Scientists hunt for clues about big Alpine Fault quake

Stuff.co.nz

GNS Science earthquake geologist Robert Langridge has been studying why the Alpine Fault is so susceptible to earthquakes - it's since been discovered that it may be the world's fastest-moving known fault line.

Geologists have dug two trenches across the Alpine Fault to find out when and how big previous earthquakes have been. 

Five geologists from GNS Science spent last week in Springs Junction, near the Lewis Pass on the West Coast, to study how the Alpine Fault has ruptured in the past and what kind of earthquake might happen in the future.

They hope to find out exactly how big the last three or four earthquakes were and when they happened. 

GNS Science earthquake geologists studying layers in the trench across the Alpine Fault at Springs Junction.
GNS SCIENCE

GNS Science earthquake geologists studying layers in the trench across the Alpine Fault at Springs Junction.

However, they did not find as much organic material as hoped for radiocarbon dating.

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"We did find a black beetle shell and a layer of peat that we will send off to our lab for dating.

The two trenches dug across the Alpine Fault where geologists are studying previous earthquakes.
GNS Science

The two trenches dug across the Alpine Fault where geologists are studying previous earthquakes.

"This will allow us to date when the layers were created and from there we can date the previous earthquakes. By examining the layers we can see that it is a fault and there has been active earth movement going on there," project leader Dr Rob Langridge said.

Langridge, an earthquake geologist with GNS Science, said identifying and dating the last few ruptures on the fault would proved a pointer to the size and style of earthquake that can be expected in the future. 

The Alpine Fault runs across the South Island.

GNS Science earthquake geologist Robert Langridge studying layers in the trench across the Alpine Fault at Springs ...
JOANNE CARROLL/FAIRFAX NZ

GNS Science earthquake geologist Robert Langridge studying layers in the trench across the Alpine Fault at Springs Junction. New research has found that the fault line may be the world's fastest-moving, having shifted around 700km in 25 million years.

The team dug two 20 metre-long and 2m-deep trenches at the Marble Hill campground, next to the Maruia River, which allowed them to take photos and make detailed drawings of the layers. 

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He said studies had revealed the last earthquake of the Alpine Fault happened around 1800 and caused 30cm of movement. He estimated it would have been about a 7.5 magnitude, but his studies had showed the previous earthquakes would have been even larger than that. 

A concrete wall built along the fault in 1964 has not moved at all. 

GNS Science earthquake geologist Robert Langridge studying layers in the trench across the Alpine Fault at Springs Junction.
Joanne Carroll

GNS Science earthquake geologist Robert Langridge studying layers in the trench across the Alpine Fault at Springs Junction.

"This is telling us that the fault is releasing the build up of energy in large earthquakes. I would expect that when it faults the wall will crack and move about five metres," he said. 

Previous studies found the old river terraces at Springs Junction had moved 11m horizontally and about 2m vertically as a result of three or four earthquakes in the last 1200 years.  

Langridge said aerial images obtained late last year revealed the Alpine Fault was several hundred metres away from where the fault had appeared on maps.  

GNS Science earthquake geologist Robert Langridge at the wall which was built across the Alpine Fault in 1964 at Springs ...
Joanne Carroll

GNS Science earthquake geologist Robert Langridge at the wall which was built across the Alpine Fault in 1964 at Springs Junction

The trenches have been filled in and the land returned to its previous state when the investigation was finished.

The project had the approval of the Department of Conservation, which owned the land, and was funded by the Natural Hazard Research Platform.

The study's findings were expected to be published in 2017.

GNS Science earthquake geologists Pilar Villamor and Robert Langridge studying layers in the trench across the Alpine ...
Joanne Carroll

GNS Science earthquake geologists Pilar Villamor and Robert Langridge studying layers in the trench across the Alpine Fault at Springs Junction.

 - Stuff

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