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

 

Departure Speech: Governor-General’s State Farewell Luncheon

"...Unfortunately I was unable to get to the Antarctic, the Chatham Islands and the Kermadecs. A dicky heart thwarted our travel to the Antarctic; and even though I volunteered to parachute into the Kermadecs to join the Young Blake expedition, time, commitments and officials frustrated my plans to visit the Kermadecs and Chathams." More>>

ALSO:

New Research: Most Homeless People Working Or Studying

“The cost of housing has been rising without corresponding increases in income, whilst the number of state houses per capita has been in decline. Many low-income people are missing out on housing, whether we recognise them as ‘homeless’ or not. More>>

ALSO:

Post-Traynor: New Offender Info Sharing Plan

“This Bill delivers on that step-change by moving away from name-based records held by individual agencies to a shared, anchor identity based on unalterable information, such as fingerprints and facial recognition. It also gives agencies access to the drivers’ licence photo database and birth, death and marriages information." More>>

  • NZ Law Foundation - New $2M fund for research on information challenges
  • Littoral: New Ship To Deliver Enhanced Naval Capability

    Defence Minister Gerry Brownlee says the Government has approved a Ministry of Defence and New Zealand Defence Force recommendation to request tenders for a new naval ship to support littoral operations. More>>

    July:

    After King's Labour Snub: Māori Party And Kiingitanga To Work Together

    Māori Party Co-leaders Te Ururoa Flavell and Marama Fox met with Kiingitanga representatives in Wellington yesterday to discuss working together on key issues for the betterment of Māori. More>>

    ALSO:

    Waitangi Claim On Rehabilitation: The 'Justus' System For Māori Not Good Enough

    Closing statements at the Waitangi Tribunal case against Corrections called for immediate steps and a comprehensive review to address the high rate of Māori reoffending. More>>

    ALSO:

    Advice: PM Sets Rules For Ministers' Treatment Of Public Servants

    Prime Minister John Key has laid down the law about the way ministers and public servants should interact, saying ministers may not always like the advice they receive, but they must listen to it carefully, respectfully and professionally. More>>

    Gordon Campbell: On The Funding Changes In Special Needs Education, And Uber

    The plan to strip out the educational support for older “special needs” children in order to meet the existing shortfall in funding for special needs in early childhood education is so miserly and relentlessly stupid as to defy belief… More>>

    SPECIAL EDUCATION (& More):

    Online Learning Plans:

    Get More From Scoop

     

    LATEST HEADLINES

     
     
     
     
     
     
     
     
     
    Regional
    Search Scoop  
     
     
    Powered by Vodafone
    NZ independent news