University of Otago researchers have recorded earthquakes up to 40km deep in Southland, uncovering evidence of hidden faults in one of the country's least seismically active regions.

Scientists from the university and GNS Science installed 19 temporary seismometers in Southland, alongside GeoNet's permanent network, and measured activity over a year.

Lead researcher Dr Jack Williams said they were able to more precisely triangulate signals to determine where earthquakes were happening with the seismometres spaced closer together.

"Southland is a gap in our understanding of New Zealand's faults, but that doesn't mean they aren't there and can't cause damage," he said. "What was really striking was the depth of the earthquakes we recorded."

Most earthquakes happened in the cold, brittle continental crust less than 15-20km deep, but Williams said the team found the seismically active crust extended up to twice the average depth in Southland.

"If you think of plasticine, when it's cold and you try to tear it apart, it fractures and breaks. But if you rub it and warm it up a bit, it's a lot more ductile," he said.

"The Earth's crust is similar. Near the surface where it's cold, earthquakes deform the crust by fracturing and rupturing it, but the deeper down you go, the warmer it is and the more ductile the crust becomes. This makes it less likely to tear apart and flow instead.

"With this research we essentially uncovered an extra-deep layer of the Earth where earthquakes can occur. This is possible in Southland because the lower part of the crust has an unusual composition with lots of iron rich minerals, which given the depth, make it relatively strong and brittle."

As well as the "remarkably deep" earthquakes, the seismometres picked up 85 quakes throughout the year, six times more than GeoNet, Williams said.

"Most of these shakes were too small to feel, but they give us hints about where larger earthquakes may occur in the future."

Williams said the team was also looking for evidence of earthquakes using high-resolution maps of Southland's ground surface released by Environment Southland.

"These maps show us evidence of surface ruptures caused by ancient earthquakes, giving us important new information about where Southland's fault lines are located."

The findings could be incorporated into hazard models that predicted the likelihood and strength of future earthquakes, he said.

The research was funded by the Natural Hazards Commission as part of a wider effort to better understand the earthquake risk in traditionally low seismic regions, including Otago and Auckland

Published by permission