Probabilistic Seismic Hazard Analysis (PSHA)
Also known as: PSHA, seismic hazard analysis, probabilistic earthquake hazard assessment, Cornell-McGuire method
Probabilistic Seismic Hazard Analysis (PSHA) is a quantitative engineering framework used in civil and geotechnical engineering to estimate the likelihood that ground shaking will exceed a specified intensity level at a site within a given time window. By combining earthquake source geometry, recurrence statistics, and ground-motion attenuation models, PSHA produces hazard curves and maps that inform seismic design codes, infrastructure planning, and risk management decisions.
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When to use it
PSHA is the standard method for seismic design ground-motion estimation whenever a probabilistic (rather than scenario-based) approach is required. It is appropriate for projects where the site is exposed to multiple seismic sources with uncertain characteristics, and where a specified annual exceedance probability must be linked to a design ground motion — such as for buildings, bridges, dams, nuclear facilities, and lifeline infrastructure. It should not be used when only a single well-characterised fault dominates hazard and a deterministic scenario is mandated by the client or regulator, in which case Deterministic Seismic Hazard Analysis (DSHA) is applied instead.
Strengths & limitations
- Rigorously combines multiple earthquake sources, magnitude ranges, and ground-motion uncertainties into a single, consistent probability statement.
- Output hazard curves map directly onto building-code return periods, enabling rational seismic design.
- Uncertainty decomposition (aleatory vs. epistemic) is explicit, typically handled through logic trees for epistemic uncertainty.
- Hazard disaggregation identifies the dominant earthquake scenarios, which guides selection of representative ground-motion records for dynamic analysis.
- Internationally standardised and accepted by regulatory bodies for critical infrastructure including nuclear power plants and large dams.
- Scalable from regional probabilistic hazard maps to site-specific studies with high spatial resolution.
- Strongly dependent on the completeness and accuracy of the earthquake catalog and fault inventory; sparse or short records increase epistemic uncertainty substantially.
- GMPEs are typically calibrated for active tectonic regions and may perform poorly in stable continental or subduction settings without region-specific models.
- The Poissonian (memoryless) earthquake occurrence assumption, standard in most PSHA, ignores time-dependent rupture probability and post-mainshock hazard changes.
- Results can be sensitive to the choice of maximum magnitude and the upper tail of the GMPE distribution, which are difficult to constrain observationally.
- Communicating probabilistic outputs (return periods, exceedance probabilities) to non-specialist decision-makers can be challenging.
Frequently asked
What is the difference between PSHA and DSHA?
Deterministic Seismic Hazard Analysis (DSHA) selects a single controlling earthquake scenario — typically the maximum magnitude on the closest active fault — and computes the resulting ground motion deterministically. PSHA instead integrates all possible earthquake scenarios weighted by their probability of occurrence, producing a hazard curve rather than a single number. PSHA is preferred when the risk must be expressed as an annual exceedance probability (as required by most building codes), while DSHA is sometimes used for facilities where a specific credible worst-case scenario is needed.
What does a 475-year return period mean?
A 475-year return period corresponds to a 10% probability of exceedance in 50 years, assuming Poissonian earthquake occurrence. It does not mean an earthquake will happen every 475 years; rather, in any given year, there is approximately a 1-in-475 chance that the specified ground-motion intensity will be exceeded. This return period is the standard used in many building codes for ordinary structures.
What is hazard disaggregation and why does it matter?
Hazard disaggregation decomposes the total hazard at a given exceedance probability into the relative contributions of different magnitude-distance-epsilon combinations. It answers the question: which earthquake scenario is most responsible for the hazard at my target return period? Disaggregation results guide selection of representative ground-motion records for structural analysis and help engineers understand what kind of earthquake drives the design.
How are uncertainties handled in PSHA?
PSHA distinguishes two types of uncertainty. Aleatory variability is the natural, irreducible randomness in earthquake occurrence and ground-motion amplitude; it is captured within the GMPE's sigma term and integrated directly into the hazard calculation. Epistemic uncertainty — arising from incomplete knowledge of source geometry, recurrence rates, and the best-fitting GMPE — is handled through logic trees, where alternative models are assigned weights and the resulting family of hazard curves quantifies the range of epistemic uncertainty.
Sources
- Cornell, C. A. (1968). Engineering seismic risk analysis. Bulletin of the Seismological Society of America, 58(5), 1583–1606. link ↗
- Kramer, S. L. (1996). Geotechnical Earthquake Engineering. Prentice Hall. ISBN: 978-0133749434
How to cite this page
ScholarGate. (2026, June 3). Probabilistic Seismic Hazard Analysis (PSHA). ScholarGate. https://scholargate.app/en/civil-engineering/probabilistic-seismic-hazard-analysis
Which method?
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