Two examples of seismic zonation in the San Francisco Bay region

The science of earthquakes in complex, requiring data and research in seismology, geology, soil mechanics, geophysics, hydrology, and engineering. Nevertheless, if earthquake hazards are to be reduced, earth science information must be translated from scientific and technical language into a form that can be effectively used by planners and decisionmakers.

Out of the need to use earth science information in local and regional planning and decisionmaking came an experimental program-the San Francisco Bay Region Environment and Resources Planning Study. the study, begun in 1970, was jointly supported by the U.S Geological Survey and the U.S Department of Housing and Urban Development. The Association of Bay Area Governments and the planning firm of William Spangle and Associates, Inc., participated in the study and provided a liaison and communication link with other regional agencies and with local governments.

Since 1975, the composition of the core group within the U.S Geological Survey concerned with seismic zonation has evolved to include seismoloists and engineers concerned with probabilistic approaches to earthquake damage and a planner to assist in the information transfer from scientists and engineers to city, county, and regional planning staffs.

Three events have encouraged communities to attempt seismic zonation: passage of the Alquist Priolo Act by the California Legislature requiring special studies in zones encompassing potentially and recently active faults, preparation and adoption of seismic safety plan elements by cities and counties as required by California statute, and further development of a seismic zonation method. The method has four steps: postulating an earthquake of a given size and location; grouping geological materials with similar physical properties; predicting effects of the postulated earthquakes for each geologic unit by type of hazard of failure, namely surface rupture, ground shaking, flooding, liquefaction potential, and landsliding; and combining geologic effects by zones on the map (fig. 1).

Examples of seismic zonation at various scales by cities and counties in the San Francisco Bay region show that scientific information can be used effectively in avoiding earthquake hazards and mitigating damage. Six examples were presented at the Second International Conference on Microzonation in 1978 and reprinted in a Survey Circular on the progress on seismic zonation.

The method has been used in California by the cities of Mountain View, Novator, and San Francisco and the counties of Marin, Santa Clara, and San Mateo to develop zones which were used as a basis for their general plans, seismic safety plans, development policies or odrinances. Two of these examples, a city and a county, are described here.