Thursday, June 24, 2010

Ontario Earthquake - Fun Facts

For now, we shall leave aside the possibility that I actually caused this earthquake by releasing my anguished earthquake story 3 hours before.

This earthquake happened smack in the middle of the Western Quebec Seismic Zone. In colourful terms, I would characterize the 'fractility' as being 'Very Crunchy'. This is a zone of extremely solid rock, highly fractured, highly stressed, with high fluid activity. Normally, it would be a typical boring zone of the Canadian Shield, except for 2 things.

During the last big plate separation, there was a hotspot. Now, I have written about the hotspot controversy, but let's just say it is a local zone of hot magma rising. If you are out in the ocean plates, this sort of thing forms volcanoes, and the Hawaiian island chain, but if there is a massive lump of granite, the effect is much more subtle. Basically, it cracks the hell of the rock. This hotspot did pop out here and there, and you can see the track.

Then there was the last glaciation. This causes a great dimple in the earth through the heavy mass of kilometres of ice. Think of pressing on a beach-ball. But the very last dimple to pop out (like inflating a soccer ball), is Western Quebec.

But really, the maximum earthquake we can get there is a 'Phony Five'. I call them phony, since they put out more bark than bite. The PGV is very low, probably less than 1 cm/s at the max. They are deep, and really can't encompass a zone more than a 100 m, due to the intense fracturing. If you are on rock, and on top of this earthquake, it just disturbs you with the intense noise.

But these earthquakes ring a pure bell. If you sit on your cottage on a calm day, and watch a boat zoom across the lake, you see the planar wake. This crosses the lake without an apparent speck of decay (attenuation). We have the same with these earthquakes. The planar wave settles down to about 1 mm/s PGV, and zooms over the continent, decaying very slowly.

Now, if you are on solid ground, on the patio, drinking beer, as I was, you don't feel anything. You only feel if, if there is amplification. Most buildings will amplify by a factor of 2-5. Soft soil by a factor of 10. Soft soil in a basin, or a high rise on soft soil, a factor of 100. Smart people will start to worry if they felt this earthquake strongly in Chicago!


Harbles said...

You were mentioning QC landslides?

Anonymous said...

Love your blog. I'm a civil engineer. I live in an area of Burlington just below the escarpment. I just got the borehole logs for my street which is under re-construction. We sit on stiff silty clay till above soft queenston shale with N values in the 16-40 range and most of my neighbours didn't feel a thing. Does that make sense for our soil conditions and how much amplification would you expect us to experience in a more local quake? I have been asking everyone I know how much shaking they experienced and where they were. I

Harold Asmis said...

That's really good soil! You'll probably just get plaster damage in a real earthquake. Still, you are at 'Ground Zero', so keep up the chimney, and secure heavy items.

Anonymous said...

Thanks for the response. What do you consider a real earthquake? Believe it or not I actually have earthquake insurance but with a 5% deductible, I will probably never have a claim even if we get the "big one". Maybe I should cancel.

Harold Asmis said...

You don't really need the insurance, according to a book I once read. If your coffee spilled, you need the insurance!