Attenuation measurements of Hard rock ground and its systematization for earthquake ground motion estimation
Attenuation characteristics of near-surface rock ground, such as low-Q, were investigated based on borehole recordings of earthquake at rock site and laboratory measurements of rocks sampled near the borehole. Spectral ratios with the frequency from 1 Hz to 20 Hz using the multiple reflected S waves between different recording levels were well explained by the bi-linear frequency-dependent attenuation model considered transition frequency of 19.5 Hz. Whereas, spectral ratio with the frequency from 18 Hz to 30 Hz using the direct upgoing S wave between GL-50 m and GL-340 m were well modeled with the frequency-independent attenuation. These results clarified that the near-surface attenuation of rock was independent of frequency over about 19.5 Hz, which was consistent with the existence of 10 Hz transition. Then we estimated the attenuation of unfractured rocks by the laboratory measurements of ultrasonic wave. From the comparison of damping factors, it was demonstrated that the attenuation from borehole recordings was low-Q rather than the one from the laboratory measurements using unfractured rock. Furthermore, the standard deviation of heterogeneity (heterogeneity index) was determined from the borehole velocity-log, and a good relationship between heterogeneity index and attenuation was obtained. It therefore was suggested that the additional attenuation effect caused from the existence of rock heterogeneity and fracture was included in the estimated attenuation by using borehole recordings. Furthermore, it was demonstrated that the additional damping factor had same effect as heterogeneity modeling on site-response transfer-function estimation. Finally, the damping factor determination flow chart for the earthquake ground motion estimation was proposed.
|硬質地盤||Hard rock ground|
|地震動評価||Earthquake ground motion estimation|