The Otago Regional Council (ORC) received the report by GNS Science at a meeting of its Data and Information Committee today. The report identifies 26 active or potentially active faults through the Dunedin and Clutha districts and summarises their rupture potential and ground surface deformation hazard.

Natural Hazards Analyst Sharon Hornblow said the report refines our understanding of earthquake fault hazards and will help to pinpoint places that would benefit from further assessment and the delineation of hazard awareness areas.

“The report does not raise significant new concerns for the Dunedin and Clutha districts as the overall seismic hazard for coastal and south Otago areas is very low compared to most of New Zealand.

“The Akatore Fault, southwest of Dunedin, has ruptured twice in the past 1,300 years and has long been regarded as the most active fault in coastal Otago. It accounts for most of the overall seismic risk for Dunedin city. New data for this fault obtained in recent years by University of Otago support earlier interpretations and provide a better knowledge base of its activity.”

David Barrell, senior geologist at GNS Science and the author of the report, said large earthquakes in the past few tens of thousands of years seems to have been concentrated on a small number of faults, with tell-tale signs of those movements still evident in the landscape.

“Although there are many active or potentially active faults in coastal and south Otago, just a handful such as the Akatore, Titri, Hyde, and Blue Mountain faults seem to have been the focus of geologically-recent large earthquakes in this part of Otago. Most of the other faults show little if any recent movement and contribute relatively little to overall seismic risk.”

“These low-activity faults include the Maungatua-North Taieri Fault along the western margin of the Taieri Plain and the recently identified Kaikorai Fault, a branch of which is thought to run down Caversham valley towards the harbour basin. However, there is no indication of this fault having moved in the last 125,000 years.”

Dr Hornblow said the report benefited from improvements in the methods for mapping faults over the last fifteen years, as well as increased scientific investment in active fault field investigations since the Canterbury and Kaikoura earthquakes.

“Data capture technology has advanced significantly since the last assessment of active fault hazards was carried out in these areas, and the new report makes good use of ORC LiDAR data, and aerial imagery.”

The new information will be added to the ORC’s online Natural Hazards Database.

The level of detail is not sufficient for site-specific zoning of fault hazards, which could be the subject of future work. The information in the report is intended to help target of future investigations and create general awareness of the existence of the potential hazards.

ORC will now discuss the next steps toward reducing risk from seismic hazard with Otago’s city and district councils.

Faults in the Queenstown Lakes District and Central Otago were assessed in 2019 by GNS Science, and faults in the Waitaki District were assessed with Environment Canterbury in a 2016 study.

The “General distribution and characteristics of active faults and folds in the Clutha and Dunedin City districts, Otago” report can be viewed in the Data and Information Committee agenda, starting on page 123: https://www.orc.govt.nz/media/9951/agenda-daic-20210609.pdf

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