Deep Arrays for Earthquake Response, San Francisco Bay Area

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This action is to support the installation, in collaboration with the U.S. Geological Survey (USGS) - and possible collaboration with the California Division of Mines and Geology (CDMG) - of two deep arrays in the San Francisco Bay area, with the objective of measuring ground motions and the elevation of ground water pressure during the large earthquake that is expected to occur in the San Francisco Bay area. These recordings will provide valuable data for seismological, earthquake engineering and earthquake hazard mitigation studies: these data do not exist at present. Surface measurements from the October 17, 1989 Loma Prieta, California earthquake showed that the apparent amplification of the peak horizontal ground acceleration at deep clay sites was higher by 100% or more than would have been anticipated; yet no measurements in the soil profile showing how amplification progressed between bedrock and the ground surface are available. Nor are data of this type available at other locations for strong-motion earthquakes. There is a significant probability (67% in 30 years) that an earthquake producing greater intensity of motion than the Loma Prieta event will occur in the San Francisco Bay area; it is thus an excellent location to establish a deep accelerometer array to improve prediction capabilities. Sites are being surveyed in the area, and several promising locations have been identified. An advisory panel consisting of eminent researchers and administrators in earthquake hazard mitigation, from major universities in the United States and Canada, and from the USGS and CDMG, will provide advice on the general requirements for the sites and their instrumentation, as well as on the location of instruments to maximize the usefulness of the data recovered. The panel members have expertise in earth science, geotechnical engineering, and field data measurements. The damage caused by the Loma Prieta earthquake, as well as previous major earthquakes worldwide, point to the need to measure two different types of data so as to improve earthquake hazard mitigation methods: The measurement during strong earthquake-induced ground shaking of the amplification of ground motions in deep clay deposits; and the ground motions and the increase in ground water pressure in saturated sands during strong earthquakes. These measurements could be obtained through two separate field arrays, or a combined array at a site which has the appropriate combination of saturated sands and deep clay deposits. The establishment of these arrays follows the recommendations of the September 1988 workshop on "Designated Sites for Geotechnical Experimentation in the United States", and the November 1990 workshop on "Dynamic Soil Properties and Site Characterization". These sites will become designated geotechnical test sites, open to all interested researchers. The sites will be available for research into geotechnical phenomena not associated with earthquakes, but it is expected that the sites chosen will, in general, be more suitable for investigation of the earthquake-induced phenomena discussed.