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Magnitude 6.3 earthquake rocks Christchurch

During the 4 September 2010 Darfield Earthquake (magnitude 7.1, depth 10-11km), Christchurch was fortunate that no lives were lost and there was comparatively little damage due to the depth, location and timing of the quake. However, the location and depth of the latest quake resulted in much greater shaking on the ground surface. This, combined with the timing, has caused massive destruction – approximately 2,000 people injured (with just under 200 classified as serious) and the deaths of at least 145 people with more than 200 people still missing or unaccounted for (as at 25 February 2011).

Dr Gary Gibson, Principal Research Fellow at the School of Earth Sciences, University of Melbourne, explains the difference in the 2 quakes further: “The earthquake fault rupture in September 2010 began about 40km west of Christchurch and ruptured for another 40km heading off to the west, away from Christchurch. The [February 2011] earthquake was smaller, with a rupture of about 15 x 15km at shallow depth immediately under Christchurch, so the shaking was much stronger.”

According to Geonet, despite its destructive power, the latest quake is classed as an aftershock and is related to the ongoing series of aftershocks since September 2010. “Its occurrence was always statistically possible, but the long time interval and slow decrease in general activity had made it less likely. Unfortunately, it has happened after all and in a location that has brought the worst result.”

Three Geonet monitors in the Christchurch CBD recorded much worse ground shaking than was felt during the 4 September 2010 quake. It was expected to have caused a fissure in the ground under its epicentre in Lyttelton.

Although the quake is being studied at present, it is known that the earthquake has a reverse faulting mechanism, meaning the rock above the inclined (sloping) fault moves up.

Studies of the ruptured area will confirm the amount of displacement in due course. However, Dr Gibson says that it is usual for an earthquake of this magnitude to occur when “an active fault area approximately 15km2 ruptures, and one side moves about 1m relative to the other. Its effect depends on how close it is, and ground shaking will be severe within 10–20km of the rupture.”

When asked if another large earthquake event was likely, Dr Gibson said the September 2010 earthquake and this February 2011 earthquake will have relieved the majority of stress in the regions in which they occurred, so another larger earthquake is unlikely. “However, aftershocks will certainly occur over the next few days and weeks, which may cause further damage in weakened buildings and will be very distressing for residents.”

Update

In a midday press release from GNS Science on 25 February 2011, Geologist Dr Kelvin Berryman said the quake caused 17km of subsurface rupture in an east-west direction between Halswell and Taylors Mistake on the coast.

“The number of aftershocks in the first 24 hours was higher than expected for a magnitude 6.3 earthquake, but had since tailed off sharply and are now less frequent than aftershocks at the equivalent time after the 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.”

An expanding ‘cloud’ of aftershocks, particularly at both ends of the main fault rupture, was a familiar pattern with large earthquakes worldwide says Dr Berryman.

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.

Activity idea

This news article concerns the magnitude 6.3 earthquake in Christchurch. Students may like to try this activity to understand the difference between magnitude and intensity of earthquakes.
Earthquake intensity

Useful links

Find out more on the Geonet website.
www.geonet.org.nzexternal link

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