... wondering why the floors were shaking.

First thought: "Wow, I must be really dizzy because it feels like the floors are moving."

Second thought: "That must have been a really huge truck passing by.  Do they make 24-wheel trucks now?  Good grief." 

Third thought: "That is some serious construction going on next door.  Are they building a new sears tower?"  

Fourth thought (and the only reasonable thought of the bunch): "Hey stupid (speaking to myself), find the nearest doorframe to stand under.  Excuse me, I'm looking for a doorframe, preferably one that won't crumble on top of me."

Yep, DC experienced an earthquake.  A pretty big one, at least for DC.  5.9 magnitude.  

http://news.blogs.cnn.com/2011/08/23/quake-hits-near-washington-d-c/