I talked to CERI (Center for Earthquake Research Institute) again today and the consensus is that the shaking was caused by a Sonic Boom from a military aircraft. While there was some quarry blasting occurring in Pocahontas, it would not explain the shaking occurring as far north as Cape Girardeau and as far south as Forrest City.
In case you are interested, here are some of the helicorder displays. The location is labeled in the upper-left corner of each graph. The green line represents the mysterious shaking. More helicorder displays can be found by clicking here. Click images to enlarge.
Interesting... Any other theories out there? And yes, we still believe that military chaff was dropped from the jets. The radar images support this theory. Chuck George at our sister-station, KOLD, says that this happens quite often in Arizona.
Have a great day,