NZ geologists fear severe South Island earthquake

Geologists studying a fault line off New Zealand’s South Island said today it could cause more severe earthquakes than previously thought – in an area that draws hundreds of thousands of tourists each year.

Fresh estimates show quakes as big as magnitude 8 could occur on the South Island, compared to previous estimates of maximum magnitude 7.5, said National Institute of Water and Atmosphere geologist Philip Barnes.

“Although it doesn’t sound a lot it’s actually quite a big deal in terms of the duration of ground shaking and the size of the earthquake and the length of the fault that would rupture,” he said.

The scientists are not predicting an imminent quake but they are upgrading the region’s quake hazard ratings after undersea mapping off South Island’s Fiordland coast showed it susceptible to bigger shakes than earlier revealed.

Hundreds of thousands of tourists visit the region’s scenic wonders, including coastal fjords and sea life, snowcapped mountains and glacier-scraped valleys and World Heritage-listed rain forests.

Kelvin Berryman from the government-funded Institute of Geological and Nuclear Sciences said findings from sea floor mapping of the offshore Alpine fault “will have an impact into (the tourist towns of) Te Anau, Milford, Queenstown and to a much lesser extent into (the regional cities of) Dunedin … and Invercargill.”

A magnitude 7.1 quake on August 22, 2003, was the last major shake to hit the region and was followed by months of sharp aftershocks. All were centred near the Alpine fault below the South Island’s southwest coast.

The temblors rattled buildings, frightened tourists but caused no injuries and only minor damage in the sparsely populated farming region next to Fiordland National Park.

The worst quake to hit the South Island had a magnitude of 7.8 on June 17, 1929, near the town of Murchison on the West Coast, killing 17.

Barnes and Berryman said the new work did not help predict when to expect major quakes.

“We can’t predict earthquakes any better … but we can make some estimates about what are the likely sizes of earthquakes that can occur on the fault,” Barnes said.

About 14,000 earthquakes are recorded in and around New Zealand each year by the institute.

Most are small, but between 100 and 150 are big enough to be felt by residents.