When designing conventional building structures for seismic design, DIN 4149 [39] or DIN EN 1998 [40] gives ground response spectra as a function of rigid body

acceleration and the nature of the subsoil. The rigid body acceleration is defined based on the specific German earthquake zone map and represents the intensity at a given location at an exceedance probability of 1/475 a « 2- 10~3/a. Other European countries have their own national earthquake zone maps.

Reference earthquake standards for nuclear installations are necessarily more stringent. As opposed to DIN 4149 [39] or DIN EN 1998, KTA 2201.1 [37] requires a reference earthquake intensity for an exceedance probability of 1 — 10~5/a to be used. Establishing this calls for highly detailed studies as part of a seismological expertise.

KTA 2201.1 requires the design basis earthquake to be defined based on deterministic and probabilistic analyses. The outcome of these analyses, the requirements for which are defined in KTA 2201.1 is a ground response spectrum for both horizontal axes and one for the vertical component. These spectra are taken as free field response spectra for a reference horizon normally defined as the top of the ground.