault under pressure

Rocks and road cracks on the Pahiatua-Pongaroa Rd in the Tararua District after the 6.2 earthquake in 2014. PHOTOS/FILE

GNS predicts low chance of big quake

ELI HILL
[email protected]

The Wairarapa fault line is loaded and could deliver “giant” earthquakes to the country, an international report says.

And the “extreme large size” of previous Wairarapa earthquakes has made seismic scientists question their understanding of earthquake physics.

But a GNS expert says there is only a 1.3-7 per cent chance of a massive earthquake happening in the next 100 years.

The report was written by European scientists, funded by the French Research Agency, and published online in Scientific Reports.

The report found the Kaikoura earthquake of 2016 had “loaded’ the Wairarapa fault as it was “likely that the two fault systems are kinematically linked”.

Previous earthquakes along the Wairarapa fault were also investigated – with the most recent in the study occurring in 1855.

That earthquake – the most powerful ever recorded in the country – had a magnitude of 8.2, killed from five to nine people, and changed the landscape of the Wellington region.

“The large slips systematically produced by the Wairarapa fault earthquakes … make them the largest ever observed on a continental fault, especially since earthquake slips are generally greater at seismogenic depths,” the report said.

“Conversely, their rupture length was moderate, in effect, producing a very high earthquake slip to length ratio, and hence… the earthquakes that repeatedly ruptured the Wairarapa fault in the past were thus all extremely energetic and hence potentially very damaging.”

This makes the Wairarapa fault one of the most threatening faults in New Zealand.

The scientists inferred that the earthquakes could have a minimum magnitude of 7.9-8.253.

In contrast, the 2011 Christchurch earthquake had a magnitude of 6.54, and the Kaikoura earthquake of 2016 was magnitude 7.8.

“If a similar earthquake were to occur today, it would initiate where stresses have been most amplified by the 2016 Kaikoura earthquake.

“The possibility of a forthcoming earthquake on the Wairarapa fault in the near future thus needs to be considered.”

GNS Science natural hazards and risks theme leader Kelvin Berryman said the technical work in the paper appeared to be well founded and reinforced previously published research by GNS Science and Victoria University of Wellington.

“However, the conclusion in the paper that the fault ‘may be prone to break’ goes beyond the data the French team presented.

“While the Kaikoura earthquake did impose some additional stress in the Wairarapa region, the long history of the fault developed by GNS and VUW experts indicates great earthquakes of around magnitude eight [lsuch as the 1855 earthquake] have an average repeat time of about 1300 years.”

Using all available data, Berryman said the likelihood of the next great earthquake on the Wairarapa fault had been calculated to be in the range of only 1.3-7 per cent in the next 100 years depending on the statistical models used – a very low likelihood.