Evidence for fault under Otago Harbour
DUNEDIN has changed in the last nine million years.
For much of that time the sea level was about 150m lower than it is now.
The city’s geography was formed by a volcano, centred at Port Chalmers, which rose 1500m above its surroundings.
It was, something more akin to Hawaii than the rolling slopes we now know, Andrew Gorman, of the University of Otago says.
Over millions of years seismic faults have broken up the original structure, and rain and rivers have steadily worn it away.
Dunedin did not always have a harbour.
For most of its history, the harbour was a river valley, before the seas rose and filled it with sediment and a shallow pool of salt water.
At the centre of that valley is what Prof Gorman is pretty confident is a seismic fault.
Prof Gorman specialises in creating pictures of geology below the surface.
He uses sound sources that echo off different layers of the subsurface, to create a picture of what is there, in the same way radiographers make images of the inside of the human body.
Using the University of Otago’s research vessel, RV Polaris II, he has been towing a sound source known as a ‘‘boomer’’ through local waters.
Also towed behind the vessel was a series of microphones, known as hydrophones, which recorded the echo as it bounced back.
Over the last 15 years he and students have been mapping the Akatore and Green Island faults off the coast just to the south of Dunedin, work which fed into the report on the Kaikorai and other Dunedin faults released last week.
Prof Gorman said it was likely the Green Island fault, which runs offshore from south of Brighton to St Clair, was connected to the fault in the harbour.
The Green Island fault was thought to be behind an earthquake that shook Dunedin in the 1970s.
In the past two years he has been working in Otago Harbour, something that had proved difficult as it was so shallow.
But a student developed a system that allowed the system to be used off a small boat.
Using that system, the sediment that sat on the volcanic rock about 80m under the surface of the water near Dunedin, and about 150m at Port Chalmers, could finally be studied.
Prof Gorman said when there was an earthquake in the harbour, one side would be pushed up, and the other down, leaving ‘‘offset’’ layers.
‘‘This is what we’ve been trying to identify in the harbour.
‘‘We have a few places where we think we can see places where there are offset beds.’’
That allowed mapping of the fault that went ‘‘basically up the middle of the harbour’’.
‘‘We’re fairly confident there’s some fault there. The big question is how active it is, and we really don’t know that.’’
There was no evidence of the sea floor going up or down in the last thousand or so years.
‘‘My feeling is that it’s not any more active than any of the other faults around Dunedin.’’
But an earthquake there would have serious effects.
‘‘It would affect a lot of people on both sides of the harbour, within a kilometre of the epicentre’’.
With a displacement in the fault of a metre ‘‘you’d definitely have water displacement’’.
However, there was no evidence in Dunedin or on the Taieri of tsunamitype deposits or past waves.
Prof Gorman said the only way to find out more was to drill through the sediment, which he hoped to get funding for in the next year.
The drilling could find sediment that could be carbon dated, and if that sediment was found at different levels in the harbour, that was evidence of movement from a fault.
Prof Gorman said the idea of a fault in the harbour had been conjectured since early to midlast century, but it had been hard to prove, as it was under water in the harbour, and under sediment further south in South Dunedin.
That proof was now coming, but Prof Gorman said it did not mean people needed to be concerned.
‘‘I don’t think there’s really a reason to be fearful. We should just know there’s a fault out there.’’
He said it was not just being on a fault that was a problem in the event of an earthquake, it was the type of geology people were on that was an issue.
‘‘We certainly don’t have the liquefaction hazard that Christchurch has.’’
South Dunedin had flat land, but the land was much thinner than in Christchurch, in most cases 20m to 60m thick, compared to Christchurch where it was almost 1km thick.
The Christchurch land was made of layers of gravels and silts, amplifying the liquefaction.
There could be localised areas of liquefaction in the event of an earthquake in Dunedin, ‘‘but you’re not going to have a red zone like you have in Christchurch, where you have the whole place being written off’’.
Prof Gorman said he did not know if scientists would ever be entirely sure of the activity level of the fault.
As long, of course, as an earthquake doesn’t rattle the sides of the harbour basin.
❛ We’re fairly confident there’s some fault there. The big question is how active it is, and we really don’t know that