Thursday, August 27, 2015

Earthquake Primers - The Big Bad Earth - Part 2

We have this big roiling Earth, and what does that have to do with earthquakes?  Simply that the earth has gone with the most efficient method of heat removal which is convection, and transfers that heat to the radiator fins of the ocean crust.  It's an operating air conditioner.  The upper convection is in the mantle, just below the solid crust, and is responsible for our earthquakes.  There may be lower convection cells that give us our magnetic field, and the Earth's core may be a nuclear reactor.  :)

Who cares about deeper convection, it's the mantle that does all the interesting stuff to us on the surface.  Because of the convection all our continents move around like floating scum, and the oceanic crust gets recycled.  The whole thing is called Plate Tectonics, which is a rather surface-oriented expression.  I'd rather call it Convection Collision.

You can read all about Plate Tectonics without once thinking about the whole 3-d mechanism.  This would be rather shallow of you.  :)  As I have said, this would be all quiet grinding if it were not for the interaction of quartz and water.  That gives us earthquakes, and nobody would care about them if we could all live on high rock.

All the details of Convection Collision (my new theory) are horribly boring. There's subduction, spreading, transform faults, etc.  The only thing interesting are the formation of diamonds as the big chunks go down.

Bye for now.

Earthquake Primers - The Big Bad Earth - Part 1

I'm writing this primer series for all my fans who are making their way in first-class containers to Canada, with Internet.  We welcome you all, as long as you are willing to open up good restaurants in our horrible monoglot places such as Flin Flon or Montreal.  :)  I used to do illustrations, but it was too much work.  You can see many of my illustrations on Wikipedia, from when I was in a more productive phase.

The Big Bad Earth has a problem.  In the deep infra-red it glows like a lightbulb in space, even if we turned off the Sun for a while.  That's the heat from our natural radioactivity, and it's a huge amount of energy.  If the Earth were an onion, our atmosphere and oceans would be a molecular layer of graphene on top.  Forget global warming, this is the big stuff!

So every day the Earth must radiate this heat out into space.  On the surface, standing in your garden, it doesn't seem like much, but once you dig more than a metre or two you start getting into the Earth's heat flow.  If you covered the planet with very thick insulating foam, the whole thing would probably melt in a few hundred million years.  :)

But the Earth's been happy for a few billion years, and in all that time we've had oceans and continents, in the exact same ratio as we see today.  Obviously things have settled down to equilibrium where enough heat has been shed to keep us in a shirtsleeve environment (suitable for life).

The continents are always growing, and you can see that in huge accretion bands, such as the megathrusts that enrich the earthquakes of Oklahoma.  That's because the deep hot rock is always producing silicate 'scum' that floats to the surface.  By now we should be just one big continent, but it hasn't happened.  That's because our continents are the big foam insulator, and the Earth just can't have that.

So even if the Gorists are right and we become a big waterless desert, we will still have ocean basins.  That's where we have the main heat flow radiating out into space.  Now I hope that all of you in the far reaches have had your Grade 8 physics, which our climate-teests somehow slept through.  There are three types of heat transfer:  conduction, convection, and radiation.  Convection is orders of magnitude above the others, so we make insulating foam with super-tiny air cells that eliminate convection.  The foam is also non-transparent to stop radiation.  The Gorries envision CO2 as acting like a big foam blanket on the Earth, they have forgotten about convection.

In our thought experiment, we turn off the Sun again, and mop up all  the water.  Then we look at the Earth with a thermal camera.  All the heat is radiating out ocean basins, with some tiny bright spots for volcanoes.  The ocean basins are our radiator fins!

-to be continued

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.