INDIETRO

The scientific community has recognized the area of eastern and south-eastern Sicily as having one of the highest probability of seismic danger the world, over and as being one in which the vulnerability of a relevant number of building structures (residential, production or service areas) and infrastructures (road, railway and technological systems) remarkably exposes the population and the entire economic system to the threat of great damage, even in case of relatively low intensity events, therefore producing an extremely high seismic risk.

In this zone of Sicily, on the other hand, the risk is even higher due to the disorderly conurbation grown around the "pole" of Catania, which covers three groups of different municipalities, many of which are also subject to the effects of volcanic seismic activity, characterized by the recurrence of events which, although not very high, are very dangerous due to the surface location of the epicenters.

In a risk area such as the Sicilian one, where the intervals between events is still unforeesable, and in any case rather protracted, the only feasable approach to all prevention actions must take into account the needs of a culture that must live together with seismic risk at a humanly and economically acceptable cost.

Seismic risk can be defined as being the product of the location’s seismic danger multiplied by the vulnerability of building structures (be they single buildings or building networks), and by the damages caused to things and persons. If one obtains an approximation to zero value in one of the three factors it follows that also the risk element will be close to zero.

It is certainly impossible to intervene on the dangerousness of the location, and the realization of systematic reduction interventions of the vulnerability degrees of building structures is extremely costly and time consuming. It follows that a useful option could focus on the reduction of eventual damages to persons and things through interventions on the overall dangerousness produced by the single urban systems.

The city complex is in fact the place in which risk reaches the highest level in certain hours of the day, especially when some of its most vulnerable parts coincide with areas of great concentration and of load transportation. The most vulnerable road axes thus become those where there is high concentration of bridges, viaducts, tunnels, and where, therefore, higher is the users’ risk in case of botthenecks or slowing down of car traffic. Furthermore, one should consider vulnerable also those urban tracts in which even pedestrian use is jeopardized by the building vulnerability of wall curtains or jutting out bodies.

The systematic individuation of such parts of the urban structure is therefore necessary in order to introduce into the relative systems the opportune corrections, and to obtain a model of behaviour of the same structure, at least insofar as non necessarily catastrophic events are concerned. It is not possible, in other words, to defend oneself against the Big One.

Even those buildings realized today with the most accurate and expensive anti-seismic criteria in fifty or one hundred years would be subject to the degradation of their materials, and therefore would not be able to resist to catastrophic events. Time could actually turn out to be the worst enemy, especially where a fading of collective memory would cause to overlook ordinary upkeep and functional verification measures. Time, on the other hand, can also become a worthy ally if it is used to organize a good knowledge of the phenomenon and the consequent adequate countermeasures, in order at least to be able to avoid damages when the level of seismic intensity would not justify them.

The analysis here proposed aims at individuating the vulnerability degrees of Catania and particularly its main urban structures, within the framework of south eastern Sicily’s seismic basin (the Hiblean-Maltese fault and Mount Etna vulcanic seismism). The study would entail a systematic definition of the potential seismic vulnerability of each bulding structure in relation to its function (use, functions, users’ density); of the overall seismic vulnerability of all road infrastructures (functions, urban or extra-urban character, technical characteristics, bridges, viaducts, tunnels) in view of the possibility of various intensity earthquakes. On the basis of data interrelations, the study also permits the construction of sceneries of the system’s behaviour relative to various intensisty earthquakes.

In this context, the usefulness of this study becomes self-evident, given that it allows for the formulation of correct measures of anti-seismic improvements or adjustments on the most vulnerable individual buildings, while providing, at the same time, a reliable overall picture of the system’s resistance potential, especially in view of civil protection interventions.

Such informations in fact makes it possible to infer and formulate what the priorities of public and private intervention should be in relation to available resources, and to verify- via simulation- the effectiveness of such interventions. Furthermore, from such information it is also possible to verify a priori the potential incompatibility deriving from the introduction into the system of new building outfits, thus generating a model of preventive impact evaluation for the deduction of environmental risks.

The anti-seismic improvement of an urban structure can be achieved:

- by means of a systematic combination of interventions aimed at easing the flow of urban traffic and therefore at the reduction of the load to wich each road-infrastructure may be subject, expecially in areas of degraded and vulnerable urbanizations;

- by means of the progressive improvement and seismic adjustment of all building outfits - be they public or private, residential or productive- beginning from the most degraded;

- removal from the most densely utilized zones of the deposits and liquid or gas fuels distribution centers and of all production centers of toxic , explosive, burning materials;

-through the individuation, in the planning stage, of free and easily accessible shelter areas, located in secure spaces, which in normal times can be utilized as parking areas or public parks;

Urban structures improvements must be based on the interrelation of three scales of intervention, which will provide : - a general frame-work of the necessary mesures; - a set of "objective" intervention priorities;- a cost plan;- a set of choices in function of the economic resources.

