Characteristics of base-rock motions during earthquakes are studied based upon the multiple reflection theory. 37 accelerograms obtained at 10 strong-motion earthquake stations are selected. Incident waves traveling through the base-rock were calculated from them, and then undamped acceleration ratio response spectra of the incident waves. The spectra of the incident waves at five stations out of the 10 stations showed very good agreement to the spectra of earthquake ground motions recorded at outcrop of rock. The spectra have very sharp peaks in the period ranging from 0.15 to 0.25 second. The spectra of other two stations are not so much similar to the spectra of ground motions at the outcrop of rock as those of the previous stations are. The spectra of the final two stations are different from
them; the reason is not clear at this time.