Scratch lines in rocks indicate direction of fault rupture

Scratch lines left in rock faces along geological faults during an earthquake may help pinpoint where earthquake energy will be focused in future quakes on the same fault.

The finding comes from a GNS Science-led study of the Kekerengu Fault that ruptured during the November 2016 magnitude 7.8 Kaikōura earthquake.

The scratch lines, or striations, have been observed in fault rock faces for decades. The study, published in Geology this week, is the first time scientists have established a link between the shape, or curvature of the striations, and the direction in which a fault “un-zips” during an earthquake.

Co-author and earthquake geologist Russ van Dissen, of GNS Science, said the direction of fault rupture propagation, or un-zipping, has a major impact on where seismic energy gets focused.

He said the finding was still a hypothesis, but “we are putting it out there so it can be tested in the field.”

“The potential usefulness of our finding is significant and it is important that our hypothesis is tested on other faults.

“If scientists can confidently deduce past directions of fault rupture propagation, that will improve the characterisation of damaging earthquake ground shaking.”

For instance, if it could be shown that the Alpine Fault in the South Island had ruptured a number of times in a certain direction – either north-to-south, or south-to-north - 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, van Dissen said.

The phenomenon of ‘directivity’ was clearly evident during the Kaikōura earthquake, with Wellington experiencing much stronger ground shaking than Christchurch even though the earthquake epicentre was closer to Christchurch. This was a result of the south-to-north direction of the rupture, which sent a large amount of seismic energy to the north.

“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,” van Dissen said.

Rock striations from past earthquakes, called curved slickenlines, are preserved in rock faces underground.

“Now there’s a reason to go looking for them. It’s another tool in our toolbox and it can potentially have a big impact. It enables us to bring a more nuanced approach to the way we analyse past fault ruptures and better estimate future ground shaking,” van Dissen said.

“Maybe instead of digging up softer sites along faults and looking for material to radiocarbon date, scientists will go to bedrock sites specifically to look for these striations. That wouldn’t have happened in the past as there was no reason to do it.”

The study was the result of numerous science disciplines coming together to investigate the impacts of the Kaikōura earthquake.

“You had lots of people all focused on the same thing and inevitably there were going to be some unexpected and quite beneficial results to come out of that.”

The convergence of modellers, geophysicists, and geologists had resulted in some landmark science, he said.

Lead author on the study was PhD student Jesse Kearse, supported by co-authors Yoshihiro Kaneko and Russ van Dissen of GNS Science and Professor Tim Little of Victoria University of Wellington.

The paper can be found here: https://pubs.geoscienceworld.org/gsa/geology/article/572092/curved-slickenlines-preserve-direction-of-rupture

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