Alpine Fault's rupture stronger than thought

By Paul Harper

Liquefaction could be an issue in areas that are hundreds of kilometres away from the epicentre, says Gregory De Pascale. Photo / Geoff Sloan
Liquefaction could be an issue in areas that are hundreds of kilometres away from the epicentre, says Gregory De Pascale. Photo / Geoff Sloan

The Alpine Fault's last rupture was stronger than previously thought, according to University of Canterbury research.

Geological sciences doctoral student Gregory De Pascale, working with Dr Robert Langridge of GNS Science, found the 1717 earthquake 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 at least as far as Milford Sound and the two researchers estimate it reached a magnitude of 8.1, higher than the previously thought mean magnitude of 7.9.

However it is not known whether the fault ruptured further south as it goes out to sea at Milford Sound, Mr De Pascale said.

"If it did rupture out to sea, then the magnitude may have been bigger than the 8.1 we've calculated," he said.

"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."

The researcher's findings will be published in the September issue of the journal Geology. They focused on a portion of the 200km section of the central portion of the Alpine Fault which had not been studied before, using Lidar (airborne light detection and ranging) data to take away the vegetation to see a section of the fault not seen before.

The pair also undertook 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.

Mr De Pascale 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."

Mr De Pascale said once Christchurch is rebuilt it 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.

- Herald Online

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