Scientists struggle to gauge fault's threat to Dunedin

Scientists investigate a section of the Akatore Fault, one of the closest active geological faults to Dunedin. Photo / Supplied

One of the closest active geological faults to Dunedin is not giving up its secrets easily, something that's worrying for scientists trying to assess the structure's threat to the city.

Researchers from the University of Otago's Department of Geology and GNS Science, led by the university's Professor Mark Stirling, have been studying the recent geological history and movements of the fault, which may extend as much as 60 kilometres from south of Dunedin towards the Clutha River mouth.

They have found the Akatore Fault may have a maverick streak, behaving strangely during the past 125,000 years or so.

Master of Science student Briar Taylor-Silva, Professor Stirling and Senior GNS Scientist Dr Nicola Litchfield said there were signs of three significant "recent" earthquakes on the Akatore Fault in the past 12,000 years, following a long quiet spell possibly stretching back about 125,000 years further.

Taylor-Silva estimated the three recent quakes had magnitudes of between 6.8 and 7.4, each displacing the ground by up to two metres vertically.

The fault runs for about 20km onshore, between about Taieri Mouth and Toko Mouth, and possibly more than 40km just offshore further north and south, though its precise length is not yet known.

Fieldwork included digging a trench across the fault at Big Creek east of Milton, near the Akatore forest, and a second trench dug at Rocky Valley Stream.

The trenching sites were chosen as the streams there were dammed by fault ruptures and became swampy.

Organic material from these swampy, peaty sediments deposited following the large quakes were then carbon dated to calculate approximate dates for the events.

The first was estimated to have occurred sometime between 12,100 and 1280 years ago.

The second and third major Akatore Fault quakes then appeared to have happened very close together geologically speaking, between around 1280 years before now and 760 years ago.

"We have been trying to understand whether the fault has slipped enough during those three Holocene events or whether we should expect more earthquakes in the immediate future," Taylor-Silva said.

"More work is required to answer this question. However, I think we need to treat the Akatore Fault as a serious hazard.

"The episodic nature of the fault just makes it more difficult to estimate future earthquake hazards.

"With faults that have periodic ruptures, we can estimate how often an earthquake may be likely to occur, say, every X years, but we can't do that for the Akatore Fault very well.

"It's quite worrying for assessing the hazard the fault poses because we can't say there won't be another earthquake in the near future, as we don't really understand its behaviour."