Will Ries, GNS Science

Unusual fault rupture during Kaikōura quake

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More than 24 faults ruptured during 2016's Kaikōura earthquake, but one of those faults has turned out to be even more unusual than scientists first thought. The Papatea Fault, unmapped before the quake, ruptured along 19km and shunted areas upward by 8m in just a few seconds. A new study has shown the fault hadn't accumulated stress as is usually seen before a quake, but rather seems to have been squeezed by all the neighbouring faults around it rupturing, which could prompt a rethink about seismic hazard calculation.

Journal/conference: Science Advances

Organisation/s: GNS Science, University of Victoria, Canada; Colorado School of Mines, USA

Funder: University of Victoria, National Science and Engineering Research Council of Canada, Canada Foundation for Innovation, BC Knowledge Development Fund, Canada Research Chair, Land Information New Zealand, NZTA, Environment Canterbury, Marlborough District Council, AAM NZ.

Media Release

From: GNS Science

Scientists explain unusual behaviour of Papatea Fault

One of the 24-plus faults that ruptured in the 2016 magnitude 7.8 Kaikōura earthquake has turned out to be even more unusual than scientists first thought and it may prompt a rethink about how seismic hazard is calculated.

The Papatea Fault, unmapped before the quake and running along a similar path to the lower Clarence River in Marlborough, produced a 19km-long surface rupture and shunted a large area of mountainous country up by 8m in a matter of seconds.

A study published this week in the journal Science Advances indicates that the fault ruptured even though it hadn’t accumulated stress normally associated with fault rupturing.

Co-author and earthquake geologist at GNS Science Rob Langridge says it appears the fault suddenly became squeezed for room by the rupture of neighbouring faults causing it to break in “a very emphatic way”.

“The rupture of the Papatea Fault stands out for being one of the most dramatic elements of what was an unusual rupture sequence in the first place,” Dr Langridge said.

“It produced the largest vertical movements of all the faults that ruptured during the earthquake and it has puzzled scientists because its rupture could not be fitted to standard models of fault rupture.”

However, the paper published this week used computer analysis of LIDAR images to come up with a solution to its unusual behaviour. It was written by Canadian MSc student Anna Diederichs with colleague and geophysicist Ed Nissen, both of the University of Victoria in British Columbia. Three scientists from GNS Science, including Langridge, were co-authors.

“We discovered a number of unusual characteristics to this fault. Most unusually, the standard elastic rebound model of earthquake faulting did not fit the observed ground deformation,” Dr Nissen said.

“We’ve concluded that the Papatea Fault did not release elastically stored tectonic strain as faults normally do during a rupture.”

Dr Nissen said the findings indicate that some faults may fall outside typical fault behaviour and conventional modelling may not capture the hazard they pose.

He said earthquake forecasting is based on the elastic strain cycle model where faults gradually accumulate strain until they fail, and then the cycle is repeated.

“However, the Papatea Fault does not seem to follow this model, and such faults may still need to be accounted for in earthquake forecast models.”

Going forward, he said this research finding could be considered when assessing the risk from faults that might have a low or unclear strain accumulation signal.

The research was based on computer analysis of pre- and post-earthquake LIDAR images of the fault rupture area. Fortuitously, Environment Canterbury collected LIDAR of the Clarence Valley area several years before the Kaikōura earthquake mainly for flood protection purposes.  These images were compared with LIDAR images collected in the wake of the earthquake in 2016.

The analysis enabled the scientists to derive the 3D geometry of the fault which was a basis for numerical modelling of the rupture.

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Professor Mark Stirling, Chair of Earthquake Science, Department of Geology, University of Otago

The study has done a nice job of explaining the unusually large displacements along the relatively short length of the Papatea Fault during the Mw7.8 2016 Kaikoura earthquake. I was part of the team that worked on the rupture mapping and trenching investigations associated with the earthquake, but with a focus on the Hundalee Fault. I also worked on comparing the overall rupture characteristics of the earthquake to newly developed fault scaling relations (magnitude as a function of rupture length and slip rate), and managed a fieldtrip to the Papatea Fault about a year after the earthquake.

It has always made sense to me that the Papatea Fault produced large displacements in the earthquake because of its unusual orientation with regard to that of the greater earthquake rupture zone. The present study has now provided a workable mechanical explanation for the anomalous behaviour. I have heard of other studies that have focused on the unusual behaviour of the fault, and all share the same general theme: That the fault was a transfer structure between major seismic moment-generating faults, and the fault orientation was central to its anomalously large displacements.

The paper mentions the seismic hazard implications of faults like the Papatea Fault within complex multi-fault earthquakes. In reality the issue here is not how much these faults contribute to ground shaking, but what they mean in terms of fault displacement hazard. A 10m displacement in a built up area or beneath a critical facility (e.g. large dam) would have significant consequences.

Last updated: 02 Oct 2019 11:54am
Declared conflicts of interest:
No conflict of interest.

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