7.0 quake unrelated to Canterbury faults

Last updated 12:08 04/07/2012
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Slideshow7.0 quake off Taranaki

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A magnitude-7.0 earthquake that struck the lower North Island is unrelated to seismic activity in Canterbury, GNS Science says.

GeoNet reported the tremor, which hit yesterday at 10.36pm, was centred 230km deep and was 60km south of the Taranaki town of Opunake.

People from Bay of Plenty to Otago reported feeling it.

The first quake was followed nine minutes later by a 4.6 shake at the same depth, 70km south of Opunake.

GNS Science seismologist Steve Sherburn said today it was ''accepted scientific knowledge'' the event was unrelated to the Canterbury quakes.

''Although earthquakes in Canterbury and on the Alpine fault, and deep earthquakes under the North Island like last night, are all related to the fact the plate boundary runs through New Zealand, different parts of the plate boundary don't greatly influence other parts,'' he said.

''When [Canterbury's] large earthquakes occurred an obvious question was 'Has there been any effect on the Alpine fault' because that's relatively close. When we're talking shallow earthquakes related to a deep earthquakes in the North Island, there's science I know of that attempts to relate those to each other directly.''

A magnitude-3.9 quake in Canterbury after 11pm was ''par for the course'' and not related to the North Island event, Sherburn said.

Largest for 120 years

Last night's widely-felt magnitude-7.0 earthquake was the largest in the area for up to 120 years, says a seismologist.

People described hanging on to doorways and diving for cover when the quake struck the country.

Did you have any video of the quake, or damage from it? Email us at newstips@stuff.co.nz

GeoNet reported the tremor, which hit at 10.36pm, was centred 230km deep and was 60km south of the Taranaki town of Opunake.

People from Bay of Plenty to Otago reported feeling it.

The first quake was followed nine minutes later by a 4.6 shake at the same depth, 70km south of Opunake.

GNS Science duty seismologist Lara Bland said the 7.0 quake was the largest to strike the Taranaki Bight area for the past 100 to 120 years.

It was related to the subduction zone where the Pacific tectonic plate was being driven under the Australian plate, she said.

The quake probably happened on the Pacific plate, which dipped down deeply, she said, occurring because the plate was hard and brittle.

That was also partly why it had been felt so widely, as the energy travelled "efficiently" back up the plate to the surface.

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It was a good-sized shake but the depth and distance offshore limited its impact, Bland said.

While large quakes in the area weren't unheard of, "they aren't often that big," she said.

Magnitude-6.0 quakes had occurred in the area over the years.

GNS said in a release that it was typical with deep North Island quakes that the strongest shaking occurred to the east of the epicentre, not directly above it.

''With a depth of 230km everyone on the surface is already at 230km or a greater distance away from the earthquake's focus, so the ability for it to have caused major damage is lessened.

''One aftershock of magnitude 4.6 popped off about 10 minutes later, but we do not expect many aftershocks from earthquakes of this nature.

GNS said large, deep earthquakes were felt over a wide area but fortunately did not cause the damage that resulted when they happened at shallow depths, such as the September 2010 Darfield earthquake of a similar magnitude.

''Nevertheless a magnitude 7.0 earthquake occurs in the New Zealand region, on average, once every three years, so last night's shake is certainly newsworthy.''

GNS would try to image the behaviour of the fault, but given the lack of damage or injuries it would probably not look much more deeply into the quake.

Partly that was because many scientists were still tied up working on the Canterbury quakes.


While no major damage had so far been reported, firefighters on a callout in Waverley, near Whanganui, reported seeing cracks in the town's main street and a few "skew-whiff'' chimneys, which the Fire Service said would be further assessed this morning.

A Whanganui woman reported a crack in her ceiling and had gone to stay with her daughter overnight.

Some people were also reported to have evacuated to higher ground on the Horowhenua coast, however a Civil Defence spokesman said there had been a low tsunami risk as the quake struck so deep.

No tsunami warning was issued following the quake.

Marea Faigan, from Lower Lepper Rd, near Inglewood, said she had never felt anything like it in her life.

"It just kept going and going," she said.

"The cupboards were rattling and I grabbed hold of the door frame to steady myself. I just felt I needed to hold on to something," she said.

One night worker from Wellington said she and her colleagues felt the quake rock their building.

"We all dived under our desks just in case," she said.

New World Chaffers duty manager Mark Huddleston said no breakable items fell off shelves at the central Wellington supermarket but small items like tea bags and painkillers tumbled to the aisles.

Wellington train service operator Tranz Metro was replacing its 5am Kapiti line service between Waikanae and Paekakariki with two taxis as it waited to clear the track following the quake. A delay of 30 minutes was possible on early Masterton services due to the wait for track clearance.

Callers flooded the Taranaki Daily News switchboard within seconds of the shake, which lasted about 30 seconds.

Taranaki Civil defence senior emergency management officer Shane Briggs said he had been in contact with the Fire Service and the Ministry of Civil Defence but there had been no reports of any damage.

Oil company Shell Exploration said the quake had not damaged the Maui A and Maui B drilling platforms off the Taranaki coast, Radio New Zealand reported.

As part of standard procedures checks were made at crucial places around the rigs and no damage was found.

The GeoNet website described the intensity of the shake as moderate.

The quake was felt as far away Dunedin, Christchurch, Blenheim, Kapiti Coast and Gisborne.

Meanwhile, at 11.07pm, a 3.9 magnitude quake struck 20km east of Christchurch, GeoNet said.

It had a focal depth of 9km.

There were no immediate reports of damage.

- The Dominion Post


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