For determining an impact index which indicates the image or damage caused by an earthquake to a portfolio, an equation is stored for calculating a local earthquake intensity for a geographical location. The portfolio includes geographical locations and individual weighting factors assigned to the geographical locations. Furthermore, for the geographical locations stored are one or more impact ratio tables including impact ratios for different earthquake intensity levels. The impact index is calculated for the geographical locations by determining in each case the impact ratio for the local intensity at the respective geographical location, and adding up the impact ratios weighted in each case by the weighting factor assigned to the respective geographical location. An impact index that reflects the geographical distribution of the portfolio can be determined, without the need for a network of seismological measurement stations associated with the geographical locations.