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New research confirms major seismic events in West Otago

Professor Mark Stirling at the Nevis Fault in western Otago

New EQC-funded seismic research on the Nevis Fault in western Otago confirms that this low-seismicity area has the potential to create an earthquake similar in size to that of the 2010 Darfield Canterbury event.

“People in Otago and Southland usually only think about the Alpine Fault when they think about earthquakes, but we now know that there are major faults in low-seismicity areas that could produce a major event,” says lead researcher Mark Stirling from the University of Otago.

Professor Stirling, Postdoctoral Scholar Dr Jack Williams and a team of students and colleagues this week returned from digging two 25-metre long trenches high in the Upper Nevis Basin, south of Queenstown, to gain a better understanding of the Nevis Fault, and greater Nevis-Cardrona fault system.

The combined fault system stretches for about 100km from Lake Wanaka to near Garston in Southland, and has the potential to produce an earthquake well into the magnitude 7 range; with the new research finding evidence of at least two major prehistorical events.

“We found evidence of the sediment layers being broken up, warped and thrust over each other, which would have been produced by significant seismic events,” says Professor Stirling.Otago Univsersity scientists trenching at Nevis Rise in West OtagoEQC Research Manager Natalie Balfour says that the Queenstown Lakes District is one of the fastest growing areas in New Zealand with a huge amount of new housing and infrastructure development.

“EQC supports this type of research because it is important for developers, local councils, and the public to understand and manage potential seismic hazards,” says Balfour

The University of Otago team re-excavated near sites that had been investigated by New Zealand Geological Survey seismologists in the 1980s, but carbon dating technology in 1980s did not allow scientists to create any timelines.

“The early studies of the Nevis-Cardrona system and neighbouring faults were the first of their kind in New Zealand, and we are building on that knowledge, because we now have new sediment dating technologies to analyse the samples,” says Stirling.

Stirling says knowing the timings of the prehistorical earthquakes will be hugely valuable to better understand the fault system, but hastens to add that they need a lot more data to forecast the timing of the next event with any degree of confidence.

“To create probability models to forecast the likelihood and timing of the next event, like can be done with the Alpine Fault, you would need a lot more than just two earthquakes,” says Stirling.Trench wall sample in Nevis Rise trenchThe professor says that the faults of Otago and Southland generally have long time periods between earthquakes, and can show great variability in behaviour through time.

“But if the Nevis-Cardrona system were to rupture, we’d most definitely see a large earthquake and most of Otago and Southland would feel it.

“Everyone knows about the Alpine Fault, which is a rock star of faults and gets most of the publicity in the South Island, but people also need to be aware of local seismic hazards,” says Stirling.

He says that awareness of earthquake hazard is often not as pronounced in low seismicity areas as in high seismicity areas, and that this could really intensify the impact of a future event.

Stirling adds that the Canterbury Plains were seismically quiet until the Darfield earthquake, and he hopes that better knowledge and awareness of local seismic hazards will help Southerners be better prepared than Canterbury was in 2010.