60,000,000 3,000,0001 4,500,0001

30,000,000

'Subsurface damage to od wells; no damage to surface structures reported. Earthquakes may have hown triggered by oilfield operations.

(widespread fires, flooding of populated areas caused by failure of large dams, or release of toxic or radioactive materials) that could be more catastrophic than the initial effects of the earthquake. A more complete, nontechnical explanation of the various hazards posed by earthquakes has been presented by Blair and Spangle (1979).

Because the hazards to life and property resulting from the primary and secondary effects of earthquakes may vary substantially throughout a region, measures to reduce earthquake hazards must necessarily begin by delineating geographic areas having different potentials for fault offset or distortion of the land surface, strong ground shaking, ground failure, or other geologically controlled effects. This volume presents methods for characterizing these types of earthquake hazards on a regional scale and provides earth-science information vital to actions that could be taken in the Los Angeles region to reduce future earthquake losses.

PURPOSE AND CONTENT OF THIS VOLUME

The Los Angeles region became a major focus for seismic hazard research following the 1971 San Fernando earthquake, which stimulated national interest in earthquakes and expanded the resources available for earthquake studies. Earthquake prediction-the specifi

cation of the time, place, and magnitude of an impending earthquake is one goal of this research. Thus, some efforts have centered on monitoring the Earth's crust with a variety of instruments to detect possible fluctuations in geophysical signals that might foreshadow a major imminent southern California earthquake and permit its

prediction within a time frame of a few days, weeks, or months. Progress toward short-term prediction has been slower than scientists had hoped, but there has been substantial progress in making long-term earthquake predictions (for example, determining the probability during the next several decades of a future large earthquake on the San Andreas fault) and in better understanding how earthquakes occur (Wesson and Wallace, 1985). Earthquake hazard assessment-the specification of the potential for destructive effects of future earthquakes-is another goal. Truly remarkable progress has been made on the earth-science aspects of this subject, as evidenced by the findings presented in this volume.

All strategies for reducing future losses from earthquakes rely on the ability to predict the geographic distribution and severity of the potentially damaging geologic and seismologic effects that commonly accompany major earthquakes-strong shaking, fault offset of the land surface, and disruption of the ground by liquefaction or landsliding. These hazards are the topics of the following chapters.

This professional paper provides an overview of the principal geologically controlled earthquake hazards in the Los Angeles region and summarizes the methods that earth scientists use to evaluate and characterize those hazards. Although the volume was initially planned to address the general public as well as earth

quake experts and researchers in related fields, this goal has been only partially achieved because of the complexity of the scientific problems associated with many of the hazards and the diversity of the potential audience. Most of the chapters emphasize the scientific bases for mapping and assessing geologically controlled earthquake hazards and thus will be of greatest interest to geologists and seismologists. Some chapters treating hazard-reduction issues have been written to be understood by planners, engineers, and decisionmakers who need to use earth-science information in specific situations to reduce those hazards. Our hope is that all readers, whatever their background or expertise, will gain a better understanding of the earthquake hazards facing the Los Angeles region and of the opportunities for reducing the risk to life and property.

A glossary (p. 15) containing the principal technical terms used in this volume will assist the nonspecialist. A table for converting from metric to English units of measure is on p. 23.

Most of the earth-science research summarized in this report has been conducted by scientists and engineers of the U.S. Geological Survey, the California Division of Mines and Geology, various universities, and private consulting firms, chiefly using funds provided by the Earthquake Hazards Reduction Program of the Geological Survey. Research activities have included de tailed geologic and seismologic studies of faults, investigations of the factors that influence seismic shaking, and analysis of the geotechnical controls on liquefaction and landsliding. Many of the methods described in the following chapters have evolved from hazard-evaluation techniques first developed in a multidisciplinary study of seismic zonation in the San Francisco Bay region (Borcherdt, 1975), where the resulting earth-science products have been successfully used to reduce hazards. The studies in both the San Francisco Bay and the Los Angeles regions have significantly improved our ability to predict and map the geologic effects of future earthquakes. Because knowledge of the behavior of faults during late Quaternary time and of the distribution and character of sediments deposited during that time span is crucial to evaluating earthquake hazards, these topics have received special attention. Subdivisions of Quaternary time as used in this volume are shown on the accompanying geologic time chart (inside front cover).

This volume examines in sequence the causes of earthquakes in the Los Angeles region, the major geologic hazards likely to be associated with future earthquakes and the methods for evaluating and mapping those hazards, and examples of the impact of earthscience information on the decisionmaking processes that result in hazard reduction. The geologic factors that control the location and size of earthquakes and

their rupture characteristics are evaluated in two chapters that describe:

• The plate-tectonic framework within which earthquakes occur in the Los Angeles region (Yerkes, p. 25).

• The identification of active faults, their potential for generating earthquakes and rupturing or distorting the land surface, and estimates of the likely size and frequency of major earthquakes along them (Ziony and Yerkes, p. 43)

Strong shaking, the chief threat posed by an earthquake because of the large area commonly affected, re ceives particular attention in seven chapters describing different aspects of that hazard:

• An introduction to available methods of predicting the extent and severity of strong shaking and the applicability of different predictive techniques to reducing earthquake hazards (Bor cherdt, p. 93).

• How the late Quaternary sedimentary deposits of alluvial basins are differentiated and mapped to help characterize variations in the severity of future shaking (Tinsley and Fumal, p. 101).

• A technique for regrouping geologic units on the basis of their geotechnical properties and shearwave velocities for prediction of future shaking levels (Fumal and Tinsley, p. 127).

• Why it is useful to predict seismic intensities, how maps of predicted intensities are prepared for postulated earthquakes, and how these techniques can be applied to estimating future losses from earthquakes (Evernden and Thomson. p. 151).

• The quantitative prediction of ground-motion values by applying newly developed equations that link peak acceleration, peak velocity, and response spectra to earthquake magnitude, geologically determined fault slip rates, and site geology (Joyner and Fumal, p. 203).

• The use of ground motions from nuclear explosions in Nevada, recorded in the Los Angeles region, combined with geologic information ob tained during the course of urban development, to map the relative shaking response expected for areas having different geologic characteristics (Rogers and others. p. 221).

• The methods for simulating numerically the timehistory of ground motion at a site from a postulated earthquake, information needed by structural engineers for the earthquake-resistant design of special facilities such as large dams and nuclear powerplants (Spudich and Hartzell, p. 249).