Quake links investigated

Last updated 11:22 21/01/2014
Fairfax NZ

A 6.2-magnitude earthquake hits the Wairarapa Coast and Manawatu and is felt at both ends of the country.

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DAVID UNWIN/FAIRFAX NZ. Zoom
Palmerston North resident Jason Holding talks about the earthquake and coming outside to see the front fence had fallen down.

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Earthquake aftermath at Eketahuna

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Scientists are investigating links between yesterday's 6.2-magnitude earthquake and a nearby series of strong tremors which occurred in the past.

Dozens of significant aftershocks were forecast as unwelcome sequels to yesterday's quake that damaged homes, cracked roads, toppled cliffs and left thousands without power.

The Wellington Anniversary Day quake, which struck shortly before 4pm, was centred about 15 kilometres east of the rural Tararua town of Eketahuna at a depth of about 33km.

GeoNet spokeswoman Sarah Page said more seismometer would be sent to the Eketahuna region this week.

"Specifically, efforts are aimed at comparing the current earthquakes with a sequence of events that occurred to the north of Eketahuna in the early 1990s."

Four large earthquakes shook the region, in the south-east of the North Island, between 1990 and 1992.

Scientists were also looking at the possibility of a link to the ongoing Kapiti slow slip event - a slow release earthquake which is affecting an area spanning over 100km from Levin to the Marlborough Sounds.

The likelihood of a larger triggered event in the coming weeks is minor. However, it is possible and we should all take this opportunity to review our earthquake emergency plans," she said.

The most shaking from Monday's 6.2 quake was recorded in Woodville as about one quarter of acceleration due to gravity - or 0.26 g.

Strong shaking was also recorded on the Kapiti Coast, with up to 0.2g in Paraparaumu, while accelerations recorded in Wellington city were less than 0.05g.

By comparison, ground motions recorded during the recent Cook Strait earthquakes ranged up to 0.75g.

By 8.30pm last night, 8000 people had reported feeling the quake to Geonet.

GNS duty seismologist Caroline Little said yesterday's quake was caused by the Pacific tectonic plate "subducting" under the Australian plate, and could not be pinned on an individual fault line.

Late last year, GNS Science released a report showing a tsunami up to 35 metres high could cause as much devastation over Wellington and Napier as the 2011 waves caused in Japan.

Little confirmed the "worst case scenario" would be triggered by the same two plates that caused yesterday's quake.

However, unlike yesterday, when the epicentre was deep inside the Pacific plate, for a tsunami to be triggered the plates would have to clash at the "interface" of the two and be centred out to sea.

A larger quake would also be needed to trigger a tsunami, she said.

The report, by natural hazard scientist Graham Leonard, said waves could reach 15 metres above normal sea level in Wairarapa, Northland, Great Barrier Island, and parts of East Cape, but Wellington's steep valleys could amplify tsunami waves up to 35m, he said.

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"No part of the New Zealand coastline is free from tsunami hazard," the report warns.

Little said yesterday's quake was different from two historic quakes in the area that caused serious damage.

A magnitude 8.2 quake on the Wairarapa Fault in 1855 remains the most violent in New Zealand since European settlement.

A 10-metre tsunami struck Palliser Bay on the south Wairarapa coast, flooded Porirua Harbour, and hit Titahi Bay and the Kapiti Coast.

In Wellington, a 4 metre to 5m tsunami swept over the isthmus between Lyall Bay and Evans Bay where Wellington Airport is now located..

In June, 1942, a 7.2 quake centred near Masterton shifted houses on their foundations and caused some roofs to collapse. Damage was widespread in he town and only a few buildings survived unscathed.

In Wellington, 100 city buildings were found to be seriously damaged, about 10,000 chimneys toppled and 5000 homes needed extensive repairs.

Little said yesterday's quake was more similar to a 6.4 quake centred near Weber, Tararua in May, 1990, that caused no major damage.

It was expected the next seven days would see another quake - possibly up to five - of magnitude 5-5.9.

- The Dominion Post

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