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.