A deep M3.6 earthquake indicates lower mantle motion. More than that - the past few weeks have shown very deep earthquakes, and unusual activity at New Madrid. Last December, a remarkable series of large earthquakes also rang New Madrid, which is the best strain gauge available in the area.
In active areas, these deep strain events happen all the time, and don't mean anything. But the physics makes it almost impossible to happen here, yet here it is. Had anybody bothered to image the crust they would have found a line, called the Moho, below which there is no structure, and no earthquakes. Yet, this stuff is like Silly Putty, and is semi-rigid.
I'm only posting this because it may prove to be significant in hind-sight. I would imagine that New Madrid would have undergone some of this before 1811. It is essential for a large earthquake, yet it doesn't presage one.
I suspect this is a response from the December event. Slowly, the earthquakes got deeper and applied the strain to the putty below the Moho. It has now responded, and the earthquakes should slowly rise again. As usual, we shall see.
Thanks to Stanford and such, OK is convinced that all this is due to volume. A close examination of the rock mechanics (physics) shows this to not be the case. They are continuing to inject more fresh water as the gas wells start up, and we expect a busy season.
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