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New Zealand’s longest earthquake record… gets longer! - 03/03/2017

From the wilderness of Fiordland comes more clues about the behaviour of New Zealand’s longest onshore fault – the Alpine Fault.

Earthquake geologists from GNS Science collecting core samples in the John O’Groats River valley in Fiordland.  Photo: Kelvin Berryman, GNS Science

Earthquake geologists from GNS Science collecting core samples in the John O’Groats River valley in Fiordland. Photo: Kelvin Berryman, GNS Science

A paper out today in Earth and Planetary Science Letters describes the detailed detective work that has enabled GNS Science geologists to identify evidence of past large earthquakes at John O’Groats River, near the mouth of Milford Sound.

Project co-leader, earthquake geologist Kate Clark, says modern technology in particular high-resolution radiocarbon dating has enabled useful information to be extracted from sites that have been known about for a long time.

“We have determined the timing of seven earthquakes that were large enough to break the ground surface and temporarily block the John O’Groats River,” Dr Clark said.

The earthquake record from John O’Groats River has been integrated with the long earthquake record previously published from Hokuri Creek to make an 8000-year long history containing 27 earthquakes.

Such a long record is rare worldwide and enables a stunning view of past earthquake behaviour and likely future occurrence.

On the South-Westland section of the fault earthquakes occur on average every 300 years and the last earthquake was exactly 300 years ago – a magnitude 8.1 in 1717 AD. Because of the natural variability in timing, this translates into a 30% chance of the next large earthquake happening in the next 50 years.

Project co-leader and fellow earthquake geologist, Ursula Cochran, notes that results have been refined since they were presented in public forums in Franz Josef, Hokitika and Christchurch in 2015, but hazard estimates for this part of the fault remain essentially the same.

“2017, being the three-hundredth anniversary of the most recent earthquake, is a great year to focus on improving individual, business and community preparedness,” Dr Cochran said.

“And it is great to see continued funding for the ‘AF8’ project – a civil defence initiative to develop a co-ordinated response to an Alpine Fault earthquake.”

Funding for the Alpine Fault research was provided by the Natural Hazards Research Platform.