I know some booms, far inland, can be the result of a storm on the coast. The waves hit the rocks and the vibration travels along a strata. It's rare, but documented.
Frostquakes are loud as hell. A little frightening too. Used to hear them all the time as a kid. Think of the sound of river ice breaking up.
It might also explain some of the horns. When thick ice cracks, it makes a zing noise. Pattern recognition applied to an unexpected sound could do the rest.
Or, the molemen may finally be tired of us intruding on their turf with drills.
I heard the sounds in the late Spring and Summer....so they couldn't have been due to cold temps.
Maybe fracking....but I think not.
Maybe it's a weapons system testing pattern.
Sort of like a laser beam, but able to compress/displace all oxygen? It would help explain these large die-offs, when thousands of birds suddenly fall out of the sky, or an entire pod of whales or sea mammals beach themselves?
Something's not right though......unless the internet has better informed us of the number of events taking place, etc.......and it's been going on since Adam and Eve.