Ground Motion

Arias Intensity (Ia) is a measure of the total energy content of a ground motion, defined as the integral of the squared acceleration time history over the duration of shaking. It correlates strongly with earthquake-induced damage and is widely used in liquefaction, slope stability, and structural damage assessments.

A design spectrum is a smoothed and simplified response spectrum prescribed by building codes for use in structural design. It represents the design-level seismic demand at a site, accounting for the site's soil class, seismic hazard, and importance of the structure, and serves as the primary input for equivalent lateral force (ELF) and modal response spectrum analysis.

Duration is the length of time over which significant ground shaking occurs during an earthquake. It is a critical intensity measure because cyclic loading, energy accumulation, and fatigue-related damage all depend on how long strong motion persists, not just on peak amplitude.

A Ground Motion Prediction Equation (GMPE) is an empirical model that estimates ground motion intensity — such as PGA, PGV, or spectral acceleration — as a function of earthquake magnitude, source-to-site distance, and site conditions. GMPEs are the fundamental building blocks of probabilistic seismic hazard analysis and modern ground motion selection.

Peak Ground Acceleration (PGA) is the maximum absolute acceleration experienced by the ground surface during an earthquake, expressed as a fraction of gravitational acceleration (g). It is the most widely used intensity measure for characterizing the severity of ground shaking at a site.

Peak Ground Velocity (PGV) is the maximum absolute velocity experienced by the ground surface during an earthquake, typically expressed in cm/s. It is a key intensity measure that correlates strongly with structural damage in the intermediate-period range and is widely used in seismic hazard assessment and damage prediction.

A response spectrum is a plot of the peak response (acceleration, velocity, or displacement) of a single-degree-of-freedom system as a function of its natural period, for a given ground motion and damping ratio. It is the fundamental tool for characterizing earthquake ground motion and forms the basis of most seismic design spectra in modern building codes.

Site amplification is the phenomenon by which local soil conditions increase the amplitude of ground shaking compared to a reference rock site. Soft soils amplify low-frequency motions, while stiff soils and rock amplify high-frequency motions, and this effect is a primary reason why damage patterns vary dramatically across short distances during earthquakes.

Spectral acceleration (Sa) is the peak acceleration response of a single-degree-of-freedom oscillator with a given natural period and damping ratio when subjected to a specific ground motion. It is the primary quantity plotted in a response spectrum and the fundamental demand parameter in modern seismic design codes.

Vs30 is the time-averaged shear wave velocity of the upper 30 meters of soil or rock at a site, used as the primary parameter for site classification in modern building codes and ground motion prediction equations. It is expressed in m/s and correlates strongly with site amplification.