Christchurch quake was aftershock of 2010 tremor, scientists say
Last September's New Zealand earthquake was more energetic but caused less damage
The earthquake that brought death and destruction to Christchurch on Tuesday was almost certainly an aftershock of a larger tremor that rocked New Zealand on 3 September last year.
But while last year's earthquake was more energetic, it struck in the early hours of the morning, some 48km outside the city. The tremor that hit on Tuesday was more devastating for several reasons: it was shallower, much closer to Christchurch and arrived in the middle of local lunchtime at 12.51pm.
Earthquakes are not rare in New Zealand. The islands are shaken by noticeable tremors on average twice every three days. Seismologists at the US Geological Survey have recorded at least six earthquakes of magnitude five or more since September's magnitude 7 incident.
Tuesday's earthquake was recorded at magnitude 6.3, or roughly 11 times weaker, but it was enough to raze buildings already damaged by the previous earthquake and later aftershocks.
The earthquake was blamed for breaking a 30m-tonne chunk of ice off the Tasman glacier in Aoraki Mount Cook national park in the centre of the south island. The iceberg produced giant waves in the park's terminal lake for half an hour. More aftershocks may yet arrive.
David Rothery at the Open University's volcano dynamics group, said: "The magnitude 6.3 earthquake was a very large aftershock following the 3 September magnitude 7.0 earthquake. Its focus was at a very shallow depth of about 5km, which doubtless contributed to the severity of the ground shaking that witnesses in Christchurch describe as more severe than in September. The rupture was also much closer to the city, and this too made it worse. Other, hopefully smaller, aftershocks are to be expected. These will be a hazard to people trapped in damaged and weakened buildings and their rescuers."
New Zealand has some of the most stringent building regulations, but early reports from government scientists in the country indicate that the levels of shaking in and around Christchurch were greater than buildings are typically built to withstand.
New Zealand is in a region of extraordinary geological activity called the Ring of Fire, which stretches from Indonesia to the coast of Chile. For every 10 earthquakes on the planet, nine are in this region. New Zealand itself straddles the boundary between the Pacific and Australasian tectonic plates, which slowly grind into one another.
On the south island, where Tuesday's earthquake struck, the Pacific and Australasian plates slip past each other horizontally, producing the enormous Alpine fault that runs down the western flank of the island. The tremors may have occurred on "blind faults", which have been inactive for so long, there are no records of their existence.
Dougal Jerram at Durham University said: "Christchurch sits on what is historically a tectonically active area where the Alpine fault runs right across New Zealand's South Island. Associated with this are many fault segments. What seems to have happened is that the pressure has built up on a particular fault segment with the epicentre much closer to the city itself.
"This earthquake has also been particularly shallow, so despite measuring slightly less than the previous major earthquake in Christchurch ... more of the earthquake's energy makes it to the surface causing far greater destruction, than if the quake was deeper underground."
http://www.guardian.co.uk/world/2011/feb/22/christchurch-earthquake-aftershock-2010-tremor
