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Source: GNS Science

A new project being led by GNS Science will dig into two big faults in the South Island – the Alpine and Wairau Faults – to document the scratch lines or ‘slip striations’ to determine which way the faults will rupture in the future.

Project leader Russ Van Dissen said the direction of fault rupture propagation, or ‘unzipping’, had a major influence on where seismic energy gets focused.  

He cited the magnitude 7.8 Kaikōura earthquake in November 2016 that, because of its south to north rupture direction, sent a pulse of energy northwards that was felt strongly in the lower North Island, particularly in Wellington.

The Kekerengu Fault which was one of 20-plus faults that ruptured in the magnitude 7.8 Kaikōura earthquake in November 2016. Photo: Julian Thomson, GNS Science.

Calculating earthquake ground-shaking hazard for a particular area depends on studying past fault ruptures to determine location, magnitude, recurrence interval and the direction in which the rupture propagated. Currently scientists can determine the first three, but not the last.

“In essence, scratch marks on fault surfaces provide a signpost that points towards the epicentre of the quake.”

First, team members will dig into Marlborough’s Kekerengu Fault, which ruptured in a known direction in the Kaikōura earthquake, and document the shape and orientation of slip striations on the rocks on the rupture surface.

Large faults are like a heavyweight boxer – you know they are going to pack a wallop. So if you want to prepare yourself, it’s helpful to know which direction the energy is going to coming from

Russ Van Dissen

They will then focus on sections of the Alpine and Wairau faults and look for similar striations. The Alpine Fault runs up the spine of the South Island between Milford Sound and Marlborough and is known to be late in its rupture cycle.

The 100km-long Wairau Fault runs between Lake Rotoiti and Cloudy Bay and is also late in its rupture cycle. Both faults are capable of producing large earthquakes.

“The direction of rupture of future quakes on these faults will have a significant influence on the strength of shaking experienced across the South Island and the lower North Island,” Van Dissen said.

“If we can show that both faults have ruptured a number of times in a certain direction, then that information could be used to better plan for future earthquakes, and in designing more resilient buildings and infrastructure. Similarly with other large faults.

“Large faults are like a heavyweight boxer – you know they are going to pack a wallop. So if you want to prepare yourself, it’s helpful to know which direction the energy is going to coming from.”

He added that the project’s findings had the potential to positively influence the global discipline of seismic hazard evaluation by opening up an archive of past fault rupture directions.

Collaborating on the project are scientists from Victoria University of Wellington, Otago University, and the University of California, Riverside, in the US.  

The project has been awarded funding of $960,000 over three years by the Marsden Fund which is administered by the Royal Society Te Apārangi.

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