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Massive aftershock was only ever a matter of time


Bridie Smith



THE surprising thing about the fatal aftershock that struck Christchurch yesterday was that it took so long to arrive.

The Australian Seismological Centre director, Kevin McCue, said yesterday's 6.3-magnitude earthquake was related to the 7.1-magnitude quake that struck the city last September.

Cliff damage south of New Brighton.

Cliff damage south of New Brighton. Photo: Don Scott/The Press

The destruction was greater, as the epicentre was about 10 kilometres south of Christchurch and just four kilometres deep. By comparison September's quake struck 40 kilometres from the city centre and was 10 kilometres deep.

According to the US Geological Survey, shallow earthquakes are more damaging as the energy is released closer to the surface.

September's quake has triggered a series of aftershocks. The Canterbury region has recorded 14 of magnitude 5 or above and Mr McCue said yesterday's quake was part of this sequence, which could continue for up to a year.

''I hate to say it but this is the aftershock they had to have,'' Mr McCue said.

''This is what would normally be the biggest aftershock after a magnitude 7 earthquake, although it's unusual that it's taken so long after the main shock.''

He said the delay could be due to a strong section of the fault line failing to break in the main September earthquake, which was the biggest in nearly 80 years. Mr McCue said that section would have been stressed by recent, milder aftershocks and finally snapped.

The quake's epicentre was about 100 kilometres east of the Alpine Fault, which runs the length New Zealand's South Island between the Pacific and Australian plates. The fault line runs for 600 kilometres from the south-west corner of the South Island to the north-east.

While the Australian Plate is relatively static, the Pacific Plate moves about 50 millimetres a year - about the rate fingernails grow. This means that the main part of the South Island is being thrust over the Australian Plate on a bearing of about 250 degrees.

The two plates grind against each other with pressure building until it is released via an earthquake. The process lifts the Southern Alps, raising them some 20,000 metres in the past 12 million years. Only the fast rate of erosion keeps their highest point below 4000 metres.

Caused by a reorganisation of the stresses around the plate boundary, Mr McCue said the magnitude of yesterday's quake suggests it was the main aftershock - which typically are one magnitude unit smaller than the main earthquake.

And while most of the damage has been done, there is still more to come from yesterday's quake. A GNS engineering geologist, Dick Beetham, said the strong shake had dislodged the soft land around central Christchurch. This means properties around the Avon and Heathcote rivers face further damage, as the soil begins to slip towards the river, taking properties with it.

''Once the shaking stops it slows down … but the continuing [sideways] movement will cause damage to homes,'' he said.

Seismologists estimate that NZ experiences several magnitude 6 earthquakes a year, one 7 a decade and one 8 a century.

with The Dominion Post

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