Work is underway to better understand a tectonic transition zone on either side of the Cook Strait and the Nelson Lakes are playing a valuable role in the research.
Researchers have spent the last decade studying the Alpine Fault – the faultline that runs the length of the Southern Alps.
The fault has a 75 per cent chance of rupturing at about magnitude 8 quake in the next 50 years in a future event known as AF8.
But questions still remain for the “transition zone” between the Alpine Fault and the Hikurangi Subduction Zone – two of the country’s largest faults – both on land and under the Cook Strait.
Several faults cut through the area, including the South Island’s Awatere, Clarence, Hope, and Wairau Faults, and the North Island’s Wairarapa and Wellington Faults.
A team of 40 experts led by Jamie Howarth and Kim King from Victoria University and Caroline Orchiston from Otago University are now one year into the five-year programme – Ngā Ngaru Wakapuke – to learn more about this area.
“The reason we’re considering this is we have fault complexity, which probably speaks to more complicated earthquake behaviour,” Jamie says.
The research aims to understand how a large earthquake, such as AF8, could impact the transition zone.
“What we’re looking at in this programme is the possibility of not just one event, but a sequence of large earthquakes, magnitude 7, each with their own aftershocks, separated or sequenced relatively closely spaced in time – we’re talking years to decades.”
There is evidence of such earthquake sequences since Europeans arrived in New Zealand and the programme seeks to build up a more comprehensive data set to figure out how often such sequences might occur, to inform future investment decisions and community resilience.
“We have to grapple, potentially, with a protracted period of uncertainty and multiple phases of response and recovery.”
While an AF8 event could heavily damage Nelson Tasman, an earthquake sequence affecting other regional faults – like the Wairau Fault which runs through the Nelson Lakes area – could also cause significant local damage.
The team has spent time on Lake Rotoiti and Lake Rotoroa analysing the sediment on the lakebeds in “minute detail” to glean insights into past earthquake events and help build a comprehensive picture of the last 10,000 year of local seismic activity.
“They act as nature’s answer to the seismometer,” Jamie says. In addition, researchers have placed several seismometers atop hills in the region – which locals are asked not to disturb – and over the next 18 months two ships are collecting a “dense array” of fault data from the Cook Strait.
But while the prospect of a sequence of damaging earthquakes might sound scary, there was “good news” and the key to being prepared was not growing “complacent”.
“As a community and as individuals, preparing for one event is the same as preparing for many,” Jamie says.
“Prepare for the AF8… and if it does happen, remain prepared, because we may have another episode shortly afterwards.”
Locals are interested in the earthquake work, with about 300 people showing up to the AF8 Roadshow event in Richmond last week, though the question-and-answer section ended slightly early due to a medical event.
Hato Hone St John confirmed they were notified of an incident just before 8pm and treated one person in a minor condition who did not need to be transferred to hospital.
The local AF8 Roadshow stops in Richmond and Tapawera were hosted by the Nelson Tasman Emergency Management Group. The six South Island emergency management groups are part of the AF8 organisation alongside their Alpine Fault hazard science partners. The organisation works to build and coordinate the readiness and response capability for a major Alpine Fault event.
