French seismic zoning

French seismic zoning is the regulatory split of the national territory into five seismicity zones, from 1 (very low) to 5 (high), which sets the earthquake-resistant building rules that apply in each town. It comes from decrees No. 2010-1254 and 2010-1255 of October 22, 2010, in force since May 1, 2011. The previous zoning, from 1991, was deterministic. The 2010 map is probabilistic: each zone matches a reference peak ground acceleration on rock for a 475-year return period, a 10% probability of exceedance in fifty years. That acceleration runs from 0.4 m/s² in zone 1 to 3 m/s² in zone 5. Zone 5 covers only the French Antilles. In mainland France the top level is zone 4, in parts of the Pyrenees and the Alps. The town's zone and the building's importance class, from I to IV, decide whether Eurocode 8 applies and at what design acceleration. Zoning creates no cover of its own: an earthquake falls under the CatNat scheme, subject to an order, whatever the zone. A single-family house falls under it only from zone 3 up; a school, from zone 2. November 11, 2019: a magnitude 4.9 earthquake hit Le Teil, in the Ardèche, and damaged hundreds of buildings, many of them built long before any seismic code. The episode shows the limit of zoning for an underwriter. It says how buildings must be built since 2011; the existing stock was built earlier, often in unreinforced masonry. The hazard module of a cat model and the regulatory map also serve different purposes, and their values diverge locally. A book of old town centers in zone 3 can therefore weigh more than a book of recent construction in zone 4.