A lot higher than the river...there are two major hills in St. Louis City. One is called the Hill where a lot of Italian-Americans live the other is a mile away called Dogtown where the Irish-Americans live. I live in Dogtown, so I'm lucky to be away from the Mississippi and higher. My house is a brick home built in the early 1900's, so it's probably not built for earthquakes.
Some common retrofits people do around here are house bolting, reinforcing the chimney (especially if its brick), securing the water heater, asking the gas co. to intall an automatic gas cutoff valve, tethering large bookcases or other furniture so they don't fall over easily, and installing sturdy latches on cabinetry. The first two will cost you, the other four can be done for almost nothing. If nothing else, pick a sturdy table and jump under that sucker. I stood in doorways during quakes for the longest time until I found out sturdy tables were the way to go. At least thats what everyone says.