All ground in the Bay Area was NOT created equal. A critical factor affecting intensity at a site is the geologic material underneath that site. Deep, loose soils tend to amplify and prolong the shaking. The type of rock that least amplifies the shaking is granite. The remaining materials fall between these two extremes, with the deeper soils in the valleys shaking more than the rocks in the hills.
Most development is in the valleys.
The map opposite groups the geologic materials in the region into eight categories, each with similar amplification in earthquakes. The role of geologic materials in affecting the intensity of shaking has been known for at least twenty years. Several researchers at the U.S. Geological Survey clearly demonstrated this relationship when they examined data from the 1906 San Francisco earthquake in 1975. Other researchers have expanded this effort by examining the relationship between intensity and geologic materials. These susceptibility categories are quite similar, but not identical, to the categories recently developed for use in site-dependent building code provisions.