I currently work in downtown Los Angeles. I sit at a desk on the 43rd floor of one of the taller buildings along Flower Street. I occasionally wonder just how resistant to collapse these buildings are. How big would an earthquake have to be to bring down the building I am in? It's not an entirely idle thought. I was at work when a smallish (4.5) earthquake struck. I could look out the window and see the sway of the building. It was not a pleasant experience.