OK, that's one more new experience, and once will be enough, thanks.
I was sitting at home, about 50 feet up in this 100-yr-old brick and wood former factory building... right next to a highway and elevated RR tracks, so a bit of shaking is very normal. Thought nothing of it until I realized I hadn't heard the sounds usually associated with such an event... and then the shaking got worse.
Rut-roh...
I ran out to the kitchen, saw a couple of the housemates. "I don't know about you, but I'm out of here", I told them.
The building was still rocking as I bolted downstairs, and when I returned a few minutes later, after talking with some neighbors who'd also evacuated, a few things in my room were still settling down.
No visible damage anywhere; I routinely check out all the various cracks I know of in the building's brickwork and the 70-foot brick smokestack out back, and they seem the same as last time I looked.
But just in case we get another ripple, I moved my car away from the building and have my flight bag and and a dry bag with important documents and other stuff ready to go by the door. I hope that's all for this one... that was not fun, not at all.