Recent development of engineering has made it possible to plan and construct very large structures whose natural periods exist over a wide period range; from 10 seconds or longer to one second or shorter. An earthquake response analysis of structures of this kind requires an input ground motion which maintains acceptable reliability over the wide period range. However, accuracy of a long period components of several seconds or longer is doubtful of strong-motion accelerograms which have been used for earthquake response analyses of common structures.
　The report presents a method to synthesize an input ground motion which, over a wide period range, has reliability achievable under the present state of knowledge of this field. The method covers a period range from a static component to a short period component of 0.1 second or shorter. The method proposed here is as follows. The wide period range where input ground motions are synthesized is divided into four period bands; one including a static component, one including several tens seconds, one including a static component, one including several tens seconds, one including several seconds, and one shorter than one second. The imput ground motions are synthesized from four components of earthquake ground motions corresponding to these four period bands. The componennt of the period band including a static motion is theoretically calculated by a fault model and the rest three are formed respectively by modifying the amplitudes of seismograms of acturalearthquake motions.
　Synthesized ground motions for a magnitude 8.0 earthquake are also presented.