Nuclear power plants built in seismic zones should be designed and constructed in such a way that they produce no radiation hazard in the event of a severe earthquake. In order to find a reasonable solution to this problem the first step that needs to be taken is to make an estimation of the maximum earthquake force to be expected at the site. This paper gives the basic Japanese views on this problem. Levels of the strongest design earthquake S1 and extreme design earthquake S2 are explained and methods for deriving S1 and S2 earthquakes from active faults are also described.