Characterization and modeling of ground motion at depth in soft sedimentary rocks: Application to the Swiss Molasse Basin

Abstract

The level of earthquake motion observed on the ground surface differs from the shaking at locations buried at a depth. Characterization of the underground shaking is important for the local seismic hazard assessment at depth which is an insufficiently investigated topic so far. Advancements in the prediction of the ground motion at depth are especially important for the design of deep geological disposals of nuclear waste, buildings with deep foundations, or studies focused on soil-structure interactions. In this paper, we analyze site-specific transfer functions that relate ground motions on the surface to that at a depth at sites located within soft sedimentary rocks of the Swiss Molasse Basin. We first evaluate empirical surface-to-borehole amplification functions at six borehole sites, compare them with predictions by a theoretical model, and characterize the high-frequency surface-to-borehole amplification. Next, we perform full-waveform prediction of ground motion at depth from surface recordings of seven significant regional earthquakes with ML > 4. The comparison of predicted and observed acceleration waveforms shows a high level of similarity in a broad frequency range and well-predicted values of PGA, PGV, and response spectra. Finally, we model a scenario earthquake for 9975 years return period at a test site, and we predict associated broadband waveforms at depth. By the latter, we demonstrate that our method to predict ground motions at depth can be used as a basis for the site-specific seismic hazard of deep geological disposals of nuclear waste.