GNS Science – update on probability of future earthquakes

A press conference this afternoon held by Wellington City Council and Civil Defence on the earthquakes featured an update from GNS Science seismologist Dr Ken Gledhill.

Click on the player below to replay the audio form Dr Gledhill’s update…

Dr Ken Gledhill
Dr Ken Gledhill

The key points from the update:

– Dr Gledhill downgraded the chance of the area suffering a magnitude 6 aftershock within the next week to 19 per cent, down from 30 per cent this morning.

– There was a 39 per cent chance the region would have a large aftershock in the next year.

– There was a seven per cent chance the region would have a large aftershock in the next 24 hours.

– The three earthquakes that have struck the region since Friday were all centred between 10km and 20km below the sea floor in the Cook Strait.

–  At its maximum point last night, the quake was 14 per cent the force of gravity – by comparison, the Canterbury quake was double the force of gravity.

Source: GNS Science
Source: GNS Science


Quotes from Dr Ken Gledhill:

How common are earthquake sequences like this in the Cook Strait region?

“Unlike the Canterbury region, an event like this is not a surprise. It is something that happens every few decades if you actually look at the historical record. These things happen periodically. What we know about this series of earthquakes and particularly the one last night, was that they were in the overlying Earth’s crust.

“The first two big events on Friday and on Sunday morning were thrusting events, a pushing up motion. The one last night was a strike slip, the faults sliding past each other. What happens is when you get something that slides half a metre or more over a large area more than 10 kilometres and with some depth, you are going to strain the area around it and that is what we are seeing at the moment.

“It is following about the pattern we would expect, but in the last few years I have been surprised by volcano, tsunami and earthquake, so I am very reluctant to hang my hat on something 100 per cent.”

What was the ground acceleration force measured?

“The maximum shaking we had in Wellington… was in Karori and was 14 per cent the force of gravity. Compare that to Christchurch at its maximum and that was two times the force of gravity and in the CBD of Christchurch a little under one times the force of gravity. So that was a much more significant shake.

“The maximum we recorded was in Ward, a town near Seddon on the other side of Cook Strait, it was 21 per cent the force of gravity.”

A swarm of earthquakes – how common are they?

“That’s very normal. The first earthquake on Friday set off its own aftershock sequence. The Sunday morning one was particularly energetic and went on all day Sunday up to the main one on Sunday evening. It was a different kind of earthquake because it was a sliding past earthquake. But all of these earthquakes are past 10 – 20 kilometres depth under the sea floor.

“We’ve basically got three aftershock sequences building up on each other. It’s not main shock, aftershock, we’ve ended up with, if you like, aftershock sequence stacked on aftershock sequence.

“It changes the probability [of aftershocks]. The latest numbers take that into account.”

Do we have historic events like this to compare?

“The 2005 Seddon sequence was a series of [around] 5.5M earthquakes over 11 days. They came and went and it was a very similar kind of thing. Every time you had a 5.5M earthquake, you had an aftershock sequence for it. This is not unusual behaviour for the Cook Strait region.”

What is the risk of a tsunami from future earthquakes or aftershocks in this region?

“There has been a lot of work done on the potential for a tsunami to be generated by those faults in the Cook Strait. The potential is very low. The earthquakes have to get a lot bigger for that to be realised. The issue is that an earthquake can trigger a landslide, but if it happens near a lot of rich sediments that you can then displace, its a possibility. But landslide produced tsunamis are very directed, rather than the earthquake produced ones that cause waves and impact over a big region.”