Ultra-sensitive fibre-optic sensors extending nearly a kilometre below the Southern Alps will transform what we know about one of the biggest earthquake threats facing New Zealand.
The Alpine Fault, which runs along the spine of the South Island, is assessed by scientists to be likely to produce a large earthquake in coming decades, a potentially catastrophic event that would change the face of the country.
On average, quakes of magnitude 7.5 or larger strike along the Alpine Fault about every 300 years, and the last big event, measuring about magnitude 8.0, occurred 299 years ago.
Scientists calculate there's a 30 per cent of a big quake within the next 50 years, a fact that's put a sense of urgency behind efforts to better understand the complex system.
In a new project, just awarded a $830,000 Marsden Fund grant, researchers will make use of optical fibre equipment already installed in a 900m borehole drilled into the Alpine Fault to gain crucial insights into what's happening deep underground.
The borehole was drilled at Whataroa Valley, north of Franz Josef Glacier, in 2014 as part of the international, multi-million dollar project focused on understanding how the Alpine Fault produces earthquakes.
The borehole revealed unusually high temperatures just several hundred metres below ground.
Engineers drilling into the Alpine Fault in the Whataroa Valley, north of Franz Josef Glacier, in 2014. Photo / Supplied
Temperature is an important characteristic of the fault - hot rock brought to the surface as the Southern Alps are uplifted interacts with groundwater.
The sensor technology being developed is sensitive enough to detect subtle changes in temperature - potentially providing a way of monitoring the fault's state of health - and seismic waves produced by earthquakes.
University of Auckland physicist Professor Neil Broderick said the technology worked by picking up changes in the properties of short pulses of laser light sent down the optical fibre.
The data it produced would be detailed enough to record signals produced by small, high-frequency quakes and recently-revealed deep, low-pitched earthquakes, complementing measurements made with conventional seismometers.
Snow delineates the escarpment formed by the Alpine Fault along the Southern Alps' northwest edge, near the South Island's west coast. Photo / NASA
Broderick, who is leading the study, said the approach had been adapted from one used by the petroleum industry to detect and monitor oil drilling.
"The basic concept has been around for 20 years, but we are looking to push it to new levels of sensitivity and capability."
Victoria University of Wellington earthquake scientist Associate Professor John Townend, a lead researcher in the drilling project and the new Marsden Fund project, said the prospect of keeping a close eye on these mysterious, deep quakes was exciting.
"It would be great to be able to keep tabs on earthquakes and changing temperatures inside the fault zone - not every millisecond, perhaps, but every few minutes or several times a day."
The 2014 drilling effort encountered surprisingly high temperatures just several hundred metres into the Alpine Fault. Photo / Supplied
The new measurements offer the prospect of breakthroughs in understanding the conditions that control earthquake triggering - even if they didn't provide specific warning ahead of the next big quake, Townend said.
"We're a long way off being able to say when an earthquake will occur, but these sorts of continuous measurements will greatly improve our understanding of what factors are important and what straws break the camel's back."
The Alpine Fault: Our looming threat
• Is the on-land boundary between the Pacific and Australian tectonic plates.
• It moves about 27m horizontally every 1000 years, in three or four separate large ruptures.
• Scientists have evidence that it has ruptured 24 times in the past 8000 years, with the average interval period about every three centuries.
• It last ruptured in 1717, or 299 years ago, and has a 30 per cent probability of rupturing in the next 50 years, which is high by global standards.
• According to GNS Science, this rupture would produce "one of the biggest earthquakes since European settlement of New Zealand", and will have "a major impact on the lives of many people".
