Yikes!
There's a fair amount of apocalyptic literature on the web making connections between seismic activity and otherwise unexplained booms. Not real convincing, but intriguing.
On the other hand, I remember several years ago some very convincing (and well received professionally) research speculating that "ball lightening" was a real phenomenon that could be associated with a peizo-electric effect when large pressures build up before an earthquake. Could the boom be a similar effect, one that wouldn't show up on a seismograph but could produce the large sound? I can think of several mechanisms, but no way to test it.
Bottom line, however, stuff is impacting the rotary impeller device.
I have this theory about reality. There are times of more stuff and times of less stuff. In times of more stuff, not only do violent natural phenomenon occur more frequently and extremely, but human behavior also gets more intense. It's like there's a "volume control" on life and it just got turned up to eleven (it's one more, it's louder).