Groundmotion

Ground motion refers to the movement of the Earth's surface produced by seismic activity, such as earthquakes. It includes the ground’s displacement, velocity, and acceleration at a location, as well as the frequency content of that motion. Ground motion is recorded by seismic networks through instruments like accelerographs and seismometers and is used to understand how structures respond to earthquakes.

Commonly used parameters in engineering are the peak ground acceleration (PGA), peak ground velocity (PGV), and peak ground displacement (PGD). The response of a structure to ground motion is often summarized with response spectra, which show the maximum expected response of a system as a function of its natural period. Spectral or Fourier analyses describe the distribution of energy across frequencies and help assess how different structures will be excited by a given event.

Ground motion results from interactions among source, path, and site effects. Attenuation with distance reduces recorded intensity, while path effects like crustal heterogeneity influence frequency content. Local site conditions, including soil type, stiffness, layering, and depth to bedrock, can amplify or modify motion and produce basin resonances or other amplification phenomena.

Modeling approaches include ground-motion prediction equations (GMPEs), empirical relationships that estimate expected ground-motion parameters from magnitude, distance, and site class, along with uncertainties. Stochastic models simulate broad-band motion statistically, whereas deterministic approaches consider rupture processes. Ground motion data underpin seismic hazard analyses, structural design standards, and performance-based earthquake engineering.