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Abstract:
The computation of time series for hard-rock site conditions, either as input motion for site response calculations or for applications to installations constructed on this site type, is a crucial step for seismic hazard assessment. The current state-of-the-art is to apply physics-based corrections to surface motions to eliminate the influence of site effects for retrieving the underlying bedrock motion. Here we evaluate the application of the generalized inversion method (GIT) for deconvolving surface recordings to obtain hard-rock time series at the amplification-free seismic bedrock. The method includes an event-specific phase scaling approach on the surface recordings that takes changes in the signal duration between the surface and the seismic bedrock into account.
We choose a total of 90 KiK-net surface-downhole sites for validation which do not have a significant velocity contrast below the downhole sensors and with the latter being located at sufficient depth so that they are not significantly impaired by downgoing waves. By comparing the empirical predictions from surface recordings with time series that have been recorded at the downhole sensors, we assess the accuracy of the predictions. In contrast to empirical and one-dimensional modelling approaches, which significantly overestimate the level of hard-rock ground motion for frequencies greater than a few Hz, we find quite high correlations and small variations in both spectral shape and amplitude over the entire frequency range. This approach delivers a data set that facilitates the development of reference ground-motion models.
