I think the previous answers were accurate but I also wanted to touch on the fact that seismic zoning is typically related to soils and how they behave during seismic activity, or simply put, earthquakes. The softer the soil, the more susceptible the area would be to seismic shaking in an earthquake prone region. The potential for liquefaction would be greater in these areas. In general, you need sandy soil, near-surface groundwater, and an earthquake for liquefaction to occur.
In California, the state has released Earthquake Fault Zone maps (beginning in the early 1970's) which outline where active faults exist in the state. Over the past decade, the state has also released Seismic Hazard Maps which outline liquefaction and landslide zones. Only portions of the state (mainly Los Angeles, Orange, Ventura, San Francisco, Oakland and San Jose) have been mapped to date, but new maps are released each year.
The map Sylvia is referring to are Uniform Building Code map which give a number 1-4 to areas in the United States with 4 being the most prone and 1 being the least. All of California is in a 3 & 4 zone, for example.