Research amends size of Alpine Fault's last quake

15 August 2012

Investigations by a University of Canterbury researcher have found that the Alpine Fault’s last rupture almost 300 years ago reached a magnitude of approximately M 8.1.

Geological sciences doctoral student Gregory De Pascale, working with Dr Robert Langridge of GNS Science, has found that the 1717 event saw at least 380km of the fault rupture at a depth of 12km.

The rupture stretched from Haupiri River on the West Coast of the South Island down to Milford Sound, and the two researchers estimate it reached a magnitude of 8.1, higher than the previously thought mean magnitude of M 7.9.

“But there is a caveat to our magnitude calculations as we are not sure if the fault ruptured further south as it goes out to sea at Milford Sound,” said Greg.

“If it did rupture out to sea, then the magnitude may have been bigger than the 8.1 we’ve calculated.”

The researcher’s findings will be published in the September issue of the journal Geology. Their paper, “New on-fault evidence for a great earthquake in AD 1717, central Alpine fault, New Zealand”, is based on work the researchers carried out on a portion of the 200km section of the central portion of the Alpine Fault which had not been studied before.

Prior to their work, it was not known if the 1717 rupture had affected this area as the difficult terrain made on-site investigations difficult. However, using lidar (airborne light detection and ranging) data, they found evidence of the last rupture which helped them fill a gap in the Alpine Fault’s seismic data.

“Lidar enabled us to strip away all the vegetation and see a section of the fault that was previously undetected,” he said.

The pair also based their findings on their own field investigations in the area around Gaunt Creek, a tributary of the Waitangitaona River, where they documented faulting on the fault scarps and timing of faulting using radiocarbon dating.

“The size of an earthquake is related to the length of the fault rupture. From what we’ve found, at least 380km of the onshore fault ruptured in a single event and we’ve calculated that such a rupture would cause an earthquake of a magnitude of around M 8.1.”

Greg said their findings would have significant implications as the high magnitude potential from Alpine Fault earthquakes “makes it the greatest seismic hazard for the South Island”.

“This means a lot more energy will be released and the shaking will go on for longer in a future event. Liquefaction could be an issue in areas prone to it, even in places that are hundreds of kilometres away from the epicentre.”

Greg said once it is rebuilt, Christchurch should be one of the safest places to be in any future earthquake, but he hoped this new information would help promote infrastructure resilience in other South Island regions.

An abstract of the article can be found at http://geology.gsapubs.org/content/early/2012/06/21/G33363.1.abstract.

 

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