Alpine Fault: What scientists are learning about the Great New Zealand earthquake

Freshly installed seismometers – one of them now operating at a depth underground roughly equivalent to the height of Auckland’s Sky Tower – are giving scientists unprecedented insight into our largest Earth fault. Video / Outdoor Learning

Freshly installed seismometers – one of them now operating at a depth underground roughly equivalent to the height of Auckland’s Sky Tower – are giving scientists unprecedented insight into our largest Earth fault.

Stretching 600km on the west side of the South Island between Milford Sound and Marlborough, the Alpine Rift is one of New Zealand’s greatest natural hazards.

Recent research has suggested that the next major earthquake along the fault could block South Island highways in more than 120 locations, cut off 10,000 people and cost the economy an estimated $10 billion.

It has a clear geologic record of rupturing roughly every three centuries – and 2017 marked the 300th anniversary of what is thought to have been a magnitude 8 earthquake that moved one side of the fault about 8m in just a few seconds.

The latest estimates put the chance of a major rupture within the next 50 years at 75% – and the probability of this earthquake measuring over 8.0 to 82%.

“It is very late in its typical cycle – so it is reasonable to expect that in the coming years or decades there will be another Alpine Fault earthquake,” said Professor John Townend. , a geophysicist at the University of Victoria.

“So what’s really important to understand is what are the temperatures and the stresses acting in the fault, before an earthquake.”

A seismometer, the first of its kind, installed by GNS Science staff deep in a borehole near Whataroa late last year, now offered a high-resolution view of fault movements deep within the fault. , while helping scientists better locate earthquakes around the area. .

GNS Science remote infrastructure management specialist Tim McDougall said a few special additions were needed to lock the instrument in place at 300m.

Borehole DFDP-2B at Whataroa, showing the GeoNet seismometer in place at 300 meters.  The borehole is close to the Alpine fault and the seismometer can register small earthquakes that we would not register otherwise.  Image / Science GNS
Borehole DFDP-2B at Whataroa, showing the GeoNet seismometer in place at 300 meters. The borehole is close to the Alpine fault and the seismometer can register small earthquakes that we would not register otherwise. Image / Science GNS

“It’s basically a thin seismometer not much bigger than a can of baked beans, attached to a longer latch mechanism, and it’s the first such sensor we’ve installed in deep drilling,” McDougall said.

“We have a few sensors set up at similar depths to keep tabs on the Auckland Volcanic Field, but having one a few meters from New Zealand’s main fault line is really important.”

GNS Science scientist Tim McDougall said it was
GNS Science scientist Tim McDougall said it was “really significant” that New Zealand now has an operating sensor just meters from its main fault line. Photo/Jeff Brass/GNS Science.

The seismometer was “broadband”, meaning it was able to record seismic waves of a wide range of frequencies, and was now an integral part of the GeoNet network.

“We recently reviewed the data collected by the sensor and are very pleased with its quality,” McDougall said.

This image shows seismic traces from the same small earthquake, captured by both a newly installed borehole seismometer and another on the surface.  The signal recorded by the underground seismometer is much clearer.  Image / Science GNS
This image shows seismic traces from the same small earthquake, captured by both a newly installed borehole seismometer and another on the surface. The signal recorded by the underground seismometer is much clearer. Image / Science GNS

“This is quite exceptional given that the ambient temperature at the sensor’s location is very close to its operational temperature limit of 60°C.”

The borehole itself was drilled in 2014, as part of the Multinational Deep Rift Drilling Project.

“Installing a sensor so deep below the ground surface and next to a major fault at the end of its typical seismic cycle is very exciting,” Townend said.

Engineers drill boreholes along the Alpine Fault at Whataroa, near Franz Josef, in 2014. Photo/Supplied
Engineers drill boreholes along the Alpine Fault at Whataroa, near Franz Josef, in 2014. Photo/Supplied

It comes as scientists have stepped up seismic monitoring along the wider fault, under several major new projects.

Townend and his colleagues recently set up an array of 50 seismometers, spaced 10 km apart over about 450 km of the fault, to create a temporary array called the Southern Alps Long Skinny Array, or Salsa.

“We’ve been working with data from existing sites to test some of the analyzes we’ll be doing, once the actual data from the Salsa Network starts coming in,” he said.

“It will be quite a task, because there are a lot of measurements involved.”

Separately, scientists captured new data on boundary “micro-earthquakes” along the fault south of Haast, where past major earthquakes have been shown to sometimes stop.

These boundaries are often called “seismic gates” because they can sometimes either allow earthquakes to break up and grow, or stop them and limit their size.

“But we don’t know what physical properties change between earthquakes to open or close the gate,” said GNS Science seismologist Dr. Emily Warren-Smith, who leads the Dense Westland Arrays Researching Fault Segmentation (Dwarfs ).

“Understanding what the scenario is before the next earthquake, however, can help us better prepare.

“By installing dense seismic arrays above these fault boundaries, we can map, in unprecedented detail, the thousands of small earthquakes that occur there each year, and then use them to create images of the subterranean fault geometry, stress state and mechanical behavior.”

In just over three years, Warren-Smith’s team mapped more than 7,000 small earthquakes, providing rich new information.

“In particular, south of Haast, near the Cascade River, we saw the fault undergo a dramatic angle change at depth, from a dip of around 45 to 85 degrees over a short distance,” said she declared.

The Alpine fault, marking the boundary between the Pacific and Australian crustal plates, generates earthquakes of magnitude 8 to 8.2 approximately every 260 years.  Photo/Science GNS
The Alpine fault, marking the boundary between the Pacific and Australian crustal plates, generates earthquakes of magnitude 8 to 8.2 approximately every 260 years. Photo/Science GNS

“This major geometric complexity is very difficult for earthquakes to traverse and we believe that changes in the stress distribution on the fault, between and during earthquakes, could be the key to allowing it to propagate at times. “

At its northern end, near Inchbonnie, the Alpine Fault was cut by the Hope Fault and other smaller faults such as the Kelly and Hura Faults.

“Our analysis of small earthquakes there shows that the Alpine Fault north of this intersection is very misaligned for failure,” she said.

“On the other hand, the Hope Fault and the Alpine Fault to the southwest are both well-trending and we propose that this may indicate a preferred rupture path of Alpine Fault earthquakes propagating northward on the system of the Marlborough Fault.

“We are currently building detailed 3D models of the fault to run major earthquake simulations to test these observations and provide plausible failure scenarios to better inform future risk planning.”

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