Just days after South Island residents were shaken awake by a 5.5 magnitude Alpine Fault quake, University of Otago earthquake expert Professor Mark Stirling warns there's a bigger threat to Dunedin.
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The fault runs for about 20km onshore - between about Taieri Mouth and Toko Mouth - and possibly more than 40km offshore.
"The Akatore Fault is the most active fault in Otago, and is the highest hazard fault for Dunedin," he told Newshub.
The fault's last quakes were between 1280 years and 760 years ago and moved the ground up to two metres vertically. The next quake is predicted to be around magnitude 7-7.4.
"We are calculating peak ground accelerations of >0.5g for Dunedin. This is very strong shaking that would be damaging for Dunedin's old buildings," Prof Stirling told Newshub.
Prof Stirling warns these effects would be "much worse" for Dunedin than a major Alpine Fault quake.
"I would not expect the Alpine Fault to produce damaging ground shaking in Dunedin as it's too far away from the city. The Akatore Fault is close (20km or less)," he says.
'Eliminate entire populations'
As the last quakes lifted a large stretch of New Zealand's southeastern coast up by two-three metres, a University of Otago team has been studying the effects on populations of coastal species.
"The old Akatore earthquake would have been similar in magnitude to the 2016 Kaikoura quake, with tens of kilometres of rocky coast affected, and the old shore lifted out of the water, well beyond the reach of the waves," says Professor Dave Craw.
This was "clearly devastating" for coastal species like kelp and shellfish, which were left literally high and dry.
"The ancient Akatore uplift was big enough to eliminate entire populations of intertidal species," the researchers say.
Fortunately this was before the historic settlement of New Zealand. But if it happened again, given the population and buildings in the area, we wouldn't be so lucky.
Destructive power could be unleashed again
Scientists are now studying whether we should expect another Akatore Fault earthquake in the immediate future.
"It has had a heightened state of activity in the geologically recent period after a long period of quiescence," Prof Stirling told Newshub.
"There is no reason to assume that the heightened activity has died off, as it's been less than 1000 years since the last event.
"We do think that it's in a heightened phase of activity, and it's hard to say how long that will last. It is therefore prudent to consider the Akatore Fault as potentially the most likely source of future strong ground shaking for Dunedin."
In order to prevent massive damage to Dunedin, Prof Stirling urges authorities to put in place effective hazard and risk communication plans to warn the public, and emergency management plans that deal with an Akatore Fault earthquake scenario.