Scientists have just gained a richer picture of how big events on the Alpine Fault have changed the Southern Alps over time. Photo / Doug Sherring
Scientists have just gained a richer picture of how big events on the Alpine Fault have changed the Southern Alps over time. Photo / Doug Sherring
They're one of New Zealand's most breath-taking features – but the Southern Alps wouldn't look so beautifully craggy, if not a series of ancient, monster quakes along the country's biggest land fault.
Scientists have just gained a richer picture of how big events on the Alpine Fault have changed thealps over time, with violent shaking sculpting their peaks, and landslides driving erosion further down the mountainside.
Clever chemistry-based tools they developed for the study, just published in Science Advances, could now also point risk planners to likely danger spots for landslides following the next major quake.
The Alpine Fault, running 600km up the western side of the South Island between Milford Sound and Marlborough, poses one of New Zealand's largest natural threats – and scientists expect a quake measuring over 8.0 could strike within decades.
Over the last 12 million years, the fault has driven an incredible 20km of uplift, making the Southern Alps its most recognisable marker: only the fast pace of erosion has kept the alps' highest point below the 4000m mark.
The Alpine Fault is marked by the Southern Alps that forms the spine of the South Island. Photo / Nasa
Scientists have been trying to build a more detailed understanding of the exact relationship between large quakes and erosion on the ranges, much of it driven by tens of thousands of quake-induced landslides.
Earlier studies drawing on geological evidence captured in surrounding flood plains and lake sediments have suggested that, in the aftermath of massive Alpine Fault quakes, such erosion could carry on for decades.
But what scientists haven't had was a way to pinpoint specifically where those landslides were happening – something crucial not just for tracking quake-made changes over time, but also revealing risk areas.
Victoria University's Dr Jamie Howarth and colleagues from Otago and Durham (UK) universities went into the study working off the knowledge that quakes drove more erosion at the crests of mountains than further down, because the shaking was more intense higher up.
Over the last 12 million years, the Alpine Fault has driven an incredible 20km of uplift, making the Southern Alps its most recognisable marker. Photo / Steven McNicholl
Directly testing that hypothesis wasn't so easy because the influence of quakes on the shape of mountain ridges plays out over many quakes – and the gaps between Alpine Fault events typically spanned hundreds of years.
That led them to find an answer a different way – using tell-tale "biomarkers" within organic matter in lake sediments, to reveal the source of ancient landsliding over many quakes.
Lake Paringa, a small lake 50km north of Haast, proved the perfect place to do it.
Not only did it act as a time machine, by gradually archiving the history of events that occur in the lake catchment over thousands of years, but it had already been used by Howarth and colleagues to reconstruct past Alpine Fault quakes.
Using a large corer, they pulled up 6m-long samples of sediments that had accumulated at the bottom of the lake.
Scientists used a corer to extract 6m-long sediment samples from the bottom of Lake Paringa on the South Island's West Coast. Photo / Supplied
"Our post-doctoral research fellow Dr Jin Wang spent many meticulous hours in the chemistry laboratory slowly extracting organic compounds from the sediment that we thought might tell us something about the location of landslides."
They found that these biomarkers could indeed point them directly to where landsliding, and hence erosion, had been happening on the alps – making it the first time they'd been used in that way.
Moreover, the work confirmed their hypothesis that huge quakes caused erosion near mountain ridges.
"The implications of our work are that the location and shape of ridges are likely to change much more rapidly in mountains that experience large earthquakes than those that don't."
Looking to the next big quake, Howarth expected New Zealand would be dealing with heightened landslide hazard for a long time to come, given the alps would be likely left scarred by tens of thousands of landslides.
He saw an opportunity to apply their new biomarker tools more widely, so scientists could get a clearer idea of precisely where quake-induced landslides would most likely happen.
"When combined with computer models of sediment export by rivers that we are developing as part of an MBIE-funded project on the Kaikoura earthquake, the information on landslide location may provide important new insights into how best to plan for the next event."
