Quake brings cooler times at Welcome Flat

South Westland's celebrated Welcome Flat hot pools have become a little less welcoming.

Not only did July's big Fiordland earthquake shift the area 30 centimetres closer to Australia, but researchers have now discovered it also affected the temperature and water levels of springs and aquifers around the South Island.

GNS Science monitoring shows the hot spring, more than six hours walk up the Copland River from State Highway 6 into the Southern Alps, is 1 degree Celsius cooler.

Immediately before the July 15, 7.8 magnitude earthquake under Resolution Island in Dusky Sound, the average spring temperature was about 57.9C. During the quake it rose to 58.1C and then over the subsequent four days it dropped to an average of 56.9C, where it remains.

As a consequence, the four mud-fan pools revered by trampers which used to average about 42C are being fed colder water from the hot spring.

The temperature of the pools can vary dramatically during heavy rain, which dilutes the hot water.

GNS Science Dunedin geologists Simon Cox and Delia Strong will be presenting their discoveries at the Geological Society of New Zealand conference in Oamaru next week.

Cox said to date no New Zealand researchers had systematically observed fluctuations in temperature and water levels, and the formation of new hot springs, as a result of earthquakes.

Visiting the Copland in September, he found to his "dismay and great interest" that the hot spring had changed temperature.

"It's changed by about a degree – that's not a lot but it has been sitting quite happily at that other temperature since at least the 1970s.

"We were predicting it might change from an Alpine Fault earthquake, if one occurred, but not one as far away as the tip of Fiordland."

The Alpine Fault, which runs down the western side of the Alps, is about 12 kilometres west of the hot spring. Shaking from the earthquake had altered the permeability of the Alps.

"The whole plumbing system of the mountains has changed so the flow of cold water being pushed from higher up seems to be greater," Cox said.

Hot springs were found along much of the Alpine Fault.

"The water's hot because the rocks are being lifted up on the Alpine Fault at a rate faster than they can cool. They come to near the surface, and the water is pushed down from the mountains, down to where the rocks are hot at quite a shallow depth, maybe a couple of kilometres deep."

Liquefaction of soil, geysers and mud volcanoes were well-known close to earthquake epicentres, and new springs and fluctuations in water levels in wells had been observed after quakes even thousands of kilometres away.

Scientists were studying how major earthquakes could affect local fluid pressures and weaken faults on the other side of the world, he said.

Cox and Strong are also interested in the effect of the July quake on water in boreholes and aquifers. "Water levels appear to have been affected as far away as Hawke's Bay, although I am still verifying this ..." he said.

- © Fairfax NZ News