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Published December 1985 | Published
Journal Article Open

Spatial variation of ground motion determined from accelerograms recorded on a highway bridge


A set of time-synchronized strong-motion accelerograms, obtained on the San Juan Bautista 156/101 Separation Bridge in California during the 6 August 1979 Coyote Lake earthquake (M_L = 5.9), are used to study the spatial variation of ground motion at the bridge site, including traveling wave effects and the influence of multiple-support excitation. Analysis of the ground motion recorded at the base of two of the bridge supports (32.6 m apart) revealed the presence of a differential support excitation having a period of ≈ 3 sec, much longer than any structural periods of the bridge. This signal also appeared as a noticeable long-period component in the superstructure displacements. Analysis of the vertical and radial components of the 3-sec ground motion indicated that ground displacements were retrograde for the duration of strong shaking, with several cycles exhibiting elliptical particle motions. These findings suggest that long-period differential support motions were induced by phase delays in a Rayleigh wave traveling across the bridge site. Further support to this premise is given by the location of the bridge site near a maxima of the Rayleigh wave radiation pattern for the Coyote Lake earthquake (based on published focal mechanism data). Traveling wave effects were also detected for compressional body waves by a correlation analysis which indicated a time delay of ≈ 7 msec between P-wave arrivals at two of the bridge supports.

Additional Information

© 1985, by the Seismological Society of America. Manuscript received 15 March 1985. The authors would like to thank John Ragsdale, formerly of the California Division of Mines and Geology, for his assistance in obtaining the accelerogram records used in this study. Also, our thanks to Jim Gates of the California Department of Transportation for his help throughout the project. This research was partially supported by Grant CEE81-19962 from the National Science Foundation. J. C. W. would also like to acknowledge the support of the Natural Sciences and Engineering Research Council of Canada while at the California Institute of Technology.

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