By Veronika Meduna Veronika.Meduna@radionz.co.nz
Geologists drilling deep into the Alpine Fault are hoping to understand what goes on before a major plate boundary fault ruptures in an earthquake.
An international team, led by New Zealand scientists, has set up camp in the small West Coast town of Whataroa to drill 1.3km deep into the South Island’s high-risk fault.
While other projects have drilled into plate boundary faults after large earthquakes, project co-leader and GNS Science geologist Rupert Sutherland says the Deep Fault Drilling Project represents one of the first attempts to probe the inside of a major fault before it ruptures.
'The Alpine Fault is a special place globally if you want to understand earthquakes. We know that it fails approximately every 330 years in an earthquake so that means it may not fail in the next 30 years or even 100 years but it is at the end of its seismic cycle. The state of the fault now is the state before it ruptures.' _ Rupert Sutherland
The Alpine Fault, which runs for 650km along the spine of the Southern Alps, produces an earthquake of about magnitude 8 on average every 330 years. The last time it did this was in 1717, when it produced about 8m of horizontal movement. It now has a 28% probability of rupturing in the next 50 years, which is high by global standards.
To develop good models of earthquake processes, Rupert Sutherland says you need a “before and after shot” – and that’s exactly what the team is hoping to achieve.
Despite technical difficulties that have delayed the drilling, the bore hole is now almost half a kilometre deep, and the geologists have been able to collect data as they go. At the moment, they are analysing rock chips that come up from the bore hole to guide them about how close they are getting to the actual fault zone, which they expect to reach at about 1000 metres in depth. When they get closer, they will change the drilling equipment to begin extracting cores of rocks to study the geophysical conditions, or the architecture of the fault.
Project co-leader and University of Otago geologist Virginia Toy says geologists cannot predict when earthquakes will happen but she hopes that the rock record will help to understand what kind of earthquake the Alpine Fault will produce in the future. She says earthquakes that generate high-frequency ground shaking can cause extreme damage to buildings and infrastructure. “That’s what happened in the Christchurch earthquakes. But other earthquakes release just as much energy or more, but because the frequency of the waves might be distributed over a broader range, they don’t cause as much shaking and damage.”
'I think that it’s possible by looking at the rocks that were affected during the generation of those sorts of earthquakes … and combining this with our knowledge of the actual style of earthquakes, that we might find some systematic relationships between observations of fault rocks and the actual earthquake that was generated.' _ Virginia Toy
In this video, produced by GNS Science, she explains how the team uses the rock chips, or cuttings, to estimate how close they are to the fault zone.
One surprising, but preliminary, finding is that temperatures of rocks and fluid at depth are unexpectedly high. At previous, much shallower, bore holes temperatures also rose with depth, but this time the increase is significantly higher. Temperature is only one of many measurements the team is collecting. The team is also tracking fluid movement and changes in pressure, because scientists think that fluids may play an important role in initiating the first slipping movements as an earthquake begins.
“It seems from many different observations from around the world and from our study that there is an important interplay between fluids and rock,” says Victoria University seismologist John Townend, who is also a co-leader of the project. “In part the character of the rock is what controls the flow of fluids throughout the Earth’s crust but the resulting changes in pressure weaken the rocks.”
University of Otago geologist David Prior, who studies rocks that have been pushed up to the surface from the fault zone several kilometres deep underground, says this project will allow the team to compare the knowledge they've gleaned from outcrops with what they find at depth. In this interview he explains his work on rocks that have been exhumed from deep in the Alpine Fault.
Austin Boles is a graduate student at the University of Michigan, focusing on the role of minerals and their potential to "act as ball bearings" in an earthquake.
Ultimately, the team plans to install high-tech equipment in the bore hole for long-term monitoring and remote data gathering. Rupert Sutherland says the research will improve hazard management planning in New Zealand, and perhaps even help to develop an early warning system – “a few seconds so that you can stop a train or turn down a power plant”.
You can follow the team's progress on Rupert Sutherland's blog.