Description of the seismic risk

L'Aquila - 6 aprile 2009 - Crollo della PrefetturaSeismicity indicates the frequency and force of earthquakes and represents a physical characteristic of an area. If we know the frequency and the energy of the earthquakes that characterise a certain area and we attribute a value to the probability of a seismic event of a given magnitude occurring in a certain interval of time, we can calculate the seismic hazard. The greater the seismic hazard, the more probability there is of an earthquake occurring of great magnitude in the same interval of time.

The consequences of an earthquake also depend on the resistance of buildings to the effects of a seismic tremor. A building’s potential for damage is called vulnerability. The more vulnerable a building is (due to its type, inadequate design, poor quality materials and construction methods, lack of maintenance), the greater the consequences will be.
Finally, the number of assets exposed to risk, the possibility in other words of damage in economic terms, to cultural heritage or the loss of human lives, is called exposure.

Seismic risk, determined by the combination of hazard, vulnerability and exposure, is the measurement of the damage expected in a given interval of time, based on the type of seismicity, the resistance of buildings and anthropisation (nature, quality and quantity of assets exposed).

Italy has a medium-high seismic hazard (due to the frequency and intensity of phenomena), very high vulnerability (due to the fragility of building, infrastructural, industrial, production and service assets) and an extremely high exposure (due to population density and its historical, artistic and monumental heritage that is one of its kind in the world). Our peninsula therefore has a high seismic risk, in terms of victims, damage to buildings and direct and indirect costs expected after an earthquake.

Mappa della pericolosità sismicaAn area’s seismic hazard is given by the frequency and force of its earthquakes, in other words by its seismicity. It is defined as the probability in a given area and in a certain interval of time of an earthquake occurring that exceeds a certain threshold of intensity, magnitude or peak ground acceleration (PGA).

In Italy we have numerous studies and documents regarding the seismicity of our peninsula, representing a historic heritage that is without equal worldwide. The first considerations, often imaginary, about the origin of earthquakes and the seismic characteristics of Italy can be traced right back to works in the fifteenth century. But it was only in the nineteenth century, with the development of seismological sciences, that research into the causes and geographic distribution of earthquakes started to be published. Wider use of seismic instruments from the end of the nineteenth century and monitoring networks in the twentieth century finally provided input for studies into seismic characterisation in Italy.

Seismic hazard studies have been used, above all in recent years, to analyse local and regional areas with a view to zonation (basic hazard information for seismic classification) or microzonation (local hazard information). In the latter case, hazard assessment means identifying areas on a municipal scale that, in the event of a seismic tremor, may be subject to amplification phenomena and provide data useful for urban planning.

Hazard studies can also be used in site analysis, to locate critical buildings from a point of view of safety, risk or strategic importance (power stations, military installations, hospitals). Hazard assessment in this case means calculating the probability of an earthquake of a magnitude (or PGA) that exceeds the threshold value established by political/decisional bodies, leading to the choice of different areas if necessary.

Hazard assessment may be deterministic or probabilistic. The deterministic method is based on the study of damage observed during seismic events in the past at a given site, reconstructing the damage scenarios to determine the frequency of repetition of tremors of the same intensity. However, because this approach requires complete information to be available regarding local seismicity and its effects, analysis generally prefers a probabilistic approach. This expresses hazard as the probability of an event with certain characteristics occurring in a given interval of time. The most frequently used probabilistic method is the Cornell method, which entails identification in the area of the zones responsible for the seismic events (genetic seismic zones), quantification of their level of seismic activity and calculation of the effects caused by these zones on the area in relation to its distance from the epicentre.