Regarding River's advice about strapping a water heater: He is correct about the value of doing this, but I would advise anyone considering this task to do more than just nail a strip of plumbers tape around the water heater.
As a 40 year native of Southern California (yes, I am looking to escape) I am better acquainted than I would care to be with earthquakes and what they do. I am also a general contractor. California's requirements for bracing water heaters can be annoying, but they do exist for good reason. Anyone looking at bracing their water heater should at least take a look at what California requires. A web search will turn up a number of methods. A good page is provided by the State of California.
Note that a water heater needs to be held from moving both forward and backward. Water heater manufacturers generally specify a minimum distance to the wall. In my experience water heaters tend to be anywhere from 3" to 6" away from the wall. That's a lot of room to rock. A nailed-in strap will be pulled out of that wall within three or four rocks of the hot water heater tank.
At the very least, brace the back and use lag bolts for that strap.
Consider purchasing a California earthquake strap kit. They are produced in large quantities and generally cost less than $20. They come with directions and lag bolts. They come with enough strapping to run full loops around the water heater and instructions for adding bracing in the back.
Just my experienced two cents. - Tom in Southern California