Last Alpine Fault quake size revised
Christchurch Earthquake 2011
The most recent Alpine Fault earthquake had a likely magnitude of 8.1, Canterbury University research shows.
Geological sciences department doctoral student Greg De Pascale has calculated the great 1717 quake's magnitude, which is plus or minus 0.1 of magnitude because of uncertainties in the fault's precise shape.
Those figures will be useful in considering the extent of liquefaction in Christchurch from an Alpine Fault quake and how to plan for it.
In a paper in this month's Geology, De Pascale and co- researcher Robert Langridge, of GNS Science, reviewed evidence of the date of the fault's last quake.
The fault marks the boundary of the Pacific and Australian plates and runs from Marlborough down the western edge of the Southern Alps to the sea at the entrance to Milford Sound.
From past research, new Lidar (light detection and ranging) data and further study of where the fault crosses Gaunt Creek near Whataroa, the two scientists determined that at least 380 kilometres of the fault - from the Haupiri River in north Westland to Milford Sound - ruptured.
In comparison, the Greendale Fault that broke on September 4, 2010, is about 30km long.
No previous work on the middle part of the 200km-long central segment of the Alpine Fault had found evidence for the 1717 quake. That had come from dating organic material in trenches dug further north, near Inchbonnie, and further south close to Haast.
The paper said the magnitude-8.1 calculation assumed the 1717 rupture stopped on the fault where it went offshore.
"Ultimately, the southern termination of the 1717 rupture is unknown. However, if portions of the offshore Alpine Fault mapped . . . south of Milford Sound ruptured with the onshore portion, the 1717 earthquake was larger than the magnitude calculated," it said.
A magnitude-8.2 quake is 1.6 times bigger than a magnitude 8.0, the previous estimate of the 1717 event, and releases twice as much energy.
De Pascale told The Press that the terrain and dense West Coast bush made studying the fault "challenging".
Lidar effectively stripped away the bush and revealed the landscape, and the fault, beneath.
"The 380km is a minimum, depending on how much offshore south of Milford Sound actually ruptured," he said. "It's very hard to gauge because at this point there's no data. You'd have to do seabed coring, and that is pretty challenging.
"If it ruptured further south offshore, the magnitude calculated is actually an underestimate."
The findings were important for Christchurch's redevelopment, he said.
"The duration and intensity of shaking is much greater in an 8.2 or greater earthquake than in an 8.0," he said.
"Researchers at Canterbury University documented liquefaction over 350km away from the 2011 Japan earthquake, and there are examples from California of liquefaction hundreds of kilometres away from large to great earthquake epicentres.
"Because of the long duration of shaking, even though the shaking would be likely much less violent than what we have experienced in Canterbury over the past couple of years, liquefaction would likely be extensive in known liquefaction-prone areas of Canterbury.
"We will need to take this into consideration during the rebuild and in future earthquake-effect models."
- © Fairfax NZ News
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