Identification and Quantification of the Degree of Nonlinear Site Response by Using the H/V Spectral Ratio Method
Speaker: Yi-Wei Cheng
Abstract
We investigate nonlinear site response based on strong motion records from the 2003 off-Miyagi intraslab earthquake. First we identify the nonlinear site response by two methods; one is a spectral ratio of surface S-wave to borehole S-wave (surface/borehole) and the other is a spectral ratio of horizontal S-wave to vertical S-wave (S-H/V) for surface records. The records are obtained from vertical arrays which have been installed. These arrays respectively are LSST array, SMART2 array, Kobe Port Island array and Kik-net array. We confirm that the results of two methods are similar. After the S-H/V method is verified by comparing with the surface/borehole method. We extend to use the K-net surface stations to identify the nonlinear site response by using S-H/V method. Next we define a degree of nonlinear site response (DNL) based on both of the surface/borehole and S-H/V spectral ratio; the DNL is summation of differences between the ratio for strong motion and that for weak motion. We find that the DNL values increase with observed PGA (Peak Ground Acceleration). Finally we make a comparison between the predicted and observed PGAs during the 2003 off-Miyagi Earthquake as a function of DNL for S-H/V. The strong motion prediction is done by using empirical Green's function method. T he differences of predicted and observed PGA are limited when the DNL are small. However, t he predicted PGA tends to be overestimated as the DNL values become higher. We conclude that the reduced observed PGA is due to nonlinear site response during strong ground motion.
Reference
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