Booming over Oklahoma

At http://phys.org/print340055912.html … a number of mysterious booms have been heard in central Oklahoma – sounding much like sonic booms rather than anything under the ground. Because the booms were heard over such a wide area this has ruled out a connection with fracking, the first culprit as far as the media are concerned. No earthquakes have been noticed at seismic recording stations, and the process known as cryoseisms (frost quakes) have been ruled out. The booms appear to be at the surface or in the atmosphere.