Friday, April 13, 2012
New line for Arkansas earthquakes
So, I really thought that Arkansas was done when they stopped the original injection. Problem is that they narrowly defined the 'prohibited' area, and I don't know how that maps out. The original zone is the cluster to the SW corner. A new cluster near the lake then appeared, and I thought we were looking forward to some typical dam induced earthquakes. Now, we have this long NW line forming.
As we know, the NW line is the main driver behind the New Madrid zone. The big megathrusts are all NE, but the main horizontal stress is also along the ridges of these things, since there is stress relief when the faults shift. A NW trending thrust fault extracts the strain energy in this direction, but I am unsure how these things form. I suspect that there were 'adjustment' faults in a conjugate set to the megathrusts, just like there is adjustment at the kinks in the subduction zones (site of one of the recent Mexico earthquakes).
The Arkansas sequence started in a NW line and then developed shear wings just like New Madrid. This gives it infinitely more potential to grow than an Oklahoma or Virginia mechanism. So the latest quakes are either on their own, or they have broken out of the 'containment' zone. Either way, we are seeing the birth of a New-New Madrid!