Q+A: Seismologist on the Christchurch earthquake

Stuff.co.nz

Hundreds evacuated in central Christchurch after Sunday's earthquake.

Along with liquefaction, big earthquakes tend to throw up a lot of questions. 

Following Christchurch's Valentine's Day quake, there were more than we had time to answer in our live chat with GNS seismologist Anna Kaiser.

For those who missed it, we have put common queries in the Q+A below:

How safe do you feel in Christchurch?

Share your stories, photos and videos.

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Sunday's quake was a magnitude 5.7 centred 13km east of the city at a depth of 14km. When did we last have a quake bigger than that? 
We haven't had a quake that big for years. The last quake bigger than the one on Sunday occurred in a similar location in December 2011. That one was a magnitude 5.9.

How likely is it that we will experience another quake of that size in the near future? 
New modelling by GNS Science after Sunday's quake shows there is a 63 per cent chance of another 5 to 5.9 magnitude quake hitting Christchurch within the next 12 months. The chance of a 5 to 5.9 magnitude quake in the next week is 19 per cent and the likelihood of a 6 to 6.9 magnitude quake in the same time period is 2 per cent.

Huge clouds of dust rising in Sumner the moment the quake struck.
GRAEME JOLLIFFE

Huge clouds of dust rising in Sumner the moment the quake struck.

Was this quake related to the sequence that started in 2010? 
Yes, Sunday's quake is part of the Canterbury sequence that started in 2010 with that magnitude 7.1 Darfield shake. There is always a chance of more earthquakes of this size however it's a slim chance. 

How long can we expect the aftershocks to last? 
There were 25 aftershocks over the 12 hours since 6pm on Sunday. The biggest was magnitude 4.2 classified as "strong" at 6.27pm, and two "moderate" aftershocks were recorded overnight, a 3.9 at 8.34 pm and  a 3.5 at 4.33. The aftershocks will become less frequent, but the sequence will continue for years. 

Why can't we predict large earthquakes?
Seismologists can forecast probabilities of major shakes, but they can't predict earthquakes. In the days leading up to Sunday's quake, the seismicity was no higher than at other times during the sequence. However the probability of large earthquakes was still higher than it was before the Canterbury sequence began in 2010. Probabilities calculated prior to Sunday's quake gave a 50 per cent chance of a quake of magnitude 5 or greater in Canterbury in the next year.

The 5.7 quake made a bit of mess at Speights Ale House in Christchurch.
DANIEL BURT

The 5.7 quake made a bit of mess at Speights Ale House in Christchurch.

Which fault lines is to blame for Sunday's quake?
The quake occurred offshore in the Pegasus Bay. This is a similar location to the last major earthquake of the sequence in December 2011. An earthquake of this size ruptures a much smaller area than say a magnitude 7, which makes it difficult to associate with a particular fault line. In addition, it's more difficult to map faults underwater. 

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Will Sunday's quake have shifted stress onto other fault lines? 
Yes, earthquakes can increase stress on neighbouring faults. However Sunday's quake was moderate-sized and will have had greatest impact on stress in the immediate area (within a few kilometres) of the epicentre.

What type of earthquake was this? 
Sunday's quake had a bit of both strike-slip (side-by-side motion) and also vertical motion (reverse faulting). 

The quake left just a few metres of land remaining between one house and the edge of the cliff.
YEAHNAH PRODUCTIONS/YOUTUBE

The quake left just a few metres of land remaining between one house and the edge of the cliff.

 - Stuff

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