Peak Ground Acceleration: Quantifying Seismic Shaking Demands
Peak ground acceleration (PGA) represents the largest instantaneous acceleration experienced by the ground during an earthquake. It is typically reported in metres per second squared (m/s²), centimetres per second squared (gal), or as a fraction of standard gravity (g). Because PGA captures short-duration impulses that govern inertial forces, it is fundamental to structural design spectra, seismic hazard maps, and performance-based engineering.
While earthquake magnitude describes the total energy release, PGA describes the local intensity of shaking at a site. Combining magnitude metrics with PGA estimates enables emergency managers to prioritise inspections, while insurers tie coverage triggers to PGA exceedance probabilities.
Definition and Measurement Techniques
Strong-motion accelerographs
PGA values come from accelerometers anchored to the ground or structures. These instruments sample acceleration at 100–500 Hz, filter noise, and automatically transmit peaks to seismic networks. Agencies often express PGA in gal, linking directly to the CGS acceleration unit.
Processing and filtering
Raw accelerograms undergo baseline correction, high-pass filtering, and instrument-response removal before PGA extraction. Engineers also compute spectral acceleration (Sa) at multiple periods to develop response spectra, but PGA remains the zero-period anchor.
Relationship to g-levels
Converting PGA to g-levels (PGA/g) allows comparison with equipment qualification standards. For example, 0.35 g shaking corresponds to 343 cm/s², offering an intuitive benchmark for nonstructural bracing decisions and linking to the g-force article.
Historical Development
Early strong-motion networks
The first analog strong-motion instruments installed in California during the 1930s captured PGA data that revolutionized seismic design. Landmark events such as the 1940 El Centro earthquake provided the basis for modern response-spectrum codes.
Probabilistic seismic hazard analysis (PSHA)
Beginning in the 1970s, PSHA frameworks modelled the annual frequency of PGA exceedance by integrating fault recurrence, ground-motion prediction equations, and site amplification. These outputs feed building codes such as ASCE 7 and Eurocode 8.
Real-time applications
Today, dense sensor networks and GNSS instruments stream PGA values within seconds. Early-warning systems use these measurements to issue automated alerts, shut down trains, or isolate industrial processes before the strongest shaking arrives.
Conceptual Considerations
Site amplification and soil conditions
PGA varies with local geology: soft sediments can amplify accelerations, while hard rock sites exhibit lower PGA but higher frequency content. Site-class factors embedded in building codes adjust design PGAs accordingly.
Duration versus peak values
Short, high PGA spikes may cause limited damage if duration is brief, whereas moderate PGA sustained over time can produce severe cumulative effects. Engineers therefore pair PGA with Arias intensity, cumulative absolute velocity, or response spectra for comprehensive assessments.
Link to magnitude and distance
Ground-motion prediction equations relate PGA to magnitude, distance, and fault mechanism. Comparing these models with observed PGA data from the moment magnitude article helps refine regional hazard curves.
Applications
Building code design
Structural engineers use mapped PGA values (or derived short-period spectral accelerations) to size lateral-force-resisting systems. Seismic base isolation, dampers, and anchorage systems are all calibrated to expected PGA demand levels.
Infrastructure and lifelines
Pipelines, bridges, dams, and substations rely on PGA-based fragility curves to predict damage states and prioritize retrofits. Utilities pair PGA thresholds with automatic shutoff systems to mitigate cascading failures.
Insurance and risk transfer
Parametric insurance products often trigger payouts when recorded PGA exceeds predefined levels at specified stations. Risk models convert PGA exceedance probabilities into annualized loss estimates used by reinsurers and catastrophe-bond investors.
Importance and Communication
Public preparedness
Communicating expected PGA levels helps residents anchor heavy furniture, secure utilities, and plan for emergency power using tools like the generator runtime calculator.
Policy and finance
Municipalities use PGA-based hazard maps to justify retrofitting programs, resilience bonds, and ordinance-or-law coverage requirements. Linking PGA forecasts to the coverage-gap analyzer clarifies the financial exposure of outdated buildings.
By translating raw accelerograms into intuitive g-levels and design spectra, peak ground acceleration remains a cornerstone metric for seismic safety, insurance, and early warning.