Great quakes' debris tracked
The West Coast's recovery from the next major Alpine Fault earthquake could be threatened for up to 50 years by rockslides, landslips and changing river courses.
Research by Otago University and the Australian Nuclear Science and Technology Organisation on sediments on the floor of South Westland's Lake Paringa shows that during the five decades after previous Alpine Fault quakes, three times the quantity of rock and debris eroded from the Southern Alps compared with the usual amount.
Millions of tonnes of crushed and fractured rock shaken loose from the ranges are likely to hinder the reconstruction of bridges and vital roads, as well as damage power lines and phone networks, either immediately or across many years, when triggered by heavy rain.
The new work casts doubt on previously accepted dates of quakes of about magnitude 8.0 generated by the fault, which runs along the western spine of the Alps from Marlborough to the entrance of Milford Sound.
Radiocarbon dating of leaves and organic material in sediment cores collected from the lake indicates the 1620 quake more likely occurred between 1535 and 1596, while the 1430 quake took place between 1374 and 1405.
Earlier major quakes were between 887 and 965, and 1064 and 1120.
The date of the most recent great quake of 1717 remains unchallenged.
In a paper in the Geology journal, Otago PhD student and research fellow Jamie Howarth and co-authors say that despite the longer return period suggested by their research, nearly 300 years have passed since the last rupture.
That implied "a high probability for the next event" in the near future.
Howarth, geography department colleague Sean Fitzsimons and field assistants collected six-metre-long cores from three locations in the 58-metre-deep southern part of Lake Paringa.
Howarth told The Press the cores of mostly sand and silt were split into 1.5m segments once on land and transported to the university for refrigeration, analysis and dating.
"In the cores we get really well-preserved leaves from around the lake, so we can be pretty confident when the leaf died and its isotopic clock started ticking."
The layers of sediment in the cores for each of the five quake cycles in the past 1100 years were remarkably similar, he said.
The first - bottom - layer was composed of deposits from around the lake that collapsed and sank as a direct result of strong shaking from the quake.
On top of that was a stacked sequence of sediments that originated from landslides and other kinds of erosion from the ranges in the years after the main quake.
It was that evidence that showed the decades-long effect of the quake and the potential economic disruption the Coast would face after the next Alpine Fault quake, Howarth said.
"The impacts are going to be there for a long time."