SITE EFFECT OF VERTICAL MOTION -- AMPLIFICATION BEHAVIOR OBSERVED FROM DOWNHOLE ARRAYS.

Strong vertical ground motions recorded during earthquakes are critical to structure designs. Similar to horizontal motions, vertical motions can be amplified by the local site condition. However, the amplification behavior of vertical motion is different from that of horizontal motion because of distinct propagation mechanisms. In this study, three component records of five downhole arrays, considering different geological conditions, ground water tables, and intensity of motions, are analyzed to evaluate the differences of wave propagation in the vertical and horizontal directions. The amplification behavior of the two directions is characterized by the transfer function of the surface and downhole measurement. It is found that the location of ground water table highly influences the amplification of vertical motion (e.g., amplitude and nonlinearity) but does not affect the amplification of horizontal motion. These variances should be considered in the dynamic site response analysis. [ABSTRACT FROM AUTHOR]