Western Earthquake Hazards Program

U.S. Geological Survey
Circular 1130

Look Before You Build—Geologic Studies for Safer Land Development in the San Francisco Bay Area

By Martha Blair Tyler


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This Circular provides a general description of the types of geologic hazards that exist throughout the United States. In nontechnical language this book describes how geologic information can be incorporated in the land-use development process and contains useful discussion of several examples from the San Francisco Bay area and elsewhere in the United States of how geologic information is already being used in the development process by some cities and counties.

Forward by Gordon P. Eaton, Director, U.S. Geological Survey

When natural disasters occur, the resulting devastation is not distributed uniformly across the landscape. Certain places repeatedly experience more severe losses than others because of their susceptibility to flooding, earthquakes, and landslides. These places are generally identifiable in advance of such natural disasters through geologic studies. Flood damage is generally confined to mapped flood plains, and earthquake losses recur at those locations where geologic conditions promote strong shaking and ground failures. Landslides tend to occur in loosely consolidated, saturated soils on sloped terrain, often in conjunction with strong shaking during earthquakes.

The historical record documents countless examples of substantial losses sustained because geology was not adequately considered. Often, such historical accounts provide clear warning of future loss patterns. The effects of the October 1989 Loma Prieta earthquake in some parts of San Francisco mirror those reported in San Francisco after earthquakes in 1865, 1868, and 1906. The success stories, losses that were prevented because geology was heeded before development, are more difficult to document. However, we know that integrated earth science-economic models of earthquake-triggered landslides during the Lorna Prieta earthquake, for example, can be used to outline cost-effective land-use strategies.

The San Francisco Bay Area is a dynamic landscape, at least on geologic time scales. Coastal California, on the Pacific plate, is moving about 2 inches to the northwest each year past the rest of the State, which is resting on the North American plate. These movements, which occur during earthquakes, have created a varied topography that both nature and man have altered to produce conditions particularly susceptible to the destructive forces of nature. Communities in the San Francisco Bay Area have had to address issues of suitable land use and structure design because of the ever-present threat of devastating earthquakes and landslides. Communities throughout northern California, indeed throughout the Nation, face similar threats of natural disaster that can be blunted through the considered use of geologic information in development planning. Geologic information can be used for safer land development in all parts of the Nation. Such information is available from the U.S. Geological Survey as well as from many state and local agencies. The U.S. Geological Survey is committed to assisting local agencies in the wise use of geologic information in the development of safer communities.

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