Earthquake
A shallow local quake can produce an audible boom along with the shake.
How to tell
- Shaking lasts longer than the sound
- Hanging objects swing afterwards
- Confirmed by a USGS record at the same minute
- Felt indoors more strongly than outdoors
When it usually happens
No pattern.
Heard one just now?
Report it and we will tell you what it most likely was. More reports from different directions make it far easier to pin down.
Report what you heard