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as a measure of severity of ground motion. Peak acceleration may not be the best parameter to characterize ground motion, but it has a wide 'acceptance and familiarity in engineering as a measure of the lateral forces on high-frequency structural systems. For intermediate- and low-frequency systems, ground velocity and displacement data are more applicable. Values of peak ground acceleration, velocity, and displacement have been assembled from past earthquakes. These peak values have been used to develop relationships between distance from the source of energy release, earthquake magnitude, and MM intensity. These relationships should be used with care and experienced judgment, particularly the intensity relationships. It is conunon to assume that accelerations in the two horizontal directions are equal, while for the maxitIllm vertical a value of two-thirds or more of the maxilllllm horizontal is used. This may not always be correct and if site specific information is available, it should be used. An effective peak ground acceleration value is sometimes used if the peak ground acceleration is associated with one or more high-frequency spikes of short duration or when the total duration of the accelerogram is short. At the present time, the use of effective peak acceleration in seismic design is controversial due to the inconsistent practices used to obtain the effective peak acceleration value from an accelerogram. A similar concept holds for effective peak velocity, but it is not yet well established. 2. Duration Duration of shaking has been shown to be one of the most i~ortant parameters of ground motion causing damage. Some earthquakes have produced short, high-frequency accelerograms, bu t did not cause struc tural damage even though the peak ground accelerations were on the order of O.5g (lg = 32.2 feet/second2). Other earthquakes where damage occurred had lower peak ground accelerations but had a long duration of ground shaking. Duration plays an important role in liquefac tion potential. Bracketed duration is frequently used and is the time during which the acceleration level equals or exceeds some amplitude threshold such as O.05g. 3. Response Spectra The concept of response spec tra has evolved from atte~ts to describe the complicated nature of earthquake-induced ground motion and the complex transient response of structures. A response spectrum represents the peak response to a ground motion of an enseuble of single-degree-of-freedom systems having viscous damping. The response spectra can be graphically represented by plotting peak spectral acceleration, -14- ,