. The operating procedures are :

A. Construction of seismic risk indicators for urban structures on the basis of data collected through the seismic vulnerability survey cards.

B. Construction of an evaluation system on the basis of the planned anti-seismic objectives.

C. Construction of a priority intervention scheme on the basis of the utilization of concordance analysis applied on two different levels: - macro-level analysis, in which preference relations are examined in view of the data collected for each road axis; -micro-level analysis, from which intervention priorities are defined in relation to each road axis, so as to determine the tracts which need immediate attention.

D. Construction of seismic simulation sceneries and analysis of the crisis areas of each sub-system and of the territorial system as a whole.

These procedures offer the decision maker a simple understanding of the intervention strategies necessary for anti-seismic planning.

So far, seismic risk evaluation and eventual alleviation measures have been generally taken into consideration in occasion of catastrophic events, and therefore in reference to the capacity of single buildings to respond adequately to the soil’s dynamic pressures. This approach has therefore avoided to take into account also the damaging effects produced by the urban system as a whole (and not just by a single building) in occasion of seismic events of medium or low level intensity.

The Italian seismic context, in particular, is rather different when compared to other parts of the world or even the Mediterranean Basin, inasmuch as here it often happens that the degradation of building materials, the modifications of the original use (with the consequent variations of urban loads), the periodical occurrence of seismic events constitute multiplying risk elements for the population, even in case of events of negligible relevance. The principal element of advancement within this project resides therefore in its plan to verify the city’s composite response capacity to the soil’s dynamic actions, in order to eliminate unneeded risks and suffering to the concerned populations.

In this context, general improvement interventions cannot be considered advantageous in the short term, given the costs and the amount of time which would be necessary for their realization. It is therefore preferable, while waiting for the realization of the aforementioned improvements, to think in terms of urban plans and managements which aim at the realization of risk mitigation.

Finally, one should not overlook the fact that the very characteristics of long and unforeseeable intervals between seismic events constitute an obstacle even to the efficiency of extremely costly anti-seismic adjustment measures, where the degradation of the building materials and all other afore mentioned components might cause - i.e. in the space of fifty years- those interventions to be obsolete.

The urbanistic discipline can give a relevant contribution to the solutions of problems related to the defense of populations living in urban realities subject to major seismic dangerousness, thanks to its particular approach to the study of the relationship between one hand building structures aimed at the realization of economic, consumption and exchange activities; and on the other infrastructures used for the transportation of materials, energy and information.

Anti-seismic adjustments or improvements regarding the buildings structures of an entire city, although desirable, cannot in fact constitute simple or feasable solutions, because of their extremely high costs, their long period of realization and their objective difficulties.

At the same time, the hypotheses of solutions proposed by scientific research on seismic prevision are also not feasible. This is so not because of the actual level of scientific knowledge, but because of logistic and operational difficulties. A seismic pre-alarm would have no positive effect, in fact, in the absence of shelter areas and of adequate infrastructures, as well in the absence of certainty as to the duration of danger or the end of the alarm.

For those reasons, top preference must be given to the study of the fabric of relations between building structures and urban infrastructures, and the consequent drawing up of risk maps on a territorial and urban scale, which facilitate the definition of possible intervention priorities, toward a more rational approach to the problem. Having determined which are the parts subject to the highest risk in a city, it will then be possible to deduct various indications of actions, depending on the priorities deriving from the economic resources, and on the willingness to face up to the time and cost necessary to each possible solution.

In some cases, a different planning of the mobility system could constitute a first acceptable -although partial-result, one which would permit concrete improvements in the flow of the vehicular inner and outer traffic, a decisively useful step toward a consistent risk mitigation.

It’s obvious, however, that the intervention examples so far mentioned are not at all mutually exclusive: in fact they can be usefully applied within the context of each urban territorial system, according to opportune priorities, resource availability and political and managerial capacities.

The project certainly is of interest to the Mediterranean Basin and it can contribute to the improvement of Community politic. Other countries, besides Italy, are affected by seismic phenomena and the problems related to seismic risk (Greece, Portugal, Egypt and others).

Beyond all problems related to geo-tectonics, the project introduces in the context of planning organization of urban structures the concept of the environmental risk produced by the incompatibility in the relation between building structures and infrastructures destined to various activities.

Important are also the advantages deriving from the orientation of the project in respect to a better conservation of the existing building and urban patrimony, in light of the attention wich the proposed methodologies bestow on the built parts of the cities. Further advantages on a more general environmental level to the urban structures derive from the proposed techniques of urbanistic intervention, given that the improved functioning of the urban machine implies a more rational utilization of communal spaces, less air pollution, less reason for stress and urban psychoses, etc.

Finally, the project proposes an organization of urban structures and therefore of their users relevant in terms of prevention of seismic events. Such an approach would imply that, should scientists ever develop efficient prevision criteria, the cities would already be predisposed to hypotheses of seismic pre-alarm, thanks to the creation of shelter areas in all neighborhoods.

 INDIETRO