Velocity seismograms recorded at nine local stations from 102 small earthquakes on the Atotsugawa fault zone, central Japan, with seismic moment from 3 × 1017 to 3 × 1021 dyne cm are analyzed to obtain the S-wave spectra. A genetic algorithm is applied to determine the earthquake source parameters and site responses simultaneously. The stability of both site responses and source parameters are experimentally tested as well as the effects of different Q models. The resulting source parameters show that the stress drops appear to vary systematically with the location of earthquakes. Events located on the Mozumi fault or the junction of the Mozumi and the Atotsugawa faults have higher stress drops than those on the creeping section and southernmost part of the Atotsugawa fault. The relationship between seismic moment and corner frequency constructed by combining our results and those of Iio (1986) for other regions in Japan shows that a single scaling relation does not apply in a moment range of 1010 to 1025 dyne cm. A steplike change in scaling relationship occurs between 5 × 1018 and 5 × 1020 dyne cm corresponding to corner frequencies between 15 and 25 Hz. This observation suggests that earthquakes are most likely a multihierarchical system divided by characteristic sizes. According to Aki's (1980) model, a characteristic source dimension (∼100–200 meters) may represent a critical size of the barriers distributed in the fault zone at which the barrier effects are balanced by the dynamic effect of the source process.