New research updates old earthquake theory

A new study reveals the likelihood of a future earthquake occurring along the Nevis-Cardrona Fault in western Otago is lower than previously thought.

It also provides new constraints for estimating earthquake hazards around the rapidly developing Queenstown-Wānaka region.

Led by the University of Otago – Ōtākou Whakaihu Waka and funded by a Natural Hazards Commission (NHC) Toka Tū Ake University Research Programme grant, researchers excavated the southern part of the fault to determine when prehistoric earthquakes took place.

While doing so, they discovered new information which updates research from the 1980s.

Jack Williams

Lead author Dr Jack Williams, of the Department of Geology, says prior studies have recognised the possibility of future earthquakes, but their rate and size were unclear.

“Our new data suggests that the rate of prehistoric Nevis-Cardrona Fault earthquake is lower than estimated by these previous studies.” – Dr Jack Williams

“Our new data suggests that the rate of prehistoric Nevis-Cardrona Fault earthquake is lower than estimated by these previous studies,” he says.

“Although, it does serve as a reminder that earthquakes can happen anywhere across Aotearoa, and we should all prepare and plan accordingly.”

Mark Stirling

Chair of Earthquake Science Professor Mark Stirling says the work is also part of a wider effort by the group to characterise the dozen or so most major active faults of the Otago region.

Knowing when past earthquakes occurred creates an insight into future ones, he says.

Published in the journal Seismica, researchers analysed the different geologic layers in trenches across the fault and used dating methods to obtain the timing of prehistoric earthquakes.

Dr Williams says where they see evidence in the terrain of past surface-rupturing, they can excavate trenches perpendicular to the fault, typically 30m-40m long, 6m deep and 7m wide.

“On the trench walls, we then analyse how different sediments have been displaced and moved by the past ground-rupturing earthquakes along that fault,” he says.

“Most importantly, we want to know how many earthquakes must have occurred to explain the pattern of sediments in the trench wall, and then we collect samples of these sediments that we can date.”

To date the sediments, researchers use a technique called Optically Stimulated Luminesce dating, which measures the last time sediments were exposed to sunlight.

“From these results, we can then constrain the timings of earthquakes that deformed the sediments.”

Researchers believe the sediments from the Nevis-Cardrona Fault experienced two earthquakes, about 12,800 years ago and sometime between 20,000-40,000 years ago. Comparatively, previous work inferred there were four earthquakes in the past 20,000 years.

Dr Williams says the study’s findings differ from research forty years ago because earthquake science has progressed since then.

The previous investigations into the fault were conducted to assess earthquake hazards during the development of hydroelectricity in the Clutha River’s catchment.

“It was one of the first detailed paleoseismic studies in New Zealand and this work was, at that time, very forward-thinking,” he says.

“In the 1980s, the only way to derive the age of sediments, and hence constrain the timing of past earthquakes, was through radiocarbon dating. However, carbon is pretty rare in near-surface soils in Central Otago and so they were only able to find carbon and derive earthquake timings from one trench.

“Elsewhere, the timings of past earthquakes were an ‘educated guess’.

“With subsequent development of dating techniques, the lack of carbon is no longer a barrier to obtaining earthquake timings, and cumulatively the dates we obtained from the trenches in the Upper Nevis Valley indicate lower earthquake rates than previous estimates.”

Head of Research at NHC, Dr Natalie Balfour, says that research into understanding earthquake risk feeds into community education programmes and risk modelling, which helps communities better prepare for future events.

“While in this case the data shows that earthquakes in this part of Otago have happened less frequently than previously thought, any part of the country could experience a damaging quake or damage from a big shake farther away. Wherever you live, it’s a good idea to quake safe your home, know your insurance cover and prepare emergency supplies.”