Sunday, January 22, 2012
Earthquake in Western Quebec zone
This is a most wonderfully active seismic zone that has nothing to do with megathrusts or injection. It is funny that whenever anybody has come up with some idea on why these places have earthquakes, they are always shot down by somebody mentioning some other zone. Truth is that a lot of these zones have different mechanisms.
The wq zone is mostly driven by glacial damage to the rock. If you plot glacial uplift, you will find the greatest shear stress changes (depending on gradients) are right here, as the last little bit pops up. I've compared it to putting your thumb on a beach ball, and slowly releasing it. You can imagine that the little dimple undergoes great stress changes, as it finally pops up.
There is also the complication of a hotspot track that zooms down along the NW-SE line straight to Boston. This was responsible for all those rounded 'mounts', such as Mt. Royal. I am sure these things broke up the rock enough to make it quite permeable.
Most people think that a lot of little earthquakes 'relieve' the stress, so as to prevent larger quakes. Ah, would that be so! No, Hopeful People, it follows a fractal pattern, so that the little earthquakes just set up bigger ones, within the fractal boundary (range of smallest to largest). There have been large earthquakes in this zone.