New study says Alpine Fault quake interval shorter than thought: GNS Science

Monday, 6 March 2017

The Alpine Fault runs for about 600km along the spine of the South Island.

The Alpine Fault last ruptured three hundred years ago in 1717, shaking the South Island at an intensity of at least magnitude-8 and causing the ground to break over a distance of at least 375 kilometres.

A new study by GNS Science has modelled the recurrence rate of large magnitude earthquakes on the fault over thousands of years, and revised the average time between great quakes along the southwest end of the fault.

The GNS Science study site of the John O'Groats River (JOG) near Milford Sound.

It put the recurrence mean rate at every 291 years, plus or minus 23 years. Previously, the estimated rate was every 329 years, plus or minus 26 years.

It also meant that although hazard estimates for the part of the fault studied remained the same, there was a 30 per cent chance of a major earthquake in the next 50 years.

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GNS Science geologists studied earthquake evidence from remote sites in Fiordland near Milford Sound at John O'Groats River and Hokuri Creek, where there was an 18-metre thick naturally exposed record of 22 earthquakes.

The team combined this evidence with a 1000-year quake record from the river site to give a 27-earthquake, 8000-year record of Alpine Fault movements.

Such a long earthquake record was rare. The two sites were 20km apart.

Project co-leader and earthquake geologist Kate Clark said modern technology such as high-resolution radiocarbon dating had enabled useful information to be extracted from well known sites.

The Franz Josef Glacier falls from the greywacke zone at its head, near the Main Divide, to the schist zone at its melting snout, close to the Alpine Fault.

The study determined the timing of seven large-magnitude earthquakes with intensities that ruptured the ground and temporarily blocked the John O'Groats River, she said.

The river catchment was short and steep, rising from about 100 metres above sea level in a wetland near Milford to 1700m at a ridge crest, over a distance of only five kilometres.

Earthquakes occurred on the South Westland section of the fault, on average, every 300 years - the last quake was the magnitude 8.1 in 1717.

The GNS study said due to the natural variability of recurrence rates, this new rate translates as a 30 per cent chance of another large magnitude 8 or above in the next 50 years.

This had been roughly the estimate for the recurrence rate of an Alpine Fault quake for some time, but the new study further refined the data for the recurring sequence of quakes over thousands of years.

The team studied the southwest section of the fault, where the record shows uplift of the Australian plate. In the northern reaches of the fault, uplift involves the Pacific plate.

At the John O'Groats River site, there was evidence of large silver beech trees being broken at heights of 8m to 15m above the ground and the ages of unbroken trees were consistent with the widely accepted date of the last quake in 1717.

Project co-leader and geologist Ursula Cochran said the hazard estimates for that part of the fault remained 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,' she said.

The paper published in the Earth and Planetary Science Letters said the research had two weaknesses because the long period timeline was derived from two sites near Milford Sound - where the preserved earthquake record can be easily accessed with bore samples and other methods.

Geologists do not have similar observations along the length of the fault system but the research has implications for the study of long records spanning millennia, recurrence models, and seismic hazards.

'Given that the last earthquake occurred in 1717 AD, the elapsed time of 300 years is now longer than the mean recurrence time between earthquakes.

'Assuming a normal recurrence distribution, there is a 29 per cent probability that the next surface rupturing earthquake will occur within the next 50 years.'

The longest time between large magnitude Alpine Fault quakes was about 350 years and this earthquake occurred between 1334 and 1394AD. The shortest interval between two quakes was about 160 years, the study said.