It is not just an earthquake on the Alpine Fault which people living in the southern lakes area need to be prepared for, GNS Science says. Several other faultlines could produce a smaller, but potentially just as damaging, quake. Wanaka reporter Jessica Maddock talks to scientist Simon Cox.
People living in the Southern lakes region need to be aware of the earthquake hazard posed by local faultlines, as well as the Alpine Fault, GNS Science says.
One of the crown research institute's principal scientists, Simon Cox, a structural geologist, spoke in Queenstown recently about four faultlines in the Queenstown Lakes and Central Otago districts which do not have the public profile of the Alpine Fault, ''could be smaller, but could cause more damage''.
''People talk about the 'big one', but it's the 'big ones'.
''There are two components to the earthquake hazard in this region - there's the hazard of the Alpine Fault and there are local earthquakes that could occur on nearby faults.''
The four local faultlines are believed to be capable of producing magnitude six to seven earthquakes.
They are much smaller than the Alpine Fault, have a lower average movement rate, and are believed to have a far longer, but more erratic, recurrence interval of between 1000 and 10,000 years.
In comparison, the Alpine Fault, which could cause an earthquake of magnitude seven to eight, is described as a ''well-behaved faultline'', because it has a regular recurrence interval of between 260 and 400 years, with an average of 329 years plus or minus 68 years.
This is based on 24 earthquakes since about 5000BC.
''Because it's so regular, we don't have to go back very far in geological time to get a record of the events, because they occur so often,'' Dr Cox, who is based in Dunedin, said.
GNS Science predicts there is a 30% chance of an earthquake occurring on the Alpine Fault in the next 50 years.
''We know we're moving into the window of when we would expect it to rupture again.''
A problem with the four local faultlines was that, due to the long recurrence interval between earthquakes, information about when they last shifted significantly was ''spotty'', making it difficult to predict when another might take place.
However, his ''preferred approach is that they're guilty until proven innocent'', Dr Cox said.
''A magnitude six to seven earthquake of course can be very damaging, particularly on a fault that doesn't rupture regularly and smooth itself out.''
The four faultlines include the Nevis-Cardrona Fault, which follows the Cardrona Valley; the Pisa Fault, which follows the Pisa Range on the Lake Dunstan side; and the Dunstan Fault - the most examined of the four, due to its potential impact on the Clyde Dam - which runs from Clyde to St Bathans and the upper Manuherikia Valley.
Dr Cox said more research was needed on the fourth faultline - the Moonlight Fault, which follows Moke Lake near Queenstown to the headwaters of Skippers Canyon - because its level of activity was not known.
However, the 5.8 magnitude earthquake which rattled Wanaka on May 4 was centred near this faultline.
All of the four faultlines run from northeast to southwest, along the base of mountain ranges.
''Anywhere within the Central Otago area is within 30km of a potential magnitude six to seven earthquake source,'' Dr Cox said.
The type of shaking and the intensity of the movement would vary throughout the region, depending on the ground conditions.
There were also ''suggestions the four faultlines work together to absorb the plate motion'', meaning an earthquake on one of them could lead to earthquakes on another, but this would probably be in geological, not human, timeframes and therefore needed to be kept in perspective, Dr Cox said.
''I'd be surprised if another was triggered within one to two years. We're still talking at least 1000-year intervals. It could be a case of, if one went, stress would build up on other ones, but it takes time to build up and for that stress to actually release.''
In the southern lakes region, a significant earthquake could cause rockfall or rock avalanches, landslips, liquefaction and lateral spreading depending on the soil type, floods due to debris in waterways and potentially a tsunami on Lake Wakatipu.
