Quantification of Degree of Nonlinear Site Respons

Abstract eng:
We investigate nonlinear site response based on strong motion records form the 2003 off-Miyagi intraslab earthquake. First we identify the nonlinear site response using records obtained at KiK-net stations where vertical array has been installed. The nonlinear site response is identified 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. We confirm that two methods show similar results, so we extended nonlinear identification to records obtained at K-NET stations, where only a surface instrument has been installed, by using the S-H/V method. Next we define a degree of nonlinear site response (DNL) based on 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. The strong motion prediction is done by using empirical Green’s function method. A fairly good agreement is obtained at stations with small DNL. However, the agreement considerably decreases at stations with large DNL; the predicted PGA is much larger than the observed one. We conclude that the reduced observed PGA is due to nonlinear site response during strong ground motion.

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