Using measured and modelled shear-wave velocity profiles for the assessement of site response in Groningen, the Netherlands

The site response input in the Groningen seismic hazard assessment is based on modelled shear-wave velocity (V_S) profiles. Two sets of data were used to compare in situ (field) and model data of V_S. The first set consists of data from several blocks of ~ 400 nodes. Inversion of passive seismic data from a coarse grid of ~ 6 km x 10 km resulted in V_S profiles to a depth of 800 m and from a denser grid of ~ 1 km x 1 km more detail to a depth of 100 m. The field V_S profiles were a combination these two depth ranges. The site response analysis based on either the field or model V_S profiles showed on average similar amplification factors over periods relevant for seismic risk. The model V_S profiles are therefore a good representation. The second set consists of V_S data from MASW surveys on dwelling mounds. The local detailed field V_S profiles reach a depth of 18 m. Site response analyses using the full model V_S profiles or profiles with the top 18 m replaced by field V_S showed that the amplification on dwelling mounds is underestimated significantly, on average by 7 to 28 %. Because of this, a frequency-dependent Penalty Factor has been derived. In the risk calculations, this Factor is to be applied to buildings on dwelling mounds to transform the estimated motions at the ground surface (based on model V_S) into motions at the top of the dwelling mound.