Site-specific ground motion models for soil sites with thick sedimentary layers

摘要

As so far ground motion prediction equations (GMPE) for soil sites might be misleading if the selection of records is restricted to the uppermost 25 m or 30 m of the subsoil (and the average shear wave velocity v_(S,25) or v_(s,30)) and if the whole underlying geological depth profile is ignored. The concept of subsoil and geology-dependent ground classes (recently introduced into German Code DIN 4149: 2005) offers an alternative approach by the explicit consideration of the thickness of sedimentary layers and their effect on the site amplification as well as on the shape of site-specific spectra. As a whole, six respectively seven site-specific subsoil classes are distinguished. Despite the fact that alongside the river Rhine in Western and Southwestern Germany, the thickness of the sediments is reaching several hundred metres, site-specific data and corresponding attenuation relationships are missing due to the lack of earthquakes and strong-motion records. The commonly used subsoil class "soft soil" (C) is replaced and differentiated by three classes (C-R, C-T, C-S), where C-R stands for soft soil above rock (with high amplification factor in a small plateau range), C-S for layers with more than 100 m thickness, and C-T for a transition range. Ground class C-S is connected with reduced site soil amplification factors (which can also be derived from site response analysis).