Cathryn, the actual rupture of the quake, given the size, probably lasted 15 or 20 seconds. But, the waves would spread out travelling to your location, so would last a little longer. Then, they might echo around LA basin, and last longer...Is Westwood at the edge of the basin? They got amplification of Northridge motion in Santa Monica. Then, once your high building got swaying, it might take a little while to stop.
Did you actually look at a watch and time the motion, or read it later? At a Dam (that was damaged) they measured ground motion of, I think 2g...which means twice force of gravity, which means things can be thrown into the air. Scientists assumed there was something wrong with the site or the measurements...until Northridge, when they measured near 1 g over wide areas, and 2 g at certain sites.
Chris