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U.S. Geological Survey Open-File Report 74–222

Effects of Local Geology on Ground Motion in the San Francisco Bay Region, California—A Continued Study

By James F. Gibbs and Roger D. Borcherdt

1974

Thumbnail of and link to plate-1 PDF (5.3 MB)Abstract

Measurements of ground motion generated by nuclear explosions in Nevada have been completed for 99 locations in the San Francisco Bay region, California. The seismograms, Fourier amplitude spectra, spectral amplification curves for the signal, and the Fourier amplitude spectra of the seismic noise are presented for 60 locations. Analog amplifications, based on the maximum signal amplitude, are computed for an additional 39 locations. The recordings of the nuclear explosions show marked amplitude variations which are consistently related to the local geologic conditions of the recording site. The average spectral amplifications observed for vertical and horizontal ground motions are, respectively: (1,1) for granite, (1.5, 1.6) for the Franciscan Formation, (2.3, 2.3), for other pre-Tertiary and Tertiary rocks, (3.0, 2.7) for the Santa Clara Formation, (3.3, 4.4) for older bay sediments, and (3.7, 11.3) for younger bay mud. Spectral amplification curves define predominant ground frequencies for younger bay mud sites and for some older bay sediment sites. The predominant frequencies for most sites were not clearly defined by the amplitude spectra computed from the seismic background noise.

The intensities ascribed to various sites in the San Francisco Bay region for the California earthquake of April 18, 1906, are strongly dependent on distance from the zone of surface faulting and the geological character of the ground. Considering only those sites (approximately one square city block in size) for which there is good evidence for the degree of ascribed intensity, the intensities for 917 sites on Franciscan rocks generally decrease with the logarithm of distance as

     Intensity = 2.69 − 1.90 log (Distance Km).

For sites on other geologic units, intensity increments, derived from this empirical rela.tion, correlate strongly with the Average Horizontal Spectral Amplifications (MISA) according to the empirical relation

     Intensity Increment= 0.27 + 2.70 log(AHSA).

Average intensity increments predicted for various geologic units are −0.3 for granite, 0.2 for Franciscan Formation, 0.6 for other pre-Tertiary, Tertiary bedrock, 0.8 for Santa Clara Formation, 1 .3 for older bay sediments, 2.4 for younger bay mud. These empirical relations, together with detailed geologic maps, delineate areas in the San Francisco Bay region of potentially high intensity from future earthquakes on either the San Andreas fault or the Hayward fault.

  • This report is available only on the Web.

For additional information:
Contact Information, Earthquake Science Center, Menlo Park Science Center
U.S. Geological Survey
345 Middlefield Road, MS 977
Menlo Park, California 94025
http://earthquake.usgs.gov/

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Suggested citation:

Gibbs, J.F. and Borcherdt, R.D. 1974, Effects of local geology on ground motion in the San Francisco Bay region, California—A continued study: U.S. Geological Survey Open-File Report 74–222, 146 p., 3 oversized figures, available at http://pubs.usgs.gov/of/1974/0222/.



Contents

1. Introduction

2. Geology of the recording area

3. Seismic data

4. Amplification of ground motion by local geologic units

5. Seismic noise analysis

6. Comparison of the nuclear data with the intensity data for the 1906 earthquake

Conclusions

Acknowledgments

References

two appendixes

three oversized figures


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