Geotechnical investigations at strong-motion stations in the Imperial Valley, California

Abstract

In the early 1970's the U.S. Geological Survey (USGS) began a program accumulating geotechnical and seismic data to be used in developing methods for estimating specific ground-response characteristics during strong local earthquakes.Recent site studies in the Los Angeles and San Francisco Bay regions indicate significant correlations exist between shear-wave velocity and various physical properties of the near-surface materials; these studies further suggest that these correlations can be used to define seismicallydistinct map units.The selection of specific locations for investigation has been based on sites where ground-motion data were recorded during earthquakes (and nuclear explosions), the availability of detailed geologic maps, and the distribution of intensity data from selected earthquakes.This study involved investigations at most of the Imperial Valley, California accelerograph stations, and was done because of the unique strongmotion data set recorded during the magnitude 6.5 earthquake of October 15, 1979.The project included the following investigations: (1) electronic conepenetrometer soundings at nine stations; (2) drilling, sampling, and logging of 22 borings to depths of from about 30 to 244 m; (3) downhole P-and S-wave velocity surveys at 22 stations; (4) high-amplitude resonant column tests of undisturbed samples from several stations; and (5) numerous gamma, S-P, and resistivity logs and call per and temperature measurements at selected stations.This study is just one part of an ongoing USGS program to compile geotechnical data at selected locations in various regions and to use these data to make detailed comparisons of the geologic and seismic characteristics that will provide a means for quantitatively estimating strong ground motion at a given site and facilitate the development of seismic zonation techniques applicable to other regions.i vThis report will focus on results of the electronic cone-penetration tests and, in particular, the downhole velocity surveys.Results of other investigations undertaken as part of this project will be discussed briefly and a reference regarding the availability of additional information or a report wil 1 be given.