Local Govt | National News Video | Parliament Headlines | Politics Headlines | Search

 


GNS: 6.3 Magnitude Earthquake Part of Aftershock Sequence

25 February 2011

6.3 magnitude earthquake part of aftershock sequence

This week’s devastating magnitude 6.3 earthquake centred southeast of Christchurch was part of the aftershock sequence that has
been occurring since the September magnitude 7.1 quake near Darfield, 40km west of the city, an earthquake geologist said today.

It caused 17km of subsurface rupture in an east-west direction between Halswell and Taylors Mistake on the coast, Natural Hazards
Research Platform Manager at GNS Science, Kelvin Berryman said.

The number of aftershocks in the first 24 hours were higher than expected for a magnitude 6.3 earthquake, but had since tailed off
sharply and were now less frequent than aftershocks at the equivalent time after the magnitude 7.1 earthquake, Dr Berryman said.

In time, the rate of aftershock activity would decay back down to the level before the magnitude 6.3 earthquake, and then continue
to decrease as before.

There was no obvious underground structure directly connecting the subsurface rupture that produced Tuesday’s earthquake with
the Greendale Fault that ruptured in September’s magnitude 7.1 earthquake,

“Aftershocks have been spreading both west and east since the magnitude 7.1 Darfield earthquake in September and this has resulted
in increased stresses in the earth’s crust in the Canterbury region,” Dr Berryman said.

An expanding “cloud” of aftershocks, particularly at both ends of the main fault rupture, was a familiar pattern with large earthquakes
worldwide, he said.

Dr Berryman said seismic energy travelled in waves and could be reflected off hard surfaces, much like sound waves.

With the epicentre of Tuesday’s earthquake in the Port Hills, a large amount of energy could have been reflected off hard volcanic rock
at depth. This would have compounded the impact of the earthquake at the surface.

Geologists had suspected for some time that there were buried and unrecognised faults in Canterbury. Some of these faults might not have
moved for many thousands of years, but had been reactivated as stresses in the earth’s crust had been redistributed since September 2010.

“If you strip away the sediment and gravels of Christchurch and the Canterbury plains you would see the bedrock looking like broken glass
from millions of years of earthquake activity.”

The underlying geology of Canterbury was the western end of the Chatham Rise which was broken with many east-west trending faults.
Many geologists believed that modern-day tectonic plate motions in the South Island had reawakened some of these very old faults,
causing them to fail.

The Greendale Fault that ruptured in September’s earthquake was one of these very old faults. Dr Berryman said the magnitude 7.1 earthquake
in September was an extraordinarily complex event with up to four interconnected faults rupturing almost simultaneously.

“The pattern of aftershocks since September has also been complex, making it difficult for scientists to understand the stress-related
mechanisms occurring in the earth’s crust.”

This week’s magnitude 6.3 earthquake appears to have been a less complex event with just one fault rupturing.

The frequency of aftershocks would continue to decrease in the coming weeks. When viewed over periods of many weeks,
this reduction tended to be fairly regular, but there were often anomalies, as the magnitude 6.3 earthquake had shown.


Click for big version

Graphic showing location of main shock, aftershocks above magnitude 3, and fault ruptures in Canterbury.
Graphic by Rob Langridge and William Ries, GNS Science

ENDS

© Scoop Media

 
 
 
 
 
Parliament Headlines | Politics Headlines | Regional Headlines

Patience: Drive Safe

Be patient before passing is the AA's message for drivers this Labour weekend.

"People taking crazy risks to get past other vehicles is one of the most dangerous things on the road,” says AA spokesperson Dylan Thomsen.

“The weather is looking good for the long weekend so the roads will be busy. Unfortunately, that also increases the chances of people getting frustrated and trying a risky passing manoeuvre. When they get past, there will probably be more traffic up ahead anyway so it won’t get people there faster.” More>>

 
 

Parliament Today:

Employment Relations Bill: Govt Strains To Get Tea Break Law Through

The Government has been left with egg on its face - failing to get its much-vaunted, but hugely unpopular, meal break law passed in the first week of its new term, Labour spokesperson on Labour Issues Andrew Little says. More>>

ALSO:

Guns: Police Association Call To Arm Police Full Time

"The new minister gave his view, that Police do not need to be armed, while standing on the forecourt of parliament. The dark irony was that the interview followed immediately after breaking news of a gunman running amok in the Canadian parliament in Ottawa..." More>>

ALSO:

Gordon Campbell: On The Tokenism Of New Zealand's Role Against Islamic State

Our contribution against IS will be to send SAS forces to train the Iraqis? That’s like offering trainers to General Custer just as the 7th cavalry reached the Little Big Horn. More>>

ALSO:

Scoop Business: Shell And Todd Caught Drilling Without Approval

Multi-national oil company Shell’s New Zealand arm and local energy giant Todd Energy have breached the new law governing New Zealand’s Exclusive Economic Zone, the Environmental Protection Authority says in an Oct. 10 document released by the Green Party. More>>

ALSO:

Gordon Campbell: On Pharmac, Gough Whitlam And Sleater-Kinney

We’re not at the outset of these negotiations. The outset was six years ago, and negotiators were hoping to have some sort of ‘framework’ deal finished in time for the APEC meeting in a few weeks’ time. These ‘extreme’ positions are what we’ve reached near the intended end of the negotiations… More>>

ALSO:

PM Of Many Hats: Questions, No Answers On Whale Oil

Dr RUSSEL NORMAN (Co-Leader – Green) to the Prime Minister: How many times since November 2008 has he spoken with blogger Cameron Slater on the phone and how many times, if any, has he texted him?
Rt Hon JOHN KEY (Prime Minister): None in my capacity as Prime Minister. More>>

ALSO:

Aussie Investigation Dropped: Call On Minister McCully To Pursue The Case Of Balibo Five

West Papua Action is deeply concerned at the lack of any clear outcome from the Australian Federal Police inquiry into the 1975 deaths of the ‘Balibo Five’ including NZ journalist Gary Cunningham. More>>

ALSO:

'Feed The Kids' Bill: Metiria Turei To Lead Fight On Feeding Hungry Children

Green Party Co-leader Metiria Turei is urging all political parties to support the Feed the Kids Bill which she inherited today from Mana leader Hone Harawira. More>>

ALSO:

Parliament Today: State Opening Of Parliament

The House sat at 10.30am on Tuesday before MPs were summoned to hear the Speech from the Throne in the Legislative Council Chamber. More>>

ALSO:

Get More From Scoop

 

LATEST HEADLINES

 
 
 
 
 
 
 
 
Regional
Search Scoop  
 
 
Powered by Vodafone
NZ independent news