I've been through a total of 5 or so in the 7 years i've been in SF. 3 of which were severe enough to get my fight or flight response going to the point that i could feel the adrenaline rush more than 15 minutes afterward. I said the same thing the first time it happened... where i'm from the farking ground isn't supposed to MOVE. I'll never forget that first one... i was sitting at my computer and my monitor slowly started vibrating... I nearly shat myself.