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Analysis of seismograms from a downhole array in sediments near San Francisco Bay

Open-File Report 76-296

In cooperation with the Division of Reactor Research and Development, U.S. Atomic Energy Commission
By:
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Abstract

A four-level downhole array of three-component instruments was established on the southwest shore of San Francisco Bay to monitor the effect of the sediments on low-amplitude seismic ground motion. The deepest instrument is at a depth of 186 meters, two meters below the top of the Franciscan bedrock. Earthquake data from regional distances (29 km ≤ Δ ≤ 485 km) over a wide range of azimuths are compared with the predictions of a simple plane-layered model with material properties independently determined. Spectral ratios between the surface and bedrock computed for the one horizontal component of motion that was analyzed agree rather well with the model predictions; the model predicts the frequencies of the first three peaks within 10 percent in most cases and the height of the peaks within 50 percent in most cases. Surface time histories computed from the theoretical model predict the time variations of amplitude and frequency content reasonably well, but correlations of individual cycles cannot be made between observed and predicted traces.

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Analysis of seismograms from a downhole array in sediments near San Francisco Bay
Series title:
Open-File Report
Series number:
76-296
Year Published:
1976
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Earthquake Science Center, Menlo Park Science Center
Description:
36 p.; Appendix; Tables; Figures
Country:
United States
State:
California
City:
San Francisco
Online Only (Y/N):
Y