Wednesday, August 26, 2015

Earthquake Primers - What kills you - Part 2

Here's where I make a bold statement:  Plop down your house anywhere in the world, on solid rock, and your seismic risk of death is zero.  Said with 95% confidence, a reasonable house, and 'solid rock' defined by Canadian standards, not Callie.

The statement is backed up by an extensive study I once made at work on ground motions for rock and soil.  Once I had this study, I went into the physics.  This was to support a nuclear waste facility.  It's fun to note that everything I recommended has been ignored for the Bruce  Black Hole,  and I suspect, as well, that it will the same for the big repository.

The PGV on rock is directly related to the induced strain within the rock mass.  On the large scale (the wavelength) rock can take only a very little strain before cracking or fluid flow.  Thus, seismic motion is divided  into the static motions (near-field) and the propagating component.  There is no limit on what can happen right on top of a fault, but seismic waves can only propagate if the rock acts in a linear manner.

So, you've got to believe me that the PGV on rock is very low a reasonable distance from the fault (a few fault lengths).  If you are right on the hammer zone (hanging wall) of a shallow thrust, then rocks will fly into the air.  Not a good place to be.

If you are on solid rock during a big earthquake, you will only wake up to the screams, or the sound of transformers exploding on the deep soil basin beside you.  Soil basins amplify PGV by a factor of 10 to 100.  I suspect that this is even greater if there are standing waves.  There is only one way for your house to survive on a soil basin, and that is to have piles that go down to something solid.

You can see that 'seismic risk of death' SRD, varies more by foundation than location.  Construction method adds very little difference.  A historic brick house will survive very well on rock, but is usually hopeless elsewhere.  We have seen very strong buildings tip over on soil.  Adding dampers gives you a factor of two in seismic capacity, but this is nothing compared to the soil amplification.

Anyway, that's it for now.  If there has been a large regional earthquake and you didn't feel it, you are golden.  Sure, you're left out of all that Twitter conversation, but be happy.

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