Study on Identification of Nonlinear Site Responses of Vertical Motions Using Strong Motion Records Observed at the Ground Surface

Abstract

We show that the existence of nonlinear site responses of vertical motions is identified by velocity waveforms integrated from acceleration records observed at the ground surface using KiK-net records. In the case of the generation of nonlinear site responses of vertical motions, the velocity time history has a linear trend to the downward direction and then becomes constant or almost constant. In some cases, the velocity time history abruptly becomes constant like as a step function. The surface-to-borehole spectral ratios of vertical strong motions with such features have the peaks at the same or twice of the peak frequencies of the horizontal strong motions. The spectral ratios of vertical strong motions tend to be larger than those of weak motions in the frequency less than about 10 Hz. The peak levels of spectral ratios of horizontal strong motions at half of peak frequencies of vertical strong motions also tend to be larger than those of weak motions. Especially, the vertical motions with constant absolute values Vnon greater than 5 cm/s had relatively clear nonlinear site responses. The strong motion records with three biggest Vnon were equivalent to instrumental seismic intensity scale of 7.