Seismic zoning

If the periodicity of earthquakes in a particular zone is known, all human constructions must bear it in mind and opportune defences must be carried out, first of all the realization of anti-seismic constructions. Examples of anti-seismic constructions are extremely old, as may be observed, for example, in the walls built by the Incas in Cuzco, Peru. An anti-seismic construction, naturally, cannot resist against all possible earthquakes – in order to be truly safe, it is sufficient (and necessary) that it is able to resist an earthquake of the highest intensity registered in the region. An earthquake of an unusual intensity, however, although not very probable, may still occur, thus thwarting all the preventive efforts – for this reason, for those living in areas with a strong seismic activity, it is necessary to learn to live with a certain degree of risk. An example of this can be seen in a country like Japan, that is prepared and equipped to face most of the earthquakes, which however at times incur severe damages, notwithstanding the rigorous building regulations. Notwithstanding all the precautions, therefore, it is practically impossible to eliminate seismic risk. Furthermore, in many zones, including many areas in Italy, the criterion used to adapt anthropic structures to the seisms of maximum intensity ever recorded, would entail prohibitive costs for all human structures. Therefore the periodicity of the major earthquakes is taken into consideration. If this interval is very long, in other words hundreds of years, or even many decades, it might be economically more advantageous to build with less restrictive criteria, bearing well in mind however that the duration of the construction will not be able to exceed the periodicity of the more intense seism. This is particularly true for structures that are not designed to last in time, as for example a dam or a bridge, which however generally require updating after a few years and a great amount of maintenance. It is this criterion that will enable the construction of the much discussed bridge over the Strait of Messina. Simple, isn’t it? Unfortunately in this approach there is a terrible illusion, the periodicity that is calculated using statistics, is an average; on average, there are, let us say, two seisms of magnitude 7 in a century – one every 50 years. Therefore, what is the conclusion? Nothing is certain, as in the case of the numbers of State lottery game, the same number can be extracted twice in a row, and then not be extracted for several months, likewise two earthquakes of magnitude 7 may occur in two months, and then nothing else for the following 100 years. Therefore to decide the level of risk that is acceptable and the resistance criteria for the constructions in order to colonize an area calls for very delicate choices. Building with anti-seismic criteria is very expensive, and it is even more so to redesign existing structures, therefore in purely economic terms it could be more advantageous to let a scarcely probable event take place without having taken adequate measures or having taken measures that are not sufficiently efficient, and then reconstruct, according to the statistics perhaps, once every 100 years… The problem is to succeed in evaluating the probability factor correctly, and on the other hand consider the weight of the social costs : the loss of human lives, in fact, has no price, not even once every 100 years…

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