Description of the seismic risk
Seismicity 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.
An 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.
Seismic vulnerability is a building’s potential for a given level of damage due to a seismic event of a given intensity.
One of the main causes of death during an earthquake is building collapse. To reduce the loss of human lives, buildings must be made safe. Laws governing construction in seismic zones today state that buildings must not be damaged by low-intensity earthquakes, must not be structurally damaged by medium-intensity earthquakes and must not collapse in the event of severe earthquakes despite suffering serious damage.
A building may suffer structural damage to its load-bearing parts (pillars, beams) and/or non-structural parts that do not affect its instability (chimneys, cornices, partitions). The kind of damage depends on: the structure of the building, its age, materials, location, vicinity to other buildings and non-structural elements. When an earthquake occurs, the ground moves horizontally and/or vertically, pushing a building backwards and forwards. The building thus starts to sway and deform. If the structure is flexible and therefore able to undergo great deformation, despite suffering great damage it will not collapse. The damage also depends on the duration and intensity of the earthquake.
After an earthquake, to assess a building’s vulnerability, it is enough to inspect the damage caused, associating it with the intensity of the tremor. Whereas assessment of building vulnerability before a seismic event occurs is more complex. This is why statistical and mechanistic methods have been perfected, in conjunction with expert opinions.
Statistical methods classify buildings according to their construction materials and techniques, based on damage observed in previous earthquakes to the same kind of buildings. This technique requires damage data from past earthquakes, which is not always available, and cannot be used to assess the vulnerability of individual buildings, because it is statistical in nature and not specific.
Mechanistic methods, on the other hand, use theory models that reproduce the main characteristics of the buildings being assessed for study of the damage caused by simulated earthquakes.
Finally, some methods use expert opinions to assess the seismic behaviour and vulnerability of predefined structural types or to identify the factors that determine the behaviour of buildings and assess their influence on vulnerability.
In order to assess the vulnerability of buildings throughout Italy, statistical methods must be used that adopt standard data regarding their characteristics. ISTAT census data regarding homes are available for Italy and used in the application of statistical methods.
The first objective for a general earthquake protection programme is safeguarding human life. For this reason it is very important to assess the number of people involved, dead and/or injured.
There are various different causes for loss of human life: the collapse of buildings, bridges and other constructions and also road accidents. Then there are those linked to phenomena triggered by the earthquake, such as landslides, land liquefaction, tidal waves and fires. Various statistics obtained from major earthquakes around the world have shown that around 25% of deaths in an earthquake are due to none structural damage of buildings (falling partition walls, glass, cornices, roof tiles, etc.) and phenomena caused by the earthquake.
It can generally be estimated, with a certain margin for error and especially for more severe earthquakes, how many people were involved, using calculations based on the number of collapsed or damaged buildings. Several considerations are needed to be able to make these estimates:
• the number of people living in the buildings
• the time of the earthquake
• the possibilities of escape and/or protection
• how people were affected (dead or injured)
• the possibility of dying even after aid has been given.
It is very difficult to accurately estimate the consequences of an earthquake in terms of human lives at different times of the day and year. The number of people living in a house in fact varies from region to region, from the city to the countryside and depends on the size of families. Furthermore, in the daytime, the number of people present in a building depends on its use. For example, offices have maximum presence during the middle of the day and are virtually empty during the night. On the other hand, the number of people in a city dwelling in the evening and at night is, on average, lower than those present in a house in the countryside because cities offer more alternatives at these times, both for pleasure and work, often outside the home. Reference to the kind of buildings and relative inhabitants, however, may provide a global estimate acceptable for violent earthquakes that affect large areas.