Monday, July 18, 2016

[Geology2] Could Dunedin be hit by a large, local earthquake?

Could Dunedin be hit by a large, local earthquake?


An exposure of the active Akatore Fault at Big Creek in the hills south of Taieri Mouth. University of Otago Professor ...

An exposure of the active Akatore Fault at Big Creek in the hills south of Taieri Mouth. University of Otago Professor Mark Stirling's finger is on the reverse fault, where schist rock on the right has been thrust up over younger sediments.

A team of scientists is assessing Dunedin's earthquake potential and what lies below the city, using knowledge from the Canterbury quakes. PAUL GORMAN reports.

After a mild start, Tuesday April 9, 1974 soon turned showery and cool in Dunedin, with a high of about 13 degrees Celsius.

The big news on the front of the Otago Daily Times (price 6c) that day was the city council's decision to increase parking fees by up to 50 per cent, with metered parking to cost 30c for two hours. There were doubts about the origins of historic ship timbers found in the Portobello mudflats and reports of deadlock over efforts to improve pedestrian safety outside St Paul's Cathedral at the top of the Octagon. 

Active and inactive faults in the Dunedin area, including the location and depth of the April 1974 magnitude 5.0 ...

Active and inactive faults in the Dunedin area, including the location and depth of the April 1974 magnitude 5.0 earthquake and other recent quakes.

It was just two months after the end of the hugely successful Commonwealth Games up the road in Christchurch. A pint of milk cost 4c and Labour prime minister Norman Kirk had about four months to live.

Darkness fell about 6.45pm and many switched on their black-and-white televisions to watch the one channel available. 

A few seconds before 7.50pm an alarming jolt was felt across the city, followed seconds later by a larger, longer quake that brought down chimneys, cut power and knocked items from shelves. Later calculated as magnitude 5.0, it was generated on an offshore fault a little more than 5km south of the beach suburb of St Clair.

Dunedin's largest known near-source quake, which led to about 3000 claims to the Earthquake and War Damage Commission (EQC), was followed by several magnitude 3.7 aftershocks later that evening. 

Scientists are still uncertain which fault was responsible for the quakes, given the main shock's epicentre was further offshore than the roughly parallel Akatore and Green Island faults.

Many Dunedin residents experienced the larger Canterbury quakes of recent years, and the big, rolling Fiordland shakes that come round every decade or so are clearly felt in the city. Yet there has always been a belief that Dunedin sits in a seismically quiet corner of the country. 

The April 1974 quake and the magnitude 4.7 Lee Stream event about 30km west of Dunedin on June 1 last year show the city is definitely at risk from nearby faults. In fact, many more faults are known to exist in the Dunedin area than were thought to be around Christchurch prior to September 2010.

A keen-eyed observer might notice the Otago Harbour, coastline and Kaikorai Valley have the same rough northeast-southwest orientation as the active Titri, Akatore and Green Island faults further down the coast and offshore.

Dunedin's underground secrets are being probed in a pilot study of active faults in urban areas, following a recommendation from the Canterbury Earthquakes Royal Commission for more research to paint a better picture of the quake risk to residents in cities and towns.

A multidisciplinary team from GNS Science, the University of Otago, EQC and the Natural Hazards Platform is about halfway through the study. Led by Dr Pilar Villamor of GNS Science, the research strands are due to be brought together in about a year's time with a conclusion about fault activity rates and an update of the National Seismic Hazard Model.

Villamor said Dunedin was chosen because:

 – Active faults already identified might extend under the city;

– It was a compact city where several research institutions were already working on quake research; and

– It had similar heritage building stock to Christchurch, and similar losses and damage could be expected if a fault ruptured under or close to the city.

Scientists are now assessing deformation rates across the city, including where the active offshore Akatore Fault might extend northeast on to land - either along the Kaikorai Valley and into the Waitati Valley or into South Dunedin and along Otago Harbour. Any onshore extension of the Green Island Fault is also currently unknown.

Otago University's chair of earthquake science, Professor Mark Stirling, said radiocarbon dating of objects from a trench dug across the Akatore Fault at Big Creek, south of Taieri Mouth, had revealed an "interesting history".

"We know the fault has had a recent period of earthquake activity – but will that continue or will it shut off again? It has moved twice in the past 1300 years, with about 1.5 to 2 metres of slip per earthquake, but before that it may not have ruptured for about 120,000 years.

"The fault may therefore go from time of long-term quiescence to times of intense activity, and at present we don't know whether it is still in that active phase."

With lengths of about 60km, the Akatore and Titri faults could be capable of generating quakes of between magnitude 7.0 and 7.5, Stirling said.

"The Canterbury quakes - particularly the [February 22, 2011] Christchurch quake – showed you can get these events on unknown faults. It raised questions about how other cities are also vulnerable to strong earthquakes on structures that have very long recurrence intervals."

To pinpoint deformation across Dunedin, researchers are using a variety of methods, including gravity measurements, onshore and offshore seismic reflection techniques, geological mapping studies, and comparisons of GPS and satellite data.

Otago University marine geophysicist Associate Professor Andrew Gorman said there were early indications the liquefaction hazard in South Dunedin from a large, local quake might not be as high as in Christchurch. However, lateral spreading of reclaimed land around the harbour edge was still expected.

"The near-surface geology of flat-lying areas in Dunedin is very different than in Christchurch. In Dunedin, coastal and harbour sediments, mostly sand and silt, lie in a shallow basin less than 100 metres deep. Christchurch is underlain by more than 600m of alternating beds of river gravels and marine sediments containing a number of highly pressured aquifers."

It was also thought splits in the rock of the Dunedin shield volcano, which became extinct about 9 million years ago, may indicate underlying faults that caused earthquakes in the past.

"These faults would have become a focus for erosion by streams and rivers, leading to the valleys we see today," Gorman said.



Posted by: Lin Kerns <>


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