Seismic probability revision study

New constraints have been provided for estimating earthquake risks around the fast-growing Queenstown-Wanaka region, after new research revealed the likelihood of a future earthquake along the Nevis-Cardrona Fault is lower than previously thought.

The study involved researchers excavating the southern part of the fault to determine when the prehistoric earthquakes occurred.

While doing so, they discovered new information that updated research conducted in the 1980s.

Lead author and University of Otago geologist Dr Jack Williams said previous studies had recognized the possibility of future earthquakes, but their rate and magnitude were unclear.

“Our new data suggest that the rate of the prehistoric Neves-Cardrona earthquake is lower than these previous studies estimated.

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

The researchers analyzed the different geological layers in the trenches across the fault and used dating methods to obtain the timing of prehistoric earthquakes.

Dr Williams said that when they saw evidence in the topography of previous surface rupture, they could dig trenches perpendicular to the fault, typically 30m to 40m long, 6m deep and 7m wide.

“On the walls of the trench, we then analyze how different sediments have been displaced and moved by previous earthquakes that have torn up the ground along this fault.

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

To date the sediment, the researchers used a technique called optically stimulated dating, which measures the last time the sediment was exposed to sunlight.

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

Researchers believe that sediments from the Neves-Cardrona fault were exposed to two earthquakes, about 12,800 years ago and sometime between 20,000 and 40,000 years ago.

By comparison, previous research has concluded that four earthquakes have occurred in the past 20,000 years.

Dr. Williams said the study's findings differ from research conducted 40 years ago because seismology has advanced.

Previous fault investigations have been carried out to assess seismic hazards during hydropower development in the Clutha River catchment.

“It was one of the first detailed studies of ancient fossils in New Zealand, and this work was, at the time, very forward-thinking.

“In the 1980s, the only way to derive the age of sediments, and thus determine the timing of past earthquakes, was through radiocarbon dating.

“However, carbon is very scarce in the near-surface soils of Central Otago, so they were only able to find carbon and extract the timing of earthquakes from a single trench.

Elsewhere, the timings of past earthquakes have been an “educated guess.”

“With the subsequent development of dating techniques,…the dates we have obtained from trenches in the upper Nevis Valley indicate lower seismic rates than previously estimated.”

The study was funded by the Toka Tū Ake University Research Program Grant of the Natural Hazards Committee. — ABL

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