Other people had similarly terrifying recollections rush back to them. A few women on my floor (according to a fellow intern sitting near them) started freaking out given their experiences being in New York for 9/11.
The lack of mention of an earthquake or terrorist attack on various news sites calmed us, but then we were just confused. The shaking was minor, but it was noticeable. It wasn't as if we had all imagined it.
Building engineers were called, who in turn called the city to confirm that there was no construction going on near the building. And the subway certainly didn't up and move closer to our building that morning. After a few hours of repeated trembles and still no answer, we got an email from the floor manager: the mystery had been solved.
Let me start by saying that my office is in Rockefeller Center, in a building that is connected to Radio City Music Hall. Normally, the only hassle associated with the location is the herds of tourists you have to navigate to get anywhere near the building. But yesterday, we discovered one more: it turns out that rehearsals for the Rockettes have started. And when 75 Rockettes dance all together on a platform suspended over the stage, the whole building shakes. They've promised to not do it again.