Monday, November 7, 2011

Earthquakes: OK and Virginia identical twins

Using my poor eyes in the morning, I have tried to plot the aftershocks to the same scale.  If they actually had accurate depths, I would expect the aftershock sequence to deepen to the SE.  I consider them both to be identical mechanisms, caused by fluid injection, by a dam, and one by borehole.

I am surprised that the OK earthquake has a much bigger aftershock zone, and yet it was a smaller earthquake in Magnitude.  This probably reflects the state of stress in the deep rocks.  By definition, it will turn out the the VA earthquake has a much higher 'stress drop'.

Anyway, with all the dams, and all the new injection, we can expect another earthquake soon, for comparison purposes -- Ohio.

PS - The OK earthquake is listed as pure strike-slip.  Either there is a wide uncertainty band, or we can expect a thrust earthquake (or one has already happened in the distant past).  An isolated pure S-S earthquake cannot exist mechanically in this area.

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