ChCh braces for a bumpy Xmas

By Hayley Hannan

If you would like to help out, visit The Student Volunteer Army are planning a clean-up on the 26th and 27th of December.

A car is swamped by flooding caused by liquefaction in the suburb of New Brighton after a series of earthquakes that struck Christchurch. Photo / Simon Baker
A car is swamped by flooding caused by liquefaction in the suburb of New Brighton after a series of earthquakes that struck Christchurch. Photo / Simon Baker

Cantabrians are likely to be in for a bumpy Christmas Eve as the "intense'' aftershock sequence continues to rumble, says a seismologist.

Christchurch was yesterday shaken by a 5.8 magnitude quake at 1:58pm followed by a 6.0 earthquake at 3:18pm, which resulted in the temporary evacuation of malls, buildings and homes across the city and cutting power supply to Sumner and Redcliffs suburbs.

Following the two major shocks, the city had been rattled by "hundreds and hundreds'' of earthquakes, said GNS seismologist Bill Fry.

By 3:30 this afternoon, the GNS website listed 23 quakes in the Christchurch area since midnight and one magnitude 3.2 quake measuring 7km deep, 10km south of New Plymouth.

Of those 23 recorded in the Canterbury region, most ranged between 3.0 magnitude to a peak of a 5.1 quake, recorded 10km east of Diamond Harbour at a depth of 8km, at 6:37am today.

Mr Fry said this amount of activity was to be expected for the region, and more was likely to follow.

"[Today I've seen] the aftershocks from the event yesterday. Most of them are thrust earthquakes, which means you have one block that's thrust over another block.''

"Commonly after you get a large earthquake then along the edges of the fault that ruptured in the large earthquake, you get smaller earthquakes that are of a similar mechanism.''

He said the earthquakes in the Christchurch area appeared as if they were all occurring along a same Pacific plate faultline, but the earthquake recorded south of Taranaki occurred along the Australian plate.

He said while the aftershocks were lessening since the two major shakes yesterday, more quakes were expected as the activity was a continuation of the Darfield sequence.

"We expect big aftershocks, yeah. We'll probably or could have more 5.0, that is a possibility.

"Once you get a large earthquake, you get a spike in activity. So then the aftershocks, they increase rapidly right after the earthquake and then they decrease through time. So there's a decay in aftershocks. So what you see between say midnight and now, is a lower rate than what you saw between 3pm yesterday and midnight.''

He said the rate of aftershocks had already noticeably dropped since yesterday afternoon, but would continue for a while.

"What we do in a response situation is we go with the biggest earthquake or the most damaging.

"While the website showed many aftershocks that happened since yesterday, many were not sizeable enough to be added yet", he said.

"You should realise too that we've had hundreds and hundreds of earthquakes since yesterday, so this aftershock sequence is quite intense as are most aftershock sequences after earthquakes this big. We just don't have the capability to put [on the website] every earthquake.''

Mr Fry said the smaller quakes would be added to the site later on.


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