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What Is Probabilistic Seismic Hazard Analysis?

Probabilistic Seismic Hazard Analysis (PSHA)A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motion levels, expressing results as probability of exceeding specific shaking levels. — Probabilistic Seismic Hazard Analysis — is the methodological framework that transforms geological knowledge about earthquake sources into quantitative estimates of ground shaking probability at specific sites. Developed by C. Allin Cornell in 1968, PSHA integrates three inputs — earthquake source characterization, ground motion prediction, and site response — into a single hazard curve that expresses the annual probability of exceeding any given level of ground shaking. PSHA output drives building codes, insurance pricing, infrastructure design standards, and land-use planning across seismically active regions worldwide.

The Four Steps of PSHA

The first step identifies and characterizes all seismic sources capable of affecting the site of interest. Sources include fault linesThe trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by geologists to assess earthquake hazard for nearby communities. with known geometry and Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard., area sources representing diffuse seismicity in regions without mapped faults, and background seismicity derived from historical earthquake catalogs. Each source is assigned a magnitude-frequency distribution, typically following the Gutenberg-Richter LawA statistical law describing the relationship between earthquake frequency and magnitude: for each unit increase in magnitude, earthquakes become about 10 times less frequent. relation, or the characteristic earthquake model for well-characterized faults.

The second step uses ground motion prediction equations (GMPEs, also called attenuation relations) to predict the probability distribution of ground shaking at the site as a function of earthquake magnitude, source-to-site distance, fault mechanism, and site conditions. Modern GMPEs are derived from large empirical databases of strong motion recordings and express not just the median expected shaking but also the uncertainty (sigma) in that prediction. The large variability in observed ground motion means that sigma terms dominate hazard at very long return periods.

The third step integrates over all possible earthquakes from all sources, weighted by their probability of occurrence, to compute the annual exceedance probability of each ground shaking level. This integration is the PSHA calculation itself and is typically performed using dedicated software packages.

The fourth step expresses results as a hazard curve — annual probability of exceedance versus Peak Ground Acceleration (PGA)The maximum acceleration of the ground during an earthquake, measured in g (gravitational acceleration). A key parameter in earthquake engineering for designing structures. or spectral acceleration — or as a hazard map showing ground shaking expected at a specific exceedance probability across a geographic area.

PSHA Software Tools

Several software packages implement PSHA calculations. OpenQuake Engine, developed by the Global Earthquake Model (GEM) Foundation, is an open-source Python-based platform used by researchers and national agencies worldwide. The USGS uses Fortran-based legacy code plus newer OpenQuake-compatible tools to produce the National Seismic Hazard Maps published approximately every six years. EZ-FRISK and R-CRISIS are commercially available tools with graphical interfaces used widely in engineering practice. All implement the same fundamental PSHA mathematics with varying capabilities for complex source geometries and large-scale probabilistic calculations.

The Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk. and Design Ground Motion

The principal product of national PSHA programs is the Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk.: a gridded map showing the peak ground acceleration (or spectral acceleration) with a specified probability of exceedance in 50 years. The standard US building code design level uses 2% in 50 years (approximately a 2,475-year return period), while some codes use 10% in 50 years (475-year return period) for different performance objectives. The Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk. values feed directly into building code seismic zone designations — higher hazard zones require stronger structural systems, more detailed analysis, and more conservative design factors.

Seismic Risk AssessmentThe process of evaluating earthquake hazard, building vulnerability, and potential losses for a specific area or structure. Combines hazard maps, building inventory, and damage models. vs. PSHA

Probabilistic Seismic Hazard Analysis (PSHA)A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motion levels, expressing results as probability of exceeding specific shaking levels. produces hazard estimates (ground shaking probability); Seismic Risk AssessmentThe process of evaluating earthquake hazard, building vulnerability, and potential losses for a specific area or structure. Combines hazard maps, building inventory, and damage models. combines hazard with exposure and vulnerability to estimate probable losses. The distinction is important: a remote area may have high seismic hazard but low risk because few people or structures are exposed. A moderately hazardous urban area may have high risk because enormous concentrations of people and infrastructure are exposed to even modest shaking. Risk assessment requires Probabilistic Seismic Hazard Analysis (PSHA)A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motion levels, expressing results as probability of exceeding specific shaking levels. output as input but additionally requires building inventory databases, vulnerability functionsA mathematical function describing the probability of various damage states for a specific building type given a level of ground shaking. Essential for loss estimation models., and population data.

The Seismic Risk Checker Tool

The Seismic Risk Checker tool implements a simplified single-site PSHA calculation that allows users to estimate ground shaking probability at any location based on the USGS National Seismic Hazard Model. Entering a US zip code returns the Peak Ground Acceleration (PGA)The maximum acceleration of the ground during an earthquake, measured in g (gravitational acceleration). A key parameter in earthquake engineering for designing structures. at the 2%-in-50-year and 10%-in-50-year exceedance levels, along with the corresponding building code seismic design category. This helps property owners, architects, and engineers quickly assess the seismic design requirements applicable to a specific site without performing a full PSHA calculation.

Uncertainties in PSHA

PSHA quantifies both aleatory uncertainty (inherent randomness in earthquake occurrence and ground motion) and epistemic uncertainty (incomplete knowledge of source parameters). Epistemic uncertainty is handled through logic trees — weighted combinations of alternative source models, magnitude-frequency distributions, and GMPEs. A typical national PSHA logic tree may have thousands of branches representing different defensible scientific interpretations. The mean hazard curve averages over this epistemic uncertainty, while fractile curves (16th, 50th, 84th percentile) quantify the spread.

Site-Specific PSHA in Engineering Practice

Building codes provide general Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk. values that conservatively account for typical site conditions. For critical facilities — hospitals, nuclear power plants, dams, LNG terminals — site-specific PSHA is required. This analysis uses a detailed characterization of local seismic sources, site-specific velocity profiles for site amplification modeling, and GMPEs selected for regional appropriateness. Site-specific PSHA may produce hazard estimates substantially different (higher or lower) than the code map values, justifying custom structural designs and potentially significant cost differences.

The Seismic Hazard Model Update Cycle

National seismic hazard models are updated periodically as new data improves source characterization. The 2023 USGS National Seismic Hazard Model update incorporated GPS GeodesyThe use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter precision. Reveals how strain accumulates on faults between earthquakes. constraints on fault slip rates, expanded fault databases including many previously unmapped blind thrust faultsA thrust fault that does not reach the surface, making it invisible at ground level and harder to detect. The 1994 Northridge earthquake occurred on a blind thrust fault., updated GMPEs incorporating lessons from the 2010 Canterbury and 2011 Tohoku earthquakes, and improved Induced SeismicityEarthquakes triggered by human activities such as hydraulic fracturing (fracking), wastewater injection, mining, or reservoir impoundment. Most are small (M<4) but some have exceeded M5.5. characterization in the central and eastern United States. Each update can significantly revise hazard estimates in specific regions, driving building code revisions and insurance repricing.

Summary

Probabilistic Seismic Hazard Analysis (PSHA)A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motion levels, expressing results as probability of exceeding specific shaking levels. tools transform geological and seismological data into the probabilistic ground shaking estimates that underpin all quantitative earthquake risk management. Understanding the Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk. as a probabilistic product — not a prediction — and recognizing the role of uncertainty in Seismic Risk AssessmentThe process of evaluating earthquake hazard, building vulnerability, and potential losses for a specific area or structure. Combines hazard maps, building inventory, and damage models. allows engineers, planners, and policymakers to use these powerful tools appropriately. The Seismic Risk Checker tool provides immediate access to national PSHA results for any US location.