Amplification Behavior of Vertical Motion Observed From Downhole Arrays

Abstract eng:
Recently, very strong vertical ground motions have been recorded during several earthquakes and have been considered to be critical to structure designs. Similar to horizontal motions, vertical motions can be amplified with the local site condition. However, the amplification behavior of vertical motion is different from that of horizontal motion because of different propagation mechanisms. In this study, three component records of five downhole arrays, including different geological conditions, ground water tables, and intensity of motions, were analyzed to evaluate the differences of wave propagation in the vertical and horizontal directions. The amplification behaviors of two directions are characterized by the transfer function of the surface and downhole measurement. The findings are as follows: (1) the amplification of vertical motion is less significant than that of horizontal motion, (2) the amplification behavior of vertical motion is less dependent on the intensity of motion (i.e., soil nonlinearity) than that of horizontal motion, and (3) the amplification behavior of vertical motion is more sensitive to the location of ground water table than that of horizontal motion. These variances should be considered in the dynamic site response analysis.

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