Monday, August 24, 2015

Earthquake Primer - Part 2

I'm only giving a broad brush here, and you know I love to get into the details of stick-slip, but not today.

The second major mechanism but we have a relatively strong fault in even stronger rock under high stress.  That's the situation of our Eastern North America earthquakes and totally unknown to the Callies until they recently waded into the muck of Oklahoma.

This is my favourite type of earthquake, since it is so bizarre.  Basically, the rock is stressed to its limit by plate tectonics.  For our continent, the whole thing has settled into a cold zone, resulting in high horizontal stresses.  These stresses don't change on our pitiful time scale, so we get an earthquake when the fault weakens for some reason.

The only thing that weakens such a fault in strong granite is water.  But not any old water.  There are oceans of water in that ancient water, going down 30 km to the bottom of the crust.  This water could unclog your sink drain, and kill your dog, but it doesn't do anything to the rock, because it has been there for millions of years.

For reasons unknown to most Callies, there is fluid flow in this rock, and sometimes new water gets introduced to the deep granite.  It eats away at the quartz points and boom!  We have an earthquake.  This can happen naturally, or can be induced by man.

You can't see the strain accumulate with GPS because the regional strain rate is something like 10-6.  That has great confused most Callies.  Nevertheless, these earthquakes happen and they can be huge.  They also transmit much farther than Callie because it is real rock, not that chewed up crap.  Funny enough, this only matters to people on soil basins.  You can be on granite and never wake up for the largest Eastern earthquake.

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