There is much practical need in obtaining independent information about earthquake-source dynamic properties directly from observable data. One such dynamic parameter, the peak slip velocity during earthquake rupture, can be calculated from the corner frequencies of the source spectra, on the assumption of the validity of the ω2-source model. To obtain the source terms, observed Fourier spectra should be corrected for the site and path effects. Small-to-moderate earthquakes in Japan recorded on multiple rock sites are well suited for the application of this methodology. The results indicate that the maximum slip velocity of the selected earthquakes ranged from approximately 0.2 to 0.6 m/sec. Direct observation-based determinations of this type provide valuable physical information about the in-situ faulting processes that can be used for constraining dynamics theories of faulting or in ground-motion prediction.