And by ominous
I mean there was a feeling of dread...subsonic sound can induce that feeling... though maybe it was just knowing on some level that it was an earthquake also created that dread. The rumble I felt seemed to be in the air, not the ground.
I have been in a couple of earthquakes that could be felt... none seemed as LOUD as this minor one...
I suspect that a cave in would be a very different experience...