What the Map Shows:
So what's on the map? Seismic hazard is basically the degree of earthquake shaking that you can expect in a given place during a given time span. To simplify things, the mappers assume that your building is on solid rock and that you're interested in relatively fast shaking (0.2 second period), a frequency that strongly affects ordinary houses. Different ground and different periods are important in determining the exact seismic risk for a particular structure.
The map shows the peak ground acceleration (PGA) that a site can expect during the next 50 years with 10 percent probability. The color scale goes from very low (white) to about half the acceleration of gravity (red-brown)---remember, this is acceleration sideways. A structure of modern design and sound construction can survive that degree of shaking without killing the people in it, although it may well need to be torn down afterward.