It’s one of New Zealand’s biggest quake-makers – and now scientists have found a clever new way to reveal some of the Alpine Fault’s mightiest shakes over potentially hundreds of thousands of years.
Stretching 600km up the western side of the South Island between Milford Sound and Marlborough, the Alpine Fault is our largest on land – and one of our best-known natural hazards.
Recent research has suggested its next major rupture – of which scientists have given a 75 per cent chance of occurring within 50 years - could block South Island highways in more than 120 places and leave 10,000 people cut off.
It also has a clear geologic record of rupturing around every three centuries - and 2017 marked the 300th anniversary of what is thought to have been a magnitude 8 quake that shunted one side of the fault by about 8m in a matter of seconds.
While scientists’ understanding of the fault’s past activity has advanced remarkably in recent times – enabling more refined forecasts of its next quake – they’ve been limited in how far back into our geological history they can reach.
Waikato University geochemist Dr Adam Hartland explained other kinds of prehistoric “paleo-evidence” could allow scientists to reconstruct volcanic eruptions or climate conditions from millions of years ago.
“By contrast, we have paleo-records of Alpine Fault quakes indicating that great earthquakes have occurred quasi-periodically every 250 to 300 years for at least the past 8000 years - but that’s the current limit,” he said.
“As geologists we use information stored in Earth materials. But earthquakes are destructive and so they aren’t really adding to the geologic record; if anything, earthquakes are agents of deletion.”
While geological records were based on evidence stored in rock or long-lived minerals, paleo-seismic information stemmed from clear signs of disturbance at the Earth’s surface, and typically erased over time.
“Therefore, we have only an ephemeral window into past shaking of only a few thousand years.”
A new University of Auckland-led study, thought to be a world-first, might be about to change that.
The study, featuring PhD research by Dr Jeff Lang, found potential for the massive fault’s shaking record to be hugely extended, using ancient, tell-tale clues left deep in caves.
In speleoseismology, scientists search for evidence of past quakes using everything from visible damage, to invisible geochemical changes recorded in groundwater-formed mineral deposits called speleothems.
The researchers suspected that, when the Alpine Fault unleashed gigantic earthquakes in Aotearoa’s prehistoric wilderness, the shaking would have broken up rocks and left geochemical markers they could still find today.
“Speleothems grow continuously to semi-continuously, over tens to hundreds of thousands of years, and can be dated with exquisite precision over the same time span,” Hartland said.
“We thought that, if successful, this new approach could be used to push the envelope of the paleoseismic record by one to two orders of magnitude - way beyond where other records reach.”
They saw New Zealand as the obvious place to test their hypothesis, given its widespread presence of cave systems and famously-active seismic environment.
“We figured that, if it doesn’t work here, then it won’t work anywhere - but if it does, we can provide a new tool in a well-understood setting, which can be used both here and around the world.”
Over three years, Lang analysed samples taken from stalagmites in the West Coast’s Guillotine Cave and Wairoa’s Te Reinga Cave – sites chosen due to being respectively exposed to the Alpine Fault and the similarly-vast Hikurangi margin.
In a Waikato University lab, study co-author Chris Wood carried out experiments that effectively mimicked the conditions in a cave system after a massive shake.
“This literally involved smashing some bedrock with a hammer to mimic the indiscriminate damage caused by a quake, then measuring the amount of magnesium leaching out of the rock into the water,” Hartland said.
Their findings, presented at a recent international conference but yet to be published, confirmed what they suspected: that big ruptures indeed left long-lasting geochemical traces.
Specifically, breaking up limestone caused a huge release of magnesium, which could be preserved in the groundwater that formed stalagmites.
Further, they suspected this quake-triggered process could also geochemically “shut off” the growth of stalagmites.
“Our working hypothesis is that both the rise in magnesium in the stalagmites - and the hiatuses which ultimately follow - are both generated by earthquakes,” Hartland said.
“This appears to lead to stop-start growth of stalagmites in caves near active faults, which is a really attractive idea, because we could look for that motif in caves near faults or plate margins which appear to be less active than the Hikurangi subduction zone or Alpine Fault, for example,” he said.
“Caves might store the evidence of large but infrequent events that simply are deleted by erosion over the course of millennia.
“And importantly, this signal can be disambiguated from other signals originating from climatic processes.”
That opened the door to discovering “new” Alpine Fault earthquakes, along with others on big faults around the world.
“We can realistically extend our understanding of the frequency - and possibly the intensity - of past earthquakes from the last few thousand years, to tens, or even hundreds of thousands of years into the past.”
In a separate research project, Hartland is using a similar approach to reconstruct regional rainfall trends from warm periods in our prehistoric past to help understand how climate change will drive deluges in our future.
Lang completed the majority of his research while studying for a PhD at Auckland University under the supervision of Professor Joel Baker, with funding from his grant from the Ministry of Business, Innovation and Employment’s Smart Ideas pool.
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