Swarm 'could trigger Wellington fault'

KERRY MCBRIDE
Last updated 06:17 23/07/2013
Fairfax NZ

Wellington mayor Celia Wade-Brown gives a rundown on the latest quake information.

Wellington quake defcon
Angela Crompton Zoom
French tourist Luca Zappula photographed the White Bluffs from the Wairau Bar minutes before the quake (left), while the photo on the right shows the dust settling afterwards, with a small piece of the headland appearing to be missing.

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Seismologists say the weekend's swarm of earthquakes could be on a new fault they have not mapped before, and could possibly trigger movement on the feared Wellington fault.

The news comes as Wellington wakes from another rocky night, which included a 4.9-magnitude quake in the early hours. Through the night, a swarm of quakes occurred which were centred off Seddon in the upper South Island -
the biggest being was a 4.9 quake at 1.31am at a depth of 25km. All other quakes since midnight were 3.8 or lower.

GNS scientists and teams from the National Institute of Water and Atmospheric Research have been working to establish which fault line was the catalyst for the series of shakes, including the magnitude 6.5 quake on Sunday evening.

Seismologist John Ristau said the earthquakes were lying in an area of Cook Strait between the Vernon fault and Needles fault.

"There are a number of active faults in the area. It's an issue because they are offshore so it's difficult to get the most accurate location we can."

A third fault to the left of Needles, called the London Hill fault, was also a possible location if it was found to extend offshore.

"The other possibility is it is either on an unmapped fault or something brand new."

Pinning down the relevant fault could take a couple of days, and Niwa was expected to map between the Vernon and Needles faults later this week.

The nearby Wellington fault - capable of magnitude 8 or larger earthquakes and running through Wellington and the Hutt Valley - was not connected, but could have been affected by the ongoing activity in the region.

"It's not the same one, but when you get a large earthquake it can increase or decrease stress on nearby faults. They all interact."

The possibility of the current swarm setting off a larger earthquake along the Wellington fault could not be discounted, Dr Ristau said. "We don't know for sure but we can't entirely rule out the possibility."

While it usually took earthquakes larger than magnitude 7.5 to set off tsunamis on their own, smaller ones could cause underwater landslides, Dr Ristau said.

"Undersea landslides caused by much smaller earthquakes can trigger tsunamis. That's a distinct possibility.

"If one did trigger a landslide it would be difficult to monitor that as it's underwater and we would not know about it."

Niwa marine geologist Scott Nodder said most of the quakes, including the largest on Sunday evening, were in an area on which they had little data.

"The earthquakes are away from the Needles fault, and nearby Booboo fault. Looking at the data it seems like it could be a new one to me."

There were "numerous" active faults in the Cook Strait area, and Niwa had mapped 100 per cent of the canyons in Cook Strait but work was continuing with other parts.

Dr Ristau said that Christchurch, unlike Wellington, was built on soft sediment with a shallow water table which contributed to liquefaction in the February quake.

"In addition, the February 2011 earthquake was very shallow and close to Christchurch, and much of the energy released was directed straight towards the city," he said.

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"All of these factors combined to cause extensive liquefaction in Christchurch. These factors were not as prominent in the Cook Strait earthquake, despite the earthquake being larger."

The possibility of another big quake has now dropped to 7 per cent in the next day, or 19 per cent in the next seven days.

- The Dominion Post

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