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Geologists answer questions on Wednesday's quake Add to ...

As a magnitude 5.0 earthquake hit eastern Canada Wednesday afternoon, readers flocked to the Globe's liveblog to talk about their experiences and join in a discussion with Canadian experts. Below is an edited transcript of the Q&A:

Jen MacMillan: I've got Alexander Cruden from University of Toronto, geology professor, on the line to take some of your questions.

First up, I've asked him about the likelihood of aftershocks.

Dr. Cruden: There will be smaller aftershocks but they're normally of lower magnitude than the main event. The probability of a similar sized earthquake is not very high. That's a general rule of thumb.

Comment From Guest: when was last time Toronto [had an]earthquake? sometime 2003-2005?

Dr. Cruden: There are periodic earthquakes around Toronto. There was a 5.5 quake under Lake Ontario about 8 years ago. And there was a 3.3 in Georgian Bay about three years ago. And there are low-level magnitude earthquakes all around the Great Lakes fairly regularly.

I believe the 5.5 quake was felt all around the GTA and Hamilton.

Comment From Guest: How can an earthquake in 1732 be [calculated]on the Richter scale?

Dr. Cruden: Obviously there weren't seismologists in 1732. They rely on written records and people's descriptions of the event. There were Europeans living in the region at the time and records of the damage so they come up with a guesstimate of the magnitude.

Comment From Michael Ranney: Is it simply the depth of the quake that has resulted in such a large shake radius, or is there something else in the nature of this quake that has caused it to be felt such a large distance?

Dr. Cruden: Yes, there is a correlation between the depth and the radius of the area where you would actually feel vibrations. The information I have is between 18 km and 19 km and that's quite deep.

Comment From Guest: As a general rule, at what magnitude does an earthquake begin to cause major structural damage to buildings?

Dr. Cruden: It depends on what the buildings are sitting on. If they're on solid rock, you're probably looking at 6 and above as likely to cause damage. But if they're sitting on unconsolidated sediment with rock underneath, you have this chance of something called ground acceleration. That amplifies the seismic waves travelling at the surface. So you could potentially get damage at magnitude 5 and above if you have that kind of acceleration.

Comment From Guest: Is there a fault line here in Eastern Canada that caused this earthquake?

Dr. Cruden: I'd call it a zone of weakness. It's called the western Quebec seismic zone. It's not a specific fault like the San Andreas fault. It's a zone a few kilometres across. Maybe 50 km across. It goes from Montreal from Temiskaming.

Comment From James: How did the CN Tower hold up in the earth quake?

Dr. Cruden: Structures like that are incredibly overengineered. When it was designed, it was designed to withstand a major earthquake. The whole foundation is actually designed with that in mind, and with a collision with an aircraft in mind... The CN Tower sits on this huge concrete slab underground that covers a massive area, much bigger than the actual footprint you see of the CN Tower. It's made of bendy, reinforced concrete.

But I wouldn't have wanted to be at the top of the CN Tower today.

Comment From Heidi: How long after an earthquake can you still expect to feel aftershocks? An hour? Several hours?

Dr. Cruden: Normally, it's a matter of days.

Comment From Guest: how is the position of Ottawa downtown i.e. is it on top of solid rock or unconsolidated sediment with rock underneath

Dr. Cruden: It's a mixture - a fair amount of the city is on or close to bedrock but particularly in the Ottawa Valley there are some areas that are built on soft sediment. I know there are people at the University of Ottawa that have been specifically studying the risk for ground acceleration in the Ottawa Valley area.

Jennifer MacMillan: Dr. Cruden, thanks so much for your time today. One last question, to sum up what a lof of our commenters have been asking us: Should we be concerned about a stronger earthquake or aftershocks?

Dr. Cruden: One thing to appreciate is that the earthquake occurred in the area that is the area of highest seismic risk in Eastern Canada. Toronto is not close to it... The area in Eastern Canada pales in comparison to the seismic risk areas on the west coast of Canada and the U.S. and in Indonesia.

