Map of seismic hazard in the department of Maine-et-Loire(49)

Seismicity is not evenly distributed throughout the metropolitan area. In order to make the appropriate arrangements for the degree of exposure of each municipality, the legislator relies on a national seismic hazard mapping (seismic zoning). This mapping is proposed on the basis of a simplified classification at the level of the entire national territory (metropolicy and Antilles) and covering all the municipalities.
This zoning is applicable to buildings and structures built to accommodate populations, to perform socio-economic functions or to maintain the functioning of the State. “Normal hazard” buildings are classified by size categories whose crossing with the seismicity zone in which they are located determines the seismic requirements to be complied with (more information). The approach is different for nuclear installations. The seismic stress likely to affect them is defined on the basis of specific studies carried out for each site, in order to take account of their particular geological and seismological conditions.
On 1 May 2011, the new seismic zoning of France for conventional construction at “normal risk” entered into force, following the Order of 22 October 2010 “on the classification and rules of seismic construction applicable to buildings of the class of the normal risk” and Decrees No 2010-1254 on the prevention of seismic risk and No. 2010-1255 on the delimitation of seismic zones in the French territory.

The new mapping promulgated in 2010 (Decree of 22 October 2010 and entered into force on 1 May 2011) is based on a so-called probabilistic assessment of seismic hazard. This approach aims at estimating the vibrational seismic movement likely to be achieved or exceeded based on a fixed probability over a given period of time, e.g. 10 % over a 50-year period for conventional construction. This type of approach allows to take into account a very wide range of seismic scenarios weighted according to their probability of occurrence. IRSN participated in the development of this new map of the seismic hazard of France, as an expert for the Ministry of the Environment.

This zoning divides the French territory into 5 areas of seismicity (very small, weak, moderate, medium, strong). In zones 2 to 5, seismic construction rules apply to “normal risk” buildings and bridges. This concerns about 21 000 municipalities instead of 5,000 previously (areas Ia, II and III).

In addition, the Order of 24 January 2011 (amending the Order of 4 October 2010 on the prevention of accidental risks in installations classified for the protection of the environment subject to authorisation) lays down the seismic rules applicable to certain classified installations (excluding nuclear installations which are dealt with by a specific regulation — RFS 2001-01) on the basis of this new seismic zoning of France.