I'm in Manhattan (UWS) and live on the fourth floor of a five story walk-up. I felt the quake, but didn't realize that's what it was (I was a little confused a

I'm in Manhattan (UWS) and live on the fourth floor of a five story walk-up. I felt the quake, but didn't realize that's what it was (I was a little confused as to why my building was vibrating -- for 40 seconds or so, as big trucks can do that but not for that long) until I received an emergency alert on my phone ~45 minutes later. Nothing broken or dislodged, just wondering why my building was vibrating fairly gently.