A building's seismic vulnerability is its predisposition to suffer damage or a collapse when a seismic event occurs. The higher the vulnerability, the more damage there will be. This depends on:

- Typology, including height;

- Design;

- Quality of materials, degradation, or cracks;

- Method of construction, such as lack of attention to details, to mutual connections of components, to hammering, etc;

- Maintenance of the structure and deterioration over time.

Vulnerability can be identified by simplifying the seismic-resistant structure. This is done with a calculation model that performs specialized analyses, providing, in probabilistic terms, possible damage according to the intensity of the earthquake. The results of these analyses depend on the position and the seismic area of the building, as well as on the materials used and their quality, the construction characteristics, and the state of maintenance, as mentioned above.

Vulnerability assessments are particularly important in Italy, where most of the heritage buildings have not been fitted with anti-seismic measures. We know this through the seismic vulnerability index or seismic risk indicator (IRS / IS-V). This is the relationship between a building's resistance capacity according to the Technical Standard, the maximum ground accelerations (PGA - Peak ground acceleration), and the demand required by an earthquake: IS-V = PGAC / PGAD. If the outcome of the verification meets the minimum requirements, as defined in the Technical Standards for the site where the building is located, then the indicator is greater than or equal to 1. If there is a negative result, the value will be less than 1. As part of its security assessments, the IRS gives us an idea of ​​the limits of a building but is not exhaustive. There may be, for example, vulnerabilities in secondary elements which also need to be verified.