Experts Respond: Canadians on Kiwi connection to local quake risks

Experts have been examining the earthquake risks in Canada and how best to prepare for one – drawing on lessons learned in New Zealand’s Canterbury earthquakes.

The recent earthquakes in New Zealand and Japan have understandably led to local concern about geological hazards in other parts of the world.  Our colleagues at the Canadian SMC organised a media briefing with several earthquake experts to explain what geological risks Canada is exposed to and how the country is prepared for such an event.

Below are some of the comments from the experts. Feel free to use these quotes in your articles. If you would like to talk with a local earthquake expert please contact the NZ SMC (smc@sciencemediacentre.co.nz).

 

Dr. Denis Mitchell, Network Leader, Canadian Seismic Research Network; Professor and Department Chair of Civil Engineering at McGill University.

Can we draw any lessons in Canada from Christchurch?

It’s rather interesting that the first earthquake that occurred in 2010 in New Zealand was a magnitude 7.1 and it caused damage, however the far more devastating one was the one that occurred in Feb, Feb 22, that had a magnitude of 6.3. And the reason the smaller earthquake more devastating was that the earthquake was closer to Christchurch, and it was fairly shallow, and they got enormous vertical accelerations that hit their structures. That vertical acceleration reached two times gravity, which is a very large vertical acceleration. So you saw a lot of damage to un-reinforced masonry structures because they have difficulty surviving such large vertical acceleration, but also we saw damage to some concrete structures as well. Some of the concrete structures, the columns failed in compression because of the vertical acceleration. So this is something quite a bit different, and reminiscent of the 1994 Northridge, California earthquake where they also had very high vertical acceleration.

Now we could get significant vertical acceleration in Quebec, and this is something we have to look out for. However, our earthquakes might be a bit deeper. Gail might want to comment on that.

 

Dr. Gail Atkinson, Canada Research Chair in Earthquake Hazards and Ground Motions at the University of Western Ontario and CSRN Theme 1 leader, (Hazard Assessment).

We could have earthquakes at a range of depth, actually, in eastern Canada we have some areas where we get shallow earthquakes, and some where they might be deep. So we could in fact get quakes quite similar to the Christchurch 6.3 event that Denis was talking about.

What are the features underlying seismically active regions in Canada, and what are the risks?

In all parts of the world, including Canada, the driving force for earthquakes is plate tectonics. In the West, we are much closer to active, constantly moving tectonic plates in that the Juan de Fuca plate is being pushed under the North American plate just off the West Coast.

And that’s the potential for those really big magnitude 8.5-9.0 earthquakes like the Japan quake. That’s called a subduction quake.

Because of all that pushing at that active plate interface, there’s the potential for quakes in the subduction slab, like the Nisqually earthquake or the Seattle quake we had a number of years ago, and earthquakes in the overlying crust, or shallow crustal earthquakes. So that’s the situation on the West Coast.

For the East Coast, the plate tectonics are a bit less direct. The entire east part of the continent is being squeezed by the spreading at the Mid-Atlantic Ridge, so the basic driving feature is coming from over 1000 kilometres away. In that environment, faults that are preexisting from earlier plate tectonic activity can become reactivated.

And we have a lot of these faults that follow the St. Lawrence Valley and the Ottawa Valley and the Saguenay graben. These are ancient faults that were actually formed hundreds of millions of years ago, but as we squeeze them today, we reactivate them, so we get the potential for moderate to even large earthquakes across a fairly extensive part of Eastern Canada.

How large could quakes get in each of these areas?

On the West coast, the big quakes, the magnitude 9.0, we expect every 500 years or so. The last one was 300 years ago, but it can vary. So we don’t really know whether that next magnitude 9 quake in Cascadia will happen 2-300 years from now, or tomorrow.

More moderate quakes, like a magnitude seven might happen in the Vancouver/Victoria area, perhaps on the order of every hundred years ago. In the east we expect quakes on the magnitude 6-7 range to happen near major cities, once every hundred years or so.

What about the North?

That is a complicated question. The seismic hazard is, how strong will the shaking be, and the seismic hazard in some parts of the North is actually quite high. Areas like Baffin Island, for example, have a lot of seismic activity. In terms of risk, when we think of risk we also think of what infrastructures are there to be damaged. And because we don’t have the population centres in the North, we generally don’t think of the risk being as large in the north because of the lack of exposed and vulnerable facilities and populations. It’s plate tectonics – there’s an active plate tectonic boundary not too far off Baffin Island, for example.

 

Luc Chouinard, Associate Professor of Engineering, McGill University and CSRN Project Leader

I look at hazards, and how it translates into risk. The focus has been for major urban centres in Canada that are at high risk for seismic hazards. So the kind of work we’re doing in Montreal is also being done for the Ottawa and Vancouver area.

Part of the analysis we’re doing is based on ‘scenarios’. We position various [hypothetical] earthquakes of various magnitudes in various distances from Montreal and then what we’re looking at is the effects of these earthquakes in terms of ground shaking, and then trying to translate that as potential damage in terms of location. We will have different side effects in terms of surface geology, and also in the type of buildings on those sites.

When we have an earthquake, the waves are going to travel through the bedrock, upward into the softer sediment, and the seismic signals can get modified. So typically, when you have softer soils, the signals would be amplified, and the duration of shaking can also be affected.

The other important aspect is that when we look at seismic hazards, we also have to look at the population distribution. So we have to look at the number of people who could potentially be injured. Also, it is important looking at the number of people who could be displaced in an earthquake and need shelter, for example. So it’s important to perform those ‘scenarios’ at different times of the day, and even at different times of the year.

What about commuters in large cities? In your scenarios to determine risk, do you look at the many hours a day commuters spend on major highways?

As part of the scenarios, the that I was showing was one of the ones responding to populations that are pretty much static at 2 a.m. and 2 p.m., but one of the important scenarios is also the one during commuting, and this is ongoing work right now, and one of the important elements in this scenario is looking at all the viaducts and the bridges in the event of an earthquake during these hours.

 

Dr. Denis Mitchell, Network Leader, Canadian Seismic Research Network; Professor and Department Chair of Civil Engineering at McGill University.

The one message is that we have a lot of deficient older structures, and there is a tremendous need for us to address those issues and what we’re looking at is ways of evaluating those structures and improving the performance of those structures in preparation for the next earthquake that hits us.

How do you prioritize which ones to do first? How are they earmarked?

We’re hoping first of all the government, usually the provincial government, will take upon some method of looking at structures like schools and hospitals. If they can do that, that would be great. We’re developing what we call rapid screening procedures, which enable us to look at a lot of different structures and very quickly come up with a rapid assessment. And for those that show up as being deficient, we’d have the consulting engineer looking at that structure carry out a very detailed evaluation. So there would be two stages: The rapid assessment first, and the detailed evaluation later. And that would be how you would prioritize what has to be done.