Earthquake Ground-Motion Amplification in Southern California

The two most important factors influencing the level of earthquake ground
motion at a site are the magnitude and distance of the earthquake. The map
available here shows the influence of a third important factor, the site
effect: conditions at a particular location can increase (amplify) or decrease
the level of shaking that is otherwise expected for a given magnitude and
distance. Combining information about site effects with where and how often
earthquakes of various magnitudes are likely to occur should provide improved
assessments of seismic hazard.

[Summary provided by the USGS.]