Tragic earthquakes of the last decade in Mexico, Armenia, and the United States have re-emphasised the importance of local geologic site conditions in determining amounts of damage and consequent loss of life. Extensive data sets in the San Francisco Bay region on strong earthquake ground motions, the damage distributions from past earthquakes, and geologic materials provide the basis to quantify site condition effects for purposes of earthquake hazard mitigation. These observational data are reviewed and analysed to provide methodologies for the characterisation and predictive mapping of potential variations in strong ground shaking for seismic zonation. The methodologies are based on existing geologic maps. They provide a method for seismic zonation applicable to many urbanised seismic regions of the world.