Seismic Classification

classificazione sismicaIn order to reduce the effects of an earthquake, the State has concentrated its action on territorial classification, based on past earthquakes' intensity and frequency, and on the application of special regulations of buildings in areas classified as seismic.
Italy's anti-seismic regulations, alined with the most modern ones at international level, establishes technical rules according to which a building should bear minor earthquakes without serious damages, and major ones without collapsing, first of all safeguarding human lives.

Up to 2003 the national territory was classified in three seismic categories with different forces. Ministerial Decrees issued by the Ministry of Public Works between 1981 and 1984 had classified totally 2,965 Italian municipalities on 8,102, that correspond to the 45% of the national territory, in which the 40% of the population lives.
New criteria for seismic classification were published in 2003. They are based on recent studies and processing regarding seismic dangerousness of the territory, i.e. the analysis of the likelihood that a territory may be affected, during a given time interval - generally 50 years - by an event that exceeds a given intensity or magnitude threshold.
The Order of the President of the Council of Ministers no. 3274 of 20 March 2003 was published for this purporse on the Official Gazette no. 105 of 8 May 2003.

The legal measure contains the main principles according to which the Regions, appointed by the State to adopt the territorial seismic classificiation (Legislative Decree no 122 of 1998 and Decree of the President of the Republic no. 380 of 2001 - "Testo Unico delle Norme per l’Edilizia”), have filled out a list of municipalities with the zone each of them belongs to, with a decreasing  standard of dangerousness. The entire national territory has been classifiied according to it.

Zone 1 - It the most dangerous area, where major earthquakes may occur.
Zone 2 - Municipalities in this area may be affected by quite strong earthquakes.
Zone 3 - Municipalities in this area may be subject to modest shocks.
Zone 4 - It is the least dangerous. Municipalities of this area have a low probability of seismic damages.

De facto, there is no such thing as an “unclassified” area, that becomes zone 4 here, within which the Regions have the power of making the antiseismic planning mandatory. Moreover, each zone has a value of the seismic action useful for the above planning, expressed in terms of maximum acceleration in rock (zone 1=0,35 g, zone 2=0,25 g. zone 3=0,15 g, zone 4=0,05 g).

The carrying out of the ordinance no.3274 of 2003 allowed to reduce considerably the distance between scientific knowledge and its application in regulatory tools and to project and build safer buildings, thanks also to innovative technologies.

Novelties introduces by the ordinance have been refined further on, thanks also to the studies carried out by the competence centres (Ingv, Reluis, Eucentre). An update of the study of dangerousness at national level (Gruppo di Lavoro, 2004), provided for by the Opcm 3274/03, was adopted with the Ordinance of President of the Council of Ministers no. 3519 of 28 April 2006.
The new study, attached to the Opcm no. 3519, supplied the Regions with an updated tool for territorial classification, introducing intervals of acceleration (ag), with a probability of exceeding the threshold equal to 10% in 50 years, to be assigned to the 4 seismic areas.

Division of the seismic areas according to the acceleration of peak on rigid ground (OPCM 3519/06)

Seismic zone Acceleration with probability of exceeding equal to 10% in 50 years (ag)
1 ag >0,25
2 0,15 <ag≤ 0,25
3 0,05 <ag≤ 0,15
4 ag ≤ 0,05

Based on addresses and criteria established at national level, some Regions have classified the territory in four zones, as described in the table, and some other have classified it by adopting three zones, and introducing, in some cases, subzones to better adapt regulations to seismicity features.
Details and meanings of zonation according to each Region are contained in the regulatory regional dispositions (ita) (15 Kb).
Regardless of the regional choice, each zone or subzone has a core dangerousness value, expressed in terms of maximum acceleration on rigid ground (ag). This value does not influence planning.

Current Technical Regulations for Buildings (Ministerial Decree of 14 January 2008), in fact, have indeed modified the role that seismic classification had for planning purposes: for each zone – and thus municipal territory – a value of peak acceleration, and consequently a spectrum of elastic response, was previously supplied to calculate seismic actions.
As of 1 July 2009, 2008 Technical Regulations for Buildings came into force: each building has its own acceleration, according to geographical coordinates of the project area and to the nominal design life of a building: the degree of core dangerousness, then, can be defined for each point of the national territory, within an area of 5 sq. metres, regardless of local administrative borders.
Seismic classification (which seismic zone a municipality belongs to) is thus useful only for planning management and territorial control by relevant boards (Region, Genio, etc.).