Lasers map active fault lines
Earthquake geologists are using new laser technology to map active fault lines running through Marlborough.
The fault lines branch off the Alpine Fault and reach across the region to the northeast, with four major fault lines comprising the Marlborough Fault System.
A GNS report for the Marlborough District Council, draws on new laser technology, aerial photographs and maps from earlier studies to create a more accurate picture of where the faults lie.
One of the major fault lines runs along the Wairau River through Renwick and Grovetown, 4 kilometres north of Blenheim.
GNS Science earthquake geologist Robert Langridge said he and geomorphologist William Ries wrote the report to inform decisions on where to subdivide and build residential developments.
‘‘Geologists have known for a long time the Wairau Fault was running along the Wairau Valley and out to sea near the Wairau Diversion. So with the advent and availability of LiDAR, we are able to make very detailed maps,’’ Langridge said.
The council, EQC and the University of Southern California made results of various laser surveys available for the GNS study.
The technology, known as LiDAR, created a 3-D digital surface model of the Wairau Valley landscape with a laser scanner attached to an aircraft, Langridge said.
‘‘It’s a little bit like radar ... the plane flies along and beams energy down, and it collects the hundreds of millions of points that come back.’’
Langridge and Ries mapped the geological makeup of the ground underneath grass and trees, to find how the ground had shifted from tectonic activity over thousands of years.
Langridge said last week a possible 8.0 magnitude earthquake along the Alpine Fault would have the potential to ‘‘unzip’’ the South Island.
A rupture like that happened roughly every 300 years, with the last one being in 1717.
The Wairau Fault ruptured on average at least every 2000 years, Langridge said.
A rupture could cause a similar level of damage as the Napier earthquake in 1931 or Darfield in 2010.
It was important the council knew where the faults were, especially the Wairau Fault, so they could consider the best locations for developing land and subdividing and decrease the potential for damage should the fault rupture, Langridge said.
However, the location of about 20 per cent of the Wairau Fault was still uncertain after the study, mainly between Renwick and Grovetown, and near the Wairau Diversion.
The flow of the Wairau River had changed the geology of the area which made the fault line harder to find, Langridge said.
The report recommended the council undertook geophysical studies to identify those buried traces.
Council environment committee chairman Peter Jerram said further research would depend on more funding.
The report was funded by the Ministry of Business, Innovation and Employment under the ‘Envirolink’ grant, created to fund research regional councils did not have the budget for.
Finding the faults under the Wairau River would require a lot of digging and a lot of funding, Jerram said.
The Awatere, Waihopai, Vernon and Clarence fault lines were moderately active, rupturing every 1500 to 2000 years, and capable of generating 7.0 magnitude earthquakes or greater.
New Zealand ‘‘straddled’’ two tectonic plates and there was a myriad of fault lines through the country.
Marlborough was a moderately active region for earthquakes, Langridge said.