Jennifer MacMillan: Thanks again to Dr. Cruden for his time today, and to everyone for their questions.

<iframe width="600" height="350" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="http://www.google.com/maps/ms?ie=UTF8&hl=en&t=h&msa=0&msid=103335406819895550235.000489b85747cab41bd5a&ll=44.166445,-78.442383&spn=5.516366,13.183594&z=6&output=embed"></iframe><br /><small>View <a href="http://www.google.com/maps/ms?ie=UTF8&hl=en&t=h&msa=0&msid=103335406819895550235.000489b85747cab41bd5a&ll=44.166445,-78.442383&spn=5.516366,13.183594&z=6&source=embed" style="color:#0000FF;text-align:left">Canadians react to the earthquake</a> in a larger map</small>

Jennifer MacMillan: We received a lot of reader questions, and we're fortunate to have another expert with us to answer your questions about today's quake. Joining us is Alan Baird from Queen's University's geophysics and geology department. He studies intraplate earthquakes, which is the type of quake that occurred today. Thanks for joining us, Dr. Baird.

Comment From Barry Mercer: You might want to ask your guest if earthquakes like the one today can pose a danger to underground miners in places like Sudbury. Miners at inco are on strike so there is limited underground activity but Extrada employees are still underground and working. That quake ripped through this area as well.

Dr. Baird: If this were to happen very close to a mine in operation, it could be quite dangerous. But I think at this distance, it wouldn't be that significant. The closer you are to the source of the earthquake, the more potential damage there would be. If it happened in Sudbury, it would be very dangerous.

Comment From Patricia Tomasi: Why was the earthquake so deep?

Dr. Baird: 18 km is a typical deep intraplate earthquake. It's not that unusual. An intraplate earthquake is a quake that doesn't occur at the plate boundaries but at the interior of the plate. Most big quakes occur at the plate boundaries but we do get these occasional ones in the plate interiors, and this is an example.

Comment From Wayne: the question on everybody's mind -- but no one has asked -- what is the chance that this a prelude to a larger earthquake? Would it be sooner or later?

Dr. Baird: It's not that unusual to get quakes of that size here. The difference is that the recurrence rate is much longer in intraplate areas like here than in places at plate boundaries. There was a large quake in the same seismic zone in 1935 as well. It's not that unusual that we're seeing these quakes.

Comment From GinoC: How BIG can an intraplate eartquake get?

Dr. Baird: The biggest intraplate quake that we know of happened in New Madrid, Missouri in 1812. There's no accurate measure of it but estimates put it around magnitude 7. Some people estimate it's a bit higher or lower but probably in the 7s. Those happen quite rarely.

Comment From Guest: Was this quake a lateral quake as caused by a fault sliding along, or vertical such as an uplift on one side of the fault, or by bending (up or down?)

Dr. Baird: You can tell that from the quake's focal mechanism which has just been posted by the USGS. I'm looking at it now and that's indicating that it's a reverse slip, it's around a 45-degree angle. One side of the fault is moving up relative to the other.

Comment From Guest: What causes an intraplate earthquake? I think it is widely understood what causes earthquakes on the edges of plates (collisions/sliding/etc).

Dr. Baird: Intraplate earthquakes aren't very well understood. Basically you have built-up stresses within the plate from pushing at the boundaries and that builds stress up within the plate that has to be released in some way. We get the occasional earthquake that is relieving some of the stress that has built up over a while. You get small and moderate quakes along these ancient structures. The Ottawa graben is an ancient rift zone that is no longer actively rifting but the structures - the faults - are still there.

Comment From Guest: Was this quake in any way linked to the sink hole that occurred in Quebec recently?

Dr. Baird: No. Earthquakes can trigger landslides, but this quake isn't related to the earlier sinkhole.

Jen MacMillan: Thanks very much to Dr. Baird for taking your questions.

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