In the past decades, the necessity for detailed earthquake microzonation studies was recognized worldwide. Therefore, different approaches were established and applied. Unfortunately, the majority of these approaches are not based on pre-existing field data but require extensive seismic measurements and investigations. Furthermore, these approaches incorporate non-linearity inadequately and cannot take groundwater level changes into account. For this purpose, notably numerical models are most suitable. These models require a good knowledge of the local geological conditions (especially of the uppermost unconsolidated units), information about the geotechnical parameters of these units, and a hydrogeological model of the investigated area. Most of this information can be obtained from geotechnical investigations and surveys that have already been carried out in most densely populated areas. In a case study for Bucharest City, non-linear analyses were performed using software that is based on the visco-hypoplastic constitutive law. The results indicate that groundwater level changes have an important influence on duration and amplitude of ground response and thus should be considered for seismic microzonation studies. This approach can be used to display site effects and to identify different microzones taking different earthquake magnitudes and groundwater levels into account. Unfortunately, the majority of these approaches are not based on pre-existing field data but require extensive seismic measurements and investigations. Thus, different approaches were established and applied. Unfortunately, the majority of these approaches are not based on pre-existing field data but require extensive seismic measurements and investigations. , these approaches incorporate non-linearity inadequately and can not take groundwater level changes into account. For this purpose, notably numerical models are most suitable. These models require a good knowledge of the local geological conditions (especially the uppermost unconsolidated units), information about the geotechnical parameters of these units, and a hydrogeological model of the investigated area. Most of this information can be obtained from geotechnical investigations and surveys that have already been carried out in most densely populated areas. In a case study for Bucharest City, non- linear analyzes have performed using software that is bas ed on the visco-hypoplastic constitutive law. The results that that groundwater level changes have an important influence on duration and amplitude of ground response and thus should be be considered for seismic microzonation studies. This approach can be used to display site effects and to identify different microzones taking different earthquake magnitudes and groundwater levels into account.