Seismic hazard refers to the expected level of ground shaking at a given site within a specific time interval, caused by potential earthquakes. It represents a fundamental component in assessing seismic risk, together with the vulnerability of buildings and the exposure of people and assets.
Why is it important?
Understanding the seismic hazard of a region allows us to:
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Design new buildings safely.
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Retrofit and reinforce existing structures.
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Plan and manage emergency response more effectively.
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Educate the public on appropriate behavior during earthquakes.
How is it calculated?
Seismic hazard can be assessed using two main approaches:
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Probabilistic Seismic Hazard Analysis (PSHA): it estimates the level of ground shaking that has a specified probability of being exceeded at a given site within a certain time period, taking into account all potential earthquakes that could affect that location; depending on the time window considered, seismic hazard can be evaluated on a long-term scale (decades) or a short-term scale (days to weeks).
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Deterministic Seismic Hazard Analysis (DSHA): it evaluates the expected ground shaking at a site produced by a specific scenario earthquake, characterized by a defined magnitude and source-to-site distance.
For the seismic classification of territories and the definition of reference seismic actions in national building codes the probabilistic approach is generally adopted. Conversely, the deterministic approach—either independently or in combination with PSHA—is often used for site-specific hazard assessments, particularly in the design of strategic and critical infrastructures such as nuclear power plants and dams.
