In-Situ Measurements of Seismic Velocities in the San Francisco Bay Region—Part II

Introduction

Seismic wave velocities (compression and shear) are important parameters for determining the seismic response characteristics of various geologic units when subjected to strong earthquake ground shaking (Borcherdt, 1970; Joyner and Chen, 1975).

Seismic velocities of various units often show a strong correlation with the amounts of damage following large earthquakes and have been used as a basis for certain types of seismic zonation studies (Medvedev, 1965; Gibbs and others, 1975b).

Currently a program is in progress to measure seismic velocities in the San Francisco Bay region at an estimated 150 sites. At each site seismic travel times are measured in drill holes, normally at 2.5-m intervals to a depth of 30 m. Geologic logs are determined from drill hole cuttings, undisturbed samples, and penetometer samples. The data provided a detailed comparison of geologic and seismic characteristics and provided parameters for estimating strong earthquake ground motions quantitatively at each of the sites (Joyner and Chen, 1975). A major emphasis of this program is to obtain a detailed comparison of geologic and seismic data on a regional scale for use in seismic zonation. The broad data base available in the San Francisco Bay region suggests using the area as a pilot area for the development of general techniques applicable to other areas.

Shear wave velocities in near-surface geologic materials are of special interest for engineering seismology and seismic zonation studies, yet in general, they are difficult to measure because of contamination by compressional waves. A comparison of various In-Situ techniques by Warrick (1974) establishes the reliability of the method utilizing a "horizontal traction" source for sites underlain by bay mud and alluvium. Gibbs, et al. (1975a) present data from 12 holes and establishes the reliability of the method for sites underlain by a variety of different rock units and suggests the feasibility of making the measurements at a much larger number of sites. Data collected from the first 12 holes also provide an opportunity for developing a routine and efficient procedure for collection and reduction of the data.

Gibbs and others (1975b) report preliminary comparisons of the data with the amplification data recorded from nuclear explosions (Gibbs and Borcherdt, 1974), and the intensity data for the 1906 earthquake (Lawson, 1908). These comparisons show that correlations exist between the three data sets. Seismic data collected at additional sites should Strengthen these correlations and provide significant new data from which ground motion predictions for seismic zonation can be made. This report presents the interpretation of the data collected from an additional 23 sites.

First released in print 1976

Last Modified August 13, 2009