Wednesday, February 16, 2011

Earthquake Twins - Vancouver and Santiago

One thing that sucks in the minds of old scientists is to look for the mysticism of universality in physical laws.  I'm not Newton or Einstein, but I can look at rock, and I like the universality of wet, crushed rock.  It's magnificent in that it is absolutely the same everywhere!  You can take a sample, and apply all sorts of mechanical tests in a testing machine, and can't tell where it's from (sort of like wine, for me).  You jack up the triaxial stresses, and it jumps from static to dynamic friction when it fails.  That 'stress drop' is the juice of earthquakes, and it is universal.  Temperature, pressure, and a bit of goop might change it.

The regional driving mechanisms may change, and there are only two distinct differences.  On plate margins we ratchet up the regional strain, until the static friction is exceeded and we have an earthquake.  Then the crust goes up or down a few metres, or slides along a transform fault.  We either build up mountain ranges, or have large displacements.

On the interior of plates, we already have high stresses caused by the crust sinking into a cool spot (think of the Niagara Tunnel rock squeeze).  There, the static friction is attacked by water-based stress corrosion until it fails, and we once again have an earthquake.  We don't get mountain ranges, but rather we grow giant fracture zones, and deep water sinks.

With each type of mechanism, the geology is exactly the same, and results in the same landforms.

Here is Vancouver.

and here is Santiago, Chile.

Can you tell the difference?  No you can't.

As a blessing to humanity, subduction zones have these beautiful strips of fine land between the mountains and the sea.  Much like the fertile slopes of episodic volcanoes!

Vancouver has this, and Chile has another.  Blessings to the god of wet, crushed rock universality!

Above is Chile.  If the Laws of Rock were slightly different, we'd just have steep mountains rising out of the sea, and we wouldn't even have these two beautiful cities.

So, our settled zones rely on the fact that the rock isn't doing anything weird until it gets deep enough.  And the price for coastal civilization, is earthquakes!

Here is the regional picture for Vancouver, followed by Santiago.

The only difference might be the speed of the plates zooming in, segmentation, and even the age of the oceanic crust.  This would result in a slight difference to the maximum magnitude of the earthquakes, and the rate at which they hit you.

Right now, Chile seems to getting hit harder than Vancouver has been.  I think its mainly the length of the subduction zone, and the b-curve, which demonstrates the relationship between small and large earthquakes.  The crust only really rips when there is an M9+ earthquake.  Then we can expect a 'stress shadow' (not many earthquakes) for a few hundred years.  These M8's and such don't mean a thing!

The Chile zone might be 'rougher' in that it has a steeper b-curve, which means a lot of M8's are needed to set up an M9.  I think Vancouver is 'smooth' in that the b-curve is relatively flat, and we just have one big M9 to form a stress shadow.

Vancouver thus has a double-edged blessing.  It has fewer M7's and M8's, but thus less 'living history' warning.  They are free to do any silly thing they want to!

In the end, Santiago may understand they have 'Glass Condos', but Vancouver will go on 'la-la' until you know what.


Civdis said...

Would you live in a highrise condo in Vancouver or Toronto?

Harold Asmis said...

Ha, ha. I would go for northern Toronto. No matter how shabby these things are built, I can't see them damaged at 5 cm/s. Near the top, though, I see people losing their lunch! Vancouver is an absolute certainty to see 50 cm/s in 30 years, maybe more.