This site was recommended by a guy who does earthquake safety consulting and was offering free consults to Berkeley Parents Network members.
It provides a stunningly clear, visual explanation of how retrofitting works to keep a house from falling off the foundation when 10,000 pounds of shear force hits it during an earthquake. I learned a lot, including that the despite fact that our house is bolted down, it still would probably slide off the foundation in an earthquake. It also explains very clearly what we could do to minimize that possibility.
Spend five minutes looking at this site, then check out your foundation.