when you pull a heavy carpet up by one end and immediately swing the end back down. There's a ripple that travels along certain carpets in a line kind of like a bubble. It is very brief, but can rattle things on top of it or displace them a bit. It feels like that, but it is under your floor. It rarely displaces much earth above ground, but you might see a few photo frames fall off your walls or shelves. I actually haven't had a noticeable one in a while.

I hear Disney rides in California get evacuated quite a bit.