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.
Arias Intensity (Ia), introduced by the Chilean engineer Arturo Arias in 1970, is a scalar measure of the total energy per unit mass dissipated by a ground motion. It is defined as the integral of the squared ground acceleration over the duration of shaking, multiplied by a constant. Unlike peak measures such as PGA or PGV, which capture the maximum instantaneous demand, Ia captures the cumulative energy delivered by the motion β making it a better predictor of phenomena that depend on cyclic loading and energy accumulation.
Ia is particularly valuable for geotechnical earthquake engineering. In liquefaction assessment, Ia is used alongside cyclic stress ratio and soil resistance parameters to estimate pore pressure generation and liquefaction triggering. For seismic slope stability, Ia drives Newmark-type displacement analyses, where permanent deformations accumulate as a function of the energy delivered by the motion. In structural engineering, Ia is used as an intensity measure in fragility functions, especially for structures whose damage depends on cumulative inelastic deformation rather than peak force or drift.
Ia is sensitive to the duration and amplitude of the motion, which distinguishes it from peak-based measures. Two ground motions with identical PGA can have very different Ia values depending on how long the strong shaking persists. This makes Ia a natural intensity measure for long-duration events such as those from subduction zones. However, Ia is also more sensitive to record processing choices β baseline correction, filtering, and integration schemes β than peak measures. Modern ground motion databases typically include Ia values for each record, and empirical GMPEs have been developed to predict Ia as a function of magnitude, distance, and site conditions. Ia is expressed in units of m/s (meters per second) in SI, which can be confusing given that it represents an energy quantity.