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Source: GNS Science

Scientists currently regard the fault as having several sections, each of which exhibits a different rupture history and all of which can generate a large earthquake. 

A new study by GNS Science has found evidence for a previously unrecognised rupture in the first half of the 19th century at the northern tip of the central section. It occurred roughly 100 years after the widely accepted date of the last major rupture of the Alpine Fault which in occurred in 1717.  

Image showing the Alpine Fault running up the western side of the South Island. For much of its length, it runs through steep, inaccessible, and bush-clad country. Because of this, there are a limited number of places where scientists can have easy access to study it in detail. Just north of Hokitika it transitions into the Marlborough Fault System which extends into Cloudy Bay.

“This study indicates that the Alpine Fault is more complex in the north as it transitions into what we call the Marlborough Fault System. It shows that smaller quakes can occur on some sections of the fault in between larger ruptures,” said study leader Dr Rob Langridge of GNS Science.

“An important outcome is that towns near the junction of the central and northern sections of the fault, including Hokitika and Greymouth, may experience strong ground-shaking more frequently than other locations along the fault.

The dominant message in the past two decades is that that the Alpine Fault produces an earthquake of about magnitude 8 every 300 years or so. However, these new results point to it being more complex than that, particularly around this junction area between the Alpine Fault and the Marlborough Fault System.

Dr Rob Langridge

“It’s a reminder that we live in a geologically active country and it pays to be prepared on a personal, community, and business level. Planning and preparedness measures are already underway on the West Coast and findings such as this are part of the body of knowledge that feeds into these initiatives.”

Dr Langridge said ruptures of several hundred kilometres on the southern and central sections of the Alpine Fault were still the dominant way in which it releases seismic strain. But the identification of a post-1717 earthquake at the northern tip of the central section highlighted the need to factor in smaller, partial fault ruptures to hazard management plans.

Published this week in the Bulletin of the Seismological Society of America, the findings were derived from trenches dug across the fault on farmland southeast of Hokitika.  

Scientists radiocarbon dated organic material from the trenches which enabled them to place the earthquake between 1813 and 1848.  

As there was not enough evidence to assign a magnitude for the quake, Dr Langridge and colleagues have come up with four possible scenarios for the newly identified quake. They range from a small local quake of magnitude 5.1 near Hokitika to a magnitude 7.4 quake that produced a 120km-long surface rupture.

The earthquake probably occurred in the decades before widespread settlement of South Island/Te Wai Pounamu.

“The dominant message in the past two decades is that that the Alpine Fault produces an earthquake of about magnitude 8 every 300 years or so. However, these new results point to it being more complex than that, particularly around this junction area between the Alpine Fault and the Marlborough Fault System.”

Findings from studies such as this will be incorporated into the National Seismic Hazard Model. The model is used by government and industry to estimate the likely impact of earthquakes on the country’s land, buildings and infrastructure.

“It helps in understanding the expected level of ground-shaking that might occur in specific parts of the country over certain time periods, for example, in the next 20, 50 or 100 years.”

The Bulletin paper is the culmination of several years’ work funded by GNS Science core funding and the Natural Hazards Research Platform. Dr Langridge said the understanding of the Alpine Fault’s behaviour had evolved steadily over the past 25 years with successive investigations.

Image from NASA’s Shuttle Radar Topography Mission.

MIL OSI