The quality factor of the shear waves Qs below 1 Hz in the Osaka sedimentary basin, Japan, is inferred from borehole earthquake-motion recordings and surface earthquake-motion recordings by three-component waveform fitting. Assuming quality factor of S waves Qs to be a function of the frequency f and the shear- wave velocity Vs in meters in the form AVs f n, the waveform fitting of borehole data reveals that coefficient A is approximately 0.24, and exponent n is less than or equal to zero. To verify Qs-values from borehole data and, if possible, to confine Qs-values to a narrow range, we infer the Qs-values from surface earthquake-motion recordings by fitting them with earthquake motions simulated by the finite-difference method. It indicates that Qs-values are negatively frequency dependent and that the lower limit of Qs-values is approximately 0.24Vs f−1.

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