A while back there was another article here on HN that discussed the San Andreas Fault and one of the pictures showed how far a building can shift when the earth moves. I think it said the building moved something like 30 feet laterally from where it originally was. I thought that was crazy.
I didn't even think about the possibility of the earth suddenly sinking several feet below the ocean in a moments notice. That would be catastrophic!
My impression was that any such quake would also cause a massive tsunami. Which would (temporarily) immerse far more land than a mere 6' drop in elevation could. While converting any human-made stuff on that low-elevation land into widely-scattered rubble.