Thursday, January 19, 2012
Arkansas seismic zone grows in mysterious ways
The fun thing about this seismic mechanism was that there wasn't a speck of activity before they started injecting. They injected too long and created a self-sustaining earthquake mechanism.
To review, we recall that there are only two types of earthquakes, from a rock mechanics point of view. The standard one is that of a weak zone near tectonic plates, a la San Andreas. The fault stands at constant strength, and stresses build up as the regional plates move. The fault breaks, and new build-up starts.
The second mechanism (denied by all seismologists and geologists) is that of a highly stressed crust with faults. Something happens which weakens the fault, and there is an earthquake. Normally, that would end things as the stress is now gone, but this type of mechanism can grow and release more energy.
Now, the Precambrian crust under our feet is highly stressed, up to the limit of its general strength (when you include all the fractures). Normally, we consider this to be 'stable' since nothing is happening to this rock to tip the balance, so to speak. As well, this rock underwent huge stress changes during various glaciations, and had the stuffing knocked out of it.
But in a few spots, water is seeping in, and creating a huge drain. The water weakens the fault, big earthquake, and there is room for more water. The mechanism grows into something as big as New Madrid. We can start a new New Madrid by injecting water until the mechanism is big enough to tap other water sources, and then to fly out of the nest.
So here in Arkansas, the fracturing has gone up the NE line, reached the artificial lake and started on a NW line. As we know, NE is strike-slip, and NW is compression in this stress field. Arkansas started along the NW line, and then broke out the NE fracture, in a conjugate set. We shall now see whether this new NW line does the same.