The Nevis Fault near Queenstown could rupture and cause an earthquake similar in size to that of the 2010 Darfield event, new research shows.
The EQC-funded seismic research involved a University of Otago team digging two 25m-long trenches high in the Upper Nevis Basin, south of Queenstown, to gain a better understanding of the 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. The new research has found evidence of at least two major prehistorical events.
Lead researcher seismologist Mark Stirling says they found evidence of the sediment layers being broken up, warped and thrust over each other, which would have been produced by significant seismic events.
“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.”
The University of Otago team led by Stirling with postdoctoral scholar Dr Jack Williams and a team of students and colleagues returned from the Upper Nevis Basin in late March.
They re-excavated near sites investigated by New Zealand Geological Survey seismologists in the 1980s, when carbon dating technology did not allow scientists to create 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.’
Prof Stirling said knowing the timings of the prehistoric earthquakes would be hugely valuable to better understanding the fault system, but said they needed 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.”
He said the faults of Otago and Southland generally had long time periods between earthquakes, and could show great variability in behaviour.
“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.”
He said awareness of earthquake hazards was often less pronounced in low seismicity areas, which could intensify the impact of a future event.
Prof Stirling said the Canterbury Plains were seismically quiet until the Darfield earthquake, and he hoped better knowledge and awareness of local seismic hazards would help Southerners be better prepared than Canterbury was in 2010.