Swarm 'could trigger Wellington fault'

23:34, Jul 22 2013
Wellington earthquake
Windows broken at Quest hotel.
Wellington earthquake
Cracks at Reading Cinema.
Wellington earthquake
Miramar New World faces a big tidy up after the quake.
Wellington earthquake
Huge crack at Wellington waterfront.
Wellington earthquake
Damage in Featherston St.
Wellington earthquake
Bottles are strewn all over the floor at the New World in Thorndon .
Wellington earthquake
Seymour Square memorial clock in central Blenheim stopped at the time of the quake.
Wellington earthquake
Briscoes in Lower Hutt has had some stock destroyed.
Wellington earthquake
Cracks in the ground at the Wellington waterfront.
Central NZ hit by earthquake
Mackenzie Presten, 10, with caregiver Anthony Mills duck for cover at the ASB sports centre in Kilbirnie during the earthquake.
Central NZ hit by earthquake
The New World supermarket in Blenheim is closed after the earthquake.
6.5 quake in central
The law library at Victoria University.
Central NZ hit by earthquake
Wellington City Council building being cordoned off after the earthquake.
Wellington quake defcon
Shattered building glass on The Terrace in Wellington.
Wellington quake defcon
The Kelburn Four Square, Wellington.
Wellington quake defcon
Damage to Hannah Turvey's apartment on Taranaki St, Wellington.
Petone Clock
The Petone clock stopped at 5.10pm when the 6.5 magnitude earthquake struck.
Wellington quake defcon
Damage at the Quest hotel in central Wellington.
Wellington quake defcon
Damage at a Liquor King story on the Kapiti coast.
Wellington quake defcon
A police cordon on Wellington's Webb St near the Mercure Hotel in the background, which was evacuated after the 6.5 quake.
Wellington quake defcon
Evacuees being taken into the Wellington Church Of Christ to keep warm after the Mercure Hotel was evacuated and cordoned.
Wellington quake defcon
Severe damage at BNZ's Harbour Quays buildings. Picture posted by @tracy_earl on Twitter.
Wellington quake
Wellington Railway Station completely empty at about 8am today.
Wellington quake
The clean-up begins at Regional Wines and Spirits near the Basin Reserve.
Wellington quake defcon
Damage at Centreport, Wellington after 15 square metres of sea wall and road had slumped into the sea at the southern end of the container terminal.
Wellington quake defcon
Damage at Centreport, Wellington
Wellington quake
"Things were thrown around" the bedroom of a Thorndon, Wellington home.
Wellington quake
Bridget Thurlow cleans up her mothers Simpson Street Seddon home after last night's quake. The house sustained extensive damage. Her mother Elva Tombs was traumatised and spent the night with family in Blenheim.
Wellington quake
Property owner Kurt Flowerday inspects the damage to his Seymour Street, Seddon property.
Wellington quake
Supervalue Seddon employee Sheryll Kamat and owner Marie Flowerday (in front) clean up after building inspectors gave them the all clear to re-enter the building.
Featherston St post-quake
Workers in Wellington's Featherston St, cordoned off with quake damage.
Wellington quake
A picture of politician Steven Joyce's office post-quake. Photo from @cjsbishop.
Featherston St post-quake
Multiple windows were shattered in the old Dominion Farmers Institute building.
Featherston St post-quake
A photo taken of a crack in the ground at the Clyde Quay development on the Chaffer's Dock end of Wellington's waterfront.
Wellington quake
Lambton Quay quiet at lunchtime on a Monday: Department of Internal Affairs compliance officer, Gareth Bostock, sits on a Lambton Quay bus stop waiting for his office building to open so he can get back to work.
Wellington quake
Workers fix windows at the Harbour City Shopping Centre building in Wellington.
Wellington quake
The Commanding Officer of the Urban Search and Rescue operation in Wellington City, Graeme Mills, at his command centre on The Terrace today. Pictured with him from left to right are senior fire-fighter Matthew Salt and station officer Chris Faithfull.
Wellington quake defcon
Damage at Centreport, Wellington after 15 square metres of sea wall and road had slumped into the sea at the southern end of the container terminal.
Wellington quake
Damage to buildings on Featherston St: the building on the right is 131 Featherston.
Wellington quake
Photo taken at the MSD Contact Centre in Lower Hutt that has now been closed due to earthquake damage.
Wellington quake
Damage to a building in Wellington's Taranaki Street.
Wellington quake
A sign on Wellington's SH1 near Johnsonville asking people to avoid entering the city's CBD.
Wellington quake
Paul Lloyd-Jones, cleaning up the mess at his store Thirsty Liqour Newlands.
Wellington quake
The Wellington waterfront at lunchtime the day after the 6.5 magnitude earthquake in Wellington.
Wellington quake
Chairman Warren Larsen inspects damage to Wellington's CentrePort caused by yesterdays 6.5 earthquake.
Wellington quake
Damage to Wellington's CentrePort caused by yesterdays 6.5 earthquake.
Wellington quake
Damage to the Old Bank Arcade in Wellington.
Wellington quake
An earthquake damage assessment team at St Mary of the Angels church on Boulcott Street.
Wellington quake
An earthquake damage assessment team looks for damage at St Mary of the Angels church on Boulcott Street.
Wellington quake defcon
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.

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.

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"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.

"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