Old tectonic scars run deep: the magnitude 5.0 earthquake in Ontario

A post by Chris RowanYesterday, eastern Canada was shaken by a magnitude 5 earthquake. This is, at first glance, a rather surprising event, because if you were to ask me to point out the most likely place for an earthquake to occur in Canada, I would point west, to the plate boundary marked by the Cascadia subduction zone (which eventually links up to with the San Andreas Fault farther south). In contrast, Ontario and Quebec are several thousand miles from the nearest plate boundary. However, perhaps I shouldn’t be so surprised: the rupture is located smack in the middle of a zone of enhanced risk in the seismic hazard map for this area. A look in the USGS’s historical database reveals why: there have been a number of earthquakes of similar or greater size in this region in the last century: a magnitude 4.9 in 2005; a 5.9 in 1998; a 5.8 in 1944; a 6.1 in 1935; and a 6.2 in 1925. There are also reports of a what is possibly up to a magnitude 7 earthquake way back in 1663. Although earthquakes within plates might not quite follow the tectonic rules seen at plate boundaries, it seems in this case, at least for the moment, past seismic activity does provide some indication of where to expect future large(ish) earthquakes.

Hazard.png
Seismic hazard map for Eastern Canada. Source: Global Seismic Hazard Assessment Program.

So why is this particular region so much more earthquake-prone than the rest of eastern Canada? Earthquakes within plates occur where they are not quite strong enough to support the forces driving motion, causing them to deform slightly. These weak points are generally found in older, fault-riddled bits of the crust that have gone through the plate tectonic wringer one or more times in the geologic past. Although there is currently no active plate boundary on the East Coast of North America, in the last 1000 million years or so it has been through a continental collision (the Grenville orogeny associated with the formation of the supercontinent of Rodinia), a rifting event (the formation of the Iapetus Ocean), then another continental collision (the closure of Iapetus and the formation of the supercontient Pangaea), then another rifting event (the opening of the present Atlantic). Faults related to all of these past episodes of tectonic activity can be found along the margin, as summarised in the color-coded figure below, taken from Thomas (2006) (pdf); this concentration of structures, easily reactivated by any applied tectonic stress, are the reason that this edge of the North American plate is much more seismically active than the continental interior.

inheritance_s.png
Click image to enlarge. Source: >Thomas (2006).

This particular earthquake appears to be associated with a small extensional graben formed during the opening of Iapetus (green lines), although a little inboard of the main locus of rifting; it is thus a failed rift, similar to the one associated with the New Madrid earthquakes. Here’s a close-up, courtesy of fellow geotweeter CPPGeophysics

OttawaGrabens.jpg

It’s interesting to see how tectonic processes that operated, and ceased, hundreds of millions of years ago, can still have a profound impact on the patterns of earthquakes today.

Categories: deep time, earthquakes, geohazards, geology, structures, tectonics
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Comments (12)

  1. That’s interesting, although I am not sure I have herd about similar seismic activity in other reworked suture zones of ancient orogens. Like for instance along the Ibero-Armorican Arc, encompassing the remnants (e.g. ophiolitic tectonic slices and associated HP metamorphic rocks) of the Variscan suture in Western Europe, and interpreted as being associated to the closure of the Rheic ocean. In SW Iberia a suture zone like this (the so called Faixa Blastomilon??tica), is interpreted by some authors as having been active during the Cadomian orogenic cycle and later re-worked during the Variscan… but as far as I know (although, of course, there is always the possibility that I should know better) no Present day seismic activity of this nature has been reported.

  2. Richard Guy says:

    Earthquakes tell us that expansion is taking place. The St Lawrence like all rivers are earthquake faults. The earthquake took place approximately 60 miles from Ottowa a little north of Thurso. Expansion is taking place there. Expansion is also taking place on the Baja Gulf. The recent spate of nearly 600 earthquakes in southern California is telling us that the splitting process that is separating the Baja Peninsular from mainland Mexico is taking place.
    Look at the USGS map and you will see that the cluster of 566 tremors are alligned along the San Andreas and the San Andreas runs straight into the Baja Gulf and continues down the middle of the Gulf. This split is taking place and the earthquake cluster along the San Andreas points the way to the Baja split. See videos on http://www.widemargin2000.com and see “The Mysterious Receding Seas” videos on http://www.youtube.com
    Our Earth is expanding and Earthquakes are a manifestation of the expansion process: get used to it. Richard Guy

  3. James says:

    “…all rivers are earthquake faults.”
    I don’t know a lot of geology, but that statement is FALSE.

  4. cisko says:

    That is the funniest thing I’ve read all day. Expanding earth. +1 for originality, +2 for wackiness, -10 for implausibility.

  5. Eamon Knight says:

    Thanks for the background, Chris. I have little to add except that I live in Ottawa, and it was a fun 30 seconds or so, yesterday ;-). That, and we get a M4 about every 10 years, usually from a fault to the south of Ottawa. It’s true: you can never completely live down your history.

  6. BrianR says:

    Chris, nice explanation — when this happened I was waiting for your post, I now expect it from you, ha!

  7. hypocentre says:

    @ Richard Guy
    I think you should check out the earthquake focal mechanism.
    It is a reverse fault – caused by horizontal COMPRESSION.
    Just because the original graben was caused by extension doesn’t mean that it is extending now. The original deformation create lines of weakness that get reused.

  8. Ken says:

    hahaaha its so funny to see the expanding earth nuts spout off their dogma. So quickly they believe the easily disproven, yet so slowly if ever do they try and learn the real science, even if just to know their enemy better.
    It’s like listening to a young earth creationist who hit his head on a rock or something, resulting in the application of his ignorance onto an entirely different subject.

  9. Chris Rowan says:

    That will teach me not to talk about the focal mechanism for once. It does indeed show reverse faulting. I don’t know for sure, but given the location I suspect the source of the stress might be glacial rebound -this part of Canada was under quite a lot of ice 10-20,000 years ago.

  10. NJ says:

    OK, now you can explain the strike-slip motions in the East Tennessee Seismic zone illustrated on your first figure between the New Madrid and the Charleston, SC zones…

  11. J.C.Chang says:

    The focal plane solution, being compressive, puzzled me as well. Chris, the glacial rebound hypothesis may be plausible, but the rebound is quite fast (geologically speaking – it’s measurable via GPS or 4D gravity).
    Are there other hypotheses on the cause of slip?

  12. Chris Rowan says:

    the rebound is quite fast (geologically speaking – it’s measurable via GPS or 4D gravity).
    Well, so are plate motions, but they still result in earthquakes. As for other causes, the axis of compression is approximately E-W, which is parallel to any ridge push force from the North Atlantic Ridge. A link to larger scale tectonic forces is therefore feasible, but extremely speculative.