Walls and Ceilings - cracked plaster ceiling

We have a plaster ceiling in what is going to be the babies room. Standing on a ladder and pushing up on the ceiling -- I can get about 1/2" deflection (movement up down). I don't know if this is due to the lath coming away from the framing or if it's the framing itself (2x4 up in the attic). I'm worried that the ceiling could give way in an earthquake (live in SF Bay Area). Is this an unrealistic worry? If not, can I break the plaster away from the lath (leaving the lath in place) and sheetrock over the lath? I've seen 1 1/4" washers that they make for plaster ceilings -- you run a #8 screw through them and it snugs the plaster/lath back to the framing. Is this an alternative to replacing the plaster ceiling w/sheetrock?

Most likely the plaster has separated from the lath. Depending on your historic tendency, you can pull the plaster tight to the lath with screws and plaster washers, which will require a skim coat to cover the washers, or remove the plaster and hang drywall. You also have to ask why the plaster came loose. Earthquake? Stomping upstairs? LEAK? Removing the plaster can answer the latter, although water stains will help you there.

There is also the oppurtunity issue. With the plaster gone, rewiring is much easier. Obviously you still have to make the decision, hopefully I have given you something to help in your selection (or maybe I added new issues).

Good luck!