Background Information
	Based on ABAG "On Shaky Ground" Reports

During the last 25 years, ABAG, with funding from both the U.S. Geological Survey and the National Science Foundation, has developed a number of earthquake hazard maps for the nine-county San Francisco Bay Area. Those maps resulted in the publication in 1987 of the first On Shaky Ground report. The Loma Prieta earthquake in 1989 and the Northridge earthquake in 1994 were devastating in their effects on northern and southern California. However, they have also provided us with valuable information to test the hypotheses forming a basis for those earlier maps and to develop a better understanding of the physical processes that occur in earthquakes. The maps described and shown on this internet site are the result of this research and better understanding. They are an updated version of the maps documented in the 1987, 1995, 1998 On Shaky Ground report (as "hard" copy), as well as the more recent 1999 and 2001 revisions.

On Shaky Ground documents ABAG's shaking hazard maps to encourage appropriate planning for and mitigation of earthquake hazards.

Many people have used the maps of ground shaking. During these past years, however, questions have been raised about how ABAG produces these maps, as well as appropriate and inappropriate uses of the maps. This documentation is intended to encourage more use, and more appropriate use, of ABAG's ground shaking hazard information for the San Francisco Bay Area.

We're Talking About Ground Shaking

The discussion in this documentation focuses on the earthquake hazard of ground shaking, and its secondary impacts on buildings and ground failure. Ground shaking is the cause of the vast majority of earthquake-related damage, deaths and injuries.

Some Words of Caution

The following maps and tables attempt to show the answers to several common questions. However, as with any general assessment of what might happen in the future, the maps and tables are imperfect and incomplete. Because large earthquakes are not an everyday common occurrence, our understanding of their impacts is limited. We generally know what types of damage will occur and what types of ground will have problems, but we cannot predict the specific damage to specific buildings. This lack should not serve as an excuse to not act. There are many things that each of us can do as individuals, and working with our neighbors, offices and agencies, to reduce the risk of damage and other earthquake effects. Thus, it is very important that you read the materials explaining:

• What question each map and table is trying to answer;
• When you might want to use the map or table, and when you should not use it;
• How each type of map or chart was prepared; and
• What assumptions we needed to make to prepare the maps and charts, including what information is unknown.

jbp 10/15/03