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Rock and Stiff-Soil Site Amplification: Dependency on VS30 and Kappa (º0)

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A ground-motion prediction equation (GMPE) specific to rock and stiff- soil sites is derived using seismic motion recorded on high VS30 sites in Japan. This GMPE applies to events with 4:5 d Mw d 6:9 andVS30 ranging from 500 to 1500 m=s (stiff-soil to rock sites). The empirical site coefficients obtained and the comparison with the simulated site functions show that seismic motion on rock and stiff-soil sites does not depend only on VS30, but also on the high-frequency attenuation site proper- ties (º0). The effects of the site-specific º0 on site amplification are analyzed using stochastic simulations, with the need to take into account both of these parameters for rock-site adjustments. Adding the site-specific º0 into the GMPEs thus appears to be essential in future work. The rock-site stochastic ground-motion simulations show that the site-specific º0 controls the frequency corresponding to the maximum response spectral acceleration (famp1). This observation is used to link the peak of the response spectral shape to º0 in this specific Japanese dataset and then to add the effects of high- frequency attenuation into the previous GMPE from the peak ground acceleration and up to periods of 0.2 s. The inclusion of º0 allows the observed bias to be corrected for the intraevent residuals and thus reduces sigma. However, this º0 determination is üülimited to a minimum number of rock-site of less than 50 km.

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