The seismic zoning map for Belgium was published in the Belgian national annex (NBN EN 1998-1 ANB) to the European building code Eurocode 8 (EN 1998-1), which became effective in all European member states in 2011. This map classifies Belgian communes into five seismic zones, corresponding to different values of the reference peak ground acceleration (PGA) to be taken into account in the design of structures for earthquake resistance. In combination with the importance class of the building and the ground type, the reference PGA is used to calculate the design response spectrum defining the accelerations that the structure should be able to withstand without collapse. For further details and the precise specifications, users should consult the normative documents, which can be ordered from the Bureau for Standardisation NBN (https://www.nbn.be/en). The Eurocode-8 seismic zoning map for Belgium was established by the Royal Observatory of Belgium, based on a reimplementation of the seismic hazard map of Leynaud et al. (2000) for a return period of 475 years (equivalent to 10% probability of exceedance in a timespan of 50 years). This hazard map was calculated following the principles of probabilistic seismic hazard assessment, and based on a simple model of seismic sources and their activity in and around Belgium, and a single ground-motion model (or “attenuation law”), describing PGA in function of earthquake magnitude and epicentral distance. To date, this is the only seismic hazard map that has been published specifically for Belgium, and it is still considered as the official seismic hazard map for Belgium.

The Belgian gravimetric database consists of 69,000 measurements, made using relative spring gravimeters. This database is managed by the Royal Observatory of Belgium since 1925. Two superconducting gravimeters measure elusive gravity variations at the stations of Membach and Rochefort, with a precision better than 1E-10 of gravity at the surface of the Earth (g). Since 1996, an absolute gravimeter performs repeated gravity measurements at selected sites, with an accuracy of 1E-9 g.