Seen this on another site ...

Geo-Geek:

Throughout much of the central United States, younger sedimentary rocks conceal the craton. However in Michigan, Wisconsin, Minnesota and several other mid-west states it is well exposed. When the WI Geologist states he has run out of ways to say "I Don't Know." That is because this part of WI has a lessor amount of sedimentary materials distributed over the top of the craton that would normally muffle/silence/absorb the pops and/or booms of the local craton granite fracturing for thousands of natural geologic and geophysical reasons.

This sort of thing happens all the time, and whenever there is a shift in sedimentary distribution, underground water table, or the thousands of other localized natural geological phenomena causes craton granite to fracture. The pops/booms/noise/whatever are not heard or felt as frequently in other areas of the carton due to far larger layers of sedimentary materials covering it. These same sedimentary materials act as a cushion which absorbs/silences the sounds much like a typical household pillow does when you scream into it.