Auckland quake a one-off - scientist

By James Ihaka

The earthquake struck near Mission bay. Photo / Doug Sherring
The earthquake struck near Mission bay. Photo / Doug Sherring

Earthquake-rattled Aucklanders can relax - Friday's tremor is not associated with a faultline or volcanic activity and is unlikely to be repeated soon, a seismologist says.

The quake, which measured 2.9 on the Richter scale and struck at 9.09pm was centred 10km east of downtown Auckland, around Kohimarama at a depth of 9km.

GNS volcano geologist Michael Rosenberg said it was felt as far north as Wellsford 68km away and Papakura to the south, and prompted more than 3000 people to post reports on the GNS website.

He said the event was a one-off, and the probability of any further shocks was low.

The quake was not associated with Auckland's only active fault, the North Wairoa fault, which runs through the Hunua Ranges to the east of the city.

"It could be a very old fault buried beneath the layers but there's no evidence of a fault there - there's nothing there to link it with a faultline," said Mr Rosenberg.

"It's most likely that it is something in the shallow part of the crust and the stresses have given away just there."

He said some people might draw a connection between the earthquake and Auckland's volcanic field because the event was located within its boundaries but he doubted this too.

"We can look at the wave forms of that one earthquake and it is very characteristic of the rock-breaking relief of stress.

"The other thing is if it was something to do with something deep in the crust underneath the volcanic field we would see a whole series of earthquakes and would expect to see a swarm, a cluster of small quakes, that would continue now whereas this was a one-off."

Friday's quake was probably the most noticeable quake Aucklanders have felt since a 4.5 jolt 30km east of Orewa in February 2007.

That quake was the strongest since 1970 when a 4.7 shake centred on Coromandel rattled Auckland.

GNS Science said at the time that Auckland had had 35 earthquakes rating more than 3 on the Richter scale since 1830.

Only one caused significant damage. Most have been to the east and south.

Auckland has had about 40 small earthquakes since 2007, while Wellington has had more than 1000 in the same time.

Christchurch residents are enduring about 10 quakes of similar size every day.

Mr Rosenberg said there was a low probability of another event in that same place.

"The Auckland area is a place that doesn't experience a lot of quakes because there isn't the same amount of stress being built up in the crust as there is through the central North Island and the volcanic zone, the lower North Island and through the South Island."

GeoNet said that in suburbs near the epicentre the tremor was felt as a 5 - rated "strong" - on the 12-step Mercalli intensity scale, which measures an earthquake's effects on people and their environment.

The February 22 earthquake in Christchurch rated at 9 which is "destructive".

EARTHQUAKE

Size: 2.9 on the Richter scale
Where: 10km east of downtown Auckland, around the Kohimarama beach and Mission Bay areas
Depth: 9km
When: Friday July 1, 9.09pm

- NZ Herald

© Copyright 2014, APN New Zealand Limited

Assembled by: (static) on production bpcf03 at 31 Oct 2014 04:41:33 Processing Time: 442ms