Thursday, February 14, 2008
More SRL on cratons
You gotta love cratons! They are the oldest crust on earth, and a source of all the diamonds, and other deep minerals.
This paper makes the proposition that there is something special about the outer edges of cratons, that makes them prime areas for earthquakes. This makes sense, since if they are general stronger than the surrounding rock, then things crush up against them, like an icebreaker in pack ice.
It starts with my most favourite picture, that of the New Madrid seismicity.
Note the big thrust zone in the centre, and the two shear wings. This is the Mama of all such zones, which would be proven if only they had decent seismic monitoring (1 km accuracy) in our ENA zones. In the 1800's all three 'wings' let go in a short time!
Now, they plot it in relation to the world, and also zooming in on North America.
You'll have to click on it to get more detail. Our favourite seismicity areas (Ottawa and Hamilton) are more in the interior, but still look related. I just tend to think they cluster on the edge because that's where the craton got the most beat-up during the Wilson Cycles.