Seismic hazard

With seismic hazard we mean the expected ground shaking at a site due to an earthquake. Being mainly a probabilistic analysis mainly, a given shaking can be defined only if associated to the probability of occurrence in the near future. It is therefore not a deterministic earthquake prediction – a goal far from being achieved in the whole  world - nor of the maximum possible earthquake in the area, since the maximum earthquake has still very low probability of occurrence.

In 2004 a map of seismic hazard ( providing a framework of the most dangerous areas in Italy has been released. The seismic hazard map of the country (GdL MPS, 2004; ref. PCM Ordinance of 28 April 2006, n. 3519, Ann. 1b) is expressed in terms of horizontal acceleration of soil with a probability of exceedance of 10% in 50 years , referring to stiff soils (Vs30> 800 m / s; cat. a, paragraph 3.2.1 of the DM 14.09.2005). The PCM Ordinance No.3519/2006 has made this map an official reference tool for the country.

In 2008 the Technical Standards for Construction have been updated: for each place of the national territory the  seismic action to consider in the design is based on this estimate to an appropriately correct hazard to take account of the actual characteristics of the soil at the local level.

The colors indicate the different values of ground acceleration that have a 10% probability of being exceeded in 50 years. Indicatively the colors associated with lower accelerations indicate less dangerous areas, where the frequency of stronger earthquakes is lower than in the most dangerous ones, but that does not mean they cannot occur.

The stronger shakings, with acceleration values of the soil above 0.225 g (g = 9.81 m / s2, the acceleration of gravity), are expected in Calabria, south-east Sicily, Friuli-Venezia Giulia and along the south central Apennines. Average values are referred to the Salento Peninsula, along the Tyrrhenian coast of Tuscany and Lazio, Liguria, in much of the Po Valley and along the entire Alpine region. Sardinia is the least dangerous region with moderate expected shaking values.

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