ABSTRACT
When transmitted through a structure, ground acceleration, velocity, and displacements (referred to as ground motion) are in most cases amplified. The amplified motion can produce forces and displacements that may exceed those the structure can sustain. Many factors influence ground motion and its amplification. An understanding of how these factors influence the response of structures and equipment is essential for a safe and economical design. Earthquake ground motion is usually measured by a strongmotion accelerograph that records the acceleration of the ground at a particular location. Record accelerograms, after they are corrected for instrument errors and adjusted for baseline, are integrated to obtain velocity and displacement-time histories. The maximum values of the ground motion (peak ground acceleration, peak ground velocity, and peak ground displacement) are of interest for seismic analysis and design. These parameters, however, do not by themselves describe the intensity of shaking that structures or equipment experience. Other factors, such as the earthquake magnitude, distance from fault or epicenter, duration of strong shaking, soil condition of the site, and frequency content of the motion, also influence the response of a structure. Some of these effects, such as the amplitude of motion, duration of strong shaking, frequency content, and local soil conditions, are best represented through the response spectrum (1.1 to 1.4), which describes the maximum response of a damped, single-degree-of-freedom oscillator to various frequencies or periods. The response spectra from a number of records are often averaged and smoothed to obtain design spectra, which also represent the amplification of ground motion at various frequencies or periods of the structure. This section discusses earthquake ground motion. The influence of earthquake parameters such as earthquake magnitude, duration of strong motion, soil condition, and epicentral distance on ground motion is presented and discussed.
