Independant of the type of "shaking", if the equipment was not in place to detect a 2.7, then can it be discounted based on seismic evidence?
The signature of an underground test, or even an above ground that couples into the ground would be picked up by more sensitive equipment elsewhere. Some of the tests in the pacific in the 50's that were fired on barges in shallow water were recorded thousands of miles away on the equipment available at that time. The earth is a marvelous trasnmitter of this type of "sound." Part of the technique of seismic detection is multiple readings that allow triangulation to determine location and magnitude.