Source: Radio New Zealand
File footage, ESNZ – a diagonal fault offset exposed in a trench. Lloyd Homer
Scientists are uncovering previously unknown fault lines and signs of hidden earthquake activity across the North Island, as new research sheds light on how the ground beneath Aotearoa moves.
Studies are investigating potential faults beneath Auckland, newly identified fault lines in Wairarapa and signs of past shaking preserved in lake sediments near Hamilton.
Principal scientist Dr Graham Leonard from Earth Sciences New Zealand said the work highlighted how much remained unknown about the country’s geology.
“There are many, many thousands of fault lines across Aotearoa,” he told RNZ’s Saturday Morning.
“We’ve really only studied some hundreds of them in detail.”
In Auckland, researchers have identified one to two dozen possible fault structures beneath the city using thousands of underground drill samples originally taken for buildings and infrastructure projects.
Likely faults, possible faults and possible structures identified in the study are superimposed on a shaded relief map of Auckland and plotted alongside Auckland Volcanic Field centres and earthquake epicentres since 1988. Supplied
Scientists plan to dig trenches several metres deep across some of the most likely candidates, including structures near Pukekohe or Drury, to determine whether they are active.
By analysing exposed layers of soil and sediment, researchers can identify where past earthquakes have shifted the ground. Buried material, such as wood or charcoal, can then be carbon-dated to estimate when those quakes occurred.
“If there are bits of wood or leaves or charcoal in there, we can carbon date them,” Leonard said.
“That helps us work out when the earthquake happened and whether the fault has been active in the last 100,000 years.”
In Wairarapa, high-resolution laser mapping known as LiDAR has revealed seven previously unknown faults, including several crossing the Wairarapa Valley.
One of them – the 26-kilometre Pāpāwai Fault – has recently been trenched for the first time.
Scientists found a several-metre-wide disturbance zone, suggesting past earthquake movement may have occurred across multiple smaller fractures rather than a single break.
Further trenching work is planned near Masterton to pin down when earthquakes last occurred there.
Another study in the Hamilton Basin uncovered evidence of past earthquake shaking preserved in lake sediments.
Researchers drilled more than 160 shallow sediment cores from lakes and analysed them using medical CT scanners, allowing them to detect disturbances in layers of mud and volcanic ash caused by strong shaking.
Those disturbances, known as seismites, act as natural records of past earthquakes.
The findings suggest some shaking came from distant faults already known to scientists, but other signals likely came from faults beneath the Hamilton area itself.
Leonard said advances in technology were helping researchers detect previously hidden fault lines.
LiDAR scans landscapes with millions of laser measurements taken from aircraft, allowing scientists to digitally remove vegetation and buildings and reveal subtle steps in the ground that may mark faults.
An example of LiDAR scanning. File photo. ESNZ
Although Auckland and Hamilton experience fewer earthquakes than other parts of the country, Leonard said that did not mean they were immune to major events.
“You can still have a big earthquake anywhere in Aotearoa.”
He pointed to the 1891 Port Waikato earthquake, which shattered windows across Auckland.
Christchurch had shown how damaging earthquakes could strike areas that rarely experience strong shaking, Leonard said.
The fault responsible for the 2011 Christchurch earthquake had been previously unknown and may only rupture every several thousand years.
Research near Taupō is also examining whether earthquakes and volcanic eruptions may influence each other, after scientists found fault movements around the time of the 232AD Taupō eruption.
Leonard said combining studies like these helped improve New Zealand’s seismic hazard models, which inform building standards and risk planning.
“When we bring all these [studies] together, it helps us understand how faults interact with each other and how earthquakes might cluster in space and time.”
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand
