Assessing soil amplifications in Groningen, the Netherlands

Since the shallow part of the Dutch subsurface is practically always unconsolidated, the elastic waves generated by deeper (~3000 m) seated earthquakes will be subjected to transformation when arriving in these layers. Since, the number of induced seismic events has increased over recent decades, a better understanding of site response of the Dutch subsurface is required. Local site amplification can directly be measured due to the presence of sensors at multiple depth levels in the Groningen borehole network. Amplification factors from 73 local events have been calculated for each borehole location to quantify earthquake site response. Furthermore, horizontal-tovertical spectral ratios (HVSR) from the ambient seismic field are calculated. A relationship has been established between the composition of the upper Holocene sediments and the size of the amplitudes of HVSR and earthquake site response. Highest amplitudes are measured where the Holocene sediments are composed of clay, fine sands, silts and peat. We can conclude that HVSR from the ambient seismic field can be used as a first-order proxy to get an indication for wave amplification during a seismic event. This allows a first assessment on wave amplification at sites without sensors at multiple depth levels and without abundant local seismicity