Christchurch earthquake: Big aftershocks every month for a year

The aftershocks. Graphic / NZ Herald

A year from now, Christchurch will still be rattled by a magnitude-four aftershock at least once a month, according to a scientist.

Last Tuesday's devastating 6.3 event meant that the quake-weary city, which has experienced 4000 aftershocks since September, has a new sequence of shocks to contend with.

Several large shocks were felt yesterday morning, causing further rockfall in Sumner.

A new forecast by GNS Science geological hazard modeller Matt Gerstenberger shows that despite an initial burst of high-energy shocks, the aftershock sequence has played out as expected.

But the Canterbury Plains now has two threads of aftershocks rumbling beneath it. In 12 months, residents will still experience at least one magnitude-four quake a month. GNS models predict that in the next week Christchurch should expect between one and 10 aftershocks greater than magnitude four and within three weeks the number should drop to one magnitude four every three to four days.

The chance of another large aftershock decreases by the day, but seismologists cannot rule out another magnitude-five or six tremor, especially given that the Lyttelton quake was itself believed to be an aftershock.

"A possibility remains for the occurrence of another aftershock greater than magnitude six. However, it is more likely that such an event will not occur," said Dr Gerstenberger.

Scientists are now working to find out how a large quake erupted in an area that was not considered a "weak spot".

GNS Science natural hazards platform manager Kelvin Berryman said aftershocks from the Darfield quake had increased the stress in the earth's crust under Canterbury, especially to the east of the epicentre.

But in investigations made after the September tremor, the amount of stress under the Port Hills was not an area of immediate concern.

Seismologists identified several high-risk sites based on where aftershocks were occurring - none of these were near Lyttelton.

GNS duty seismologist John Ristau said the Lyttelton faultline might have already been near breaking point before September.

"It could be that it was a fault that already had a lot of stress built up on it beforehand, so then it wouldn't take much to push it over the edge."

Seismologists will also use computer models to investigate whether the most recent quake has unsettled nearby faultlines.

Areas which were considered high-risk after the September quake will be closely monitored.

At present, the most recent quake is not believed to have affected the Alpine Fault, which runs along the spine of the South Island and is considered the most likely source of a large quake in New Zealand.

Some contention has arisen over whether last Tuesday's quake was in fact an aftershock. International academics have suggested that the high energy of the Lyttelton quake and its distance from the Darfield faultline dissociated the two.

GNS has reported that while the two quakes were not connected underground, last week's tremor was part of the Darfield quake's seismic aftermath.

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