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Importance of local scattering in high frequency motion lessons from interpacific project sites, application to the kik-net database and derivation of new hard-rock GMPE

Abstract : The effective shear wave attenuation, often denoted $Q_{Seff}$, is an important parameter for ground motion simulations. However, this parameter is difficult to measure and is often inferred from rules of thumb (e.g., Q$_{Seff}$=V$_S$/X$_Q$, where X$_Q$ is a scalar value, frequently chosen as equal to 10). The recent work by Laurendeau et al. (2017), aiming to derive consistent hard-rock GMPEs from KiK-net surface and down-hole recordings, shows the shortcomings of such a Q$_{Seff}$-V$_S$ scaling at high frequency using the standard scaling Q$_{Seff}$=V$_S$/10 leads to an overestimation of the local amplification, computed from 1D simulations, at high frequency. We show that this issue is likely related to a too simple description of the velocity profile using a limited number of homogeneous layers. Using invasive measurements from the InterPacific project (Garofalo et al., 2017), we show that 1D simulations based on high resolution velocity profiles (as provided by the PSSL method) lead to a significantly higher effective attenuation at high frequency in comparison with the use of lower resolution velocity profiles (as provided by the down-hole method). This observation is clearly due to the back scattering induced by non-smoothed profiles.We then work on the stiff-soil-to-rock KiK-net sites used by Laurendeau et al. (2017) and we introduce a random perturbation to the former simplified profiles provided for each KiK-net site. These perturbations are consistent from a geostatistical point of view with values found in the literature (even though they were not measured on the KiK-net sites) and on the few InterPacific sites. The 1D simulations with these modified profiles, coupled with the standard Q$_{Seff}$-V$_S$ scaling, were found to well reproduce the high frequency attenuation.A new hard-rock GMPE was then derived from the KiK-net surface recordings corrected for the theoretical site response defined from this optimized simulation, and was compared to the previous results of Laurendeau et al. (2017).
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H. Shible, A. Laurendeau, P.-Y. Bard, F. Hollender. Importance of local scattering in high frequency motion lessons from interpacific project sites, application to the kik-net database and derivation of new hard-rock GMPE. 16th european conference on earthquake engineering, Jun 2018, Thessalonique, Greece. ⟨cea-02338932⟩

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