We construct an earthquake source model which provides a complete description of acceleration power spectra of direct body waves. The model is a specific form of the barrier model proposed by Aki et al. (1977). According to this specific barrier model, the fault surface is visualized as composed of an aggregate of circular cracks which represent areas of localized slip, and the strong motion is assumed to be generated by the stationary occurrence of these localized ruptures as the rupture front propagates. The acceleration power spectra of direct S waves are described by their peak value P0 and an effective bandwidth Δf. P0 scales proportionally to the width of the causative fault and to the square of local stress drop which is proportional to the ratio of maximum slip over the barrier interval. Δf is specified by the corner frequency and a cutoff frequency which presumably originates from fault nonelasticity and is considered to be inversely proportional to the cohesive zone size. From these spectra and an estimate of the duration of faulting, measures of strong motion intensity such as rms and maximum acceleration can be inferred. Results of the application of the model to strong motion observations of various events are reported in Part II.