Source parameters of 68 small earthquakes (0.6 < MW < 3.0) in western Nagano, Japan, are investigated to determine the scaling of static stress drop (Δσs) and apparent stress (σa) with seismic moment (M0). Data from the 800-m-deep borehole in the area provides clean recordings containing a wide range of frequencies. Source parameters are determined by manipulation of P and S spectra in the frequency domain using individually determined time window lengths for arrivals on each seismogram. Frequency-independent quality factors, Q, corner frequencies, f c and the amplitude spectra levels are estimated with the best-fitting Brune (1970) ω2 model. A frequency-dependent attenuation model, Q(f), is calculated by spectra normalization.
Static stress drop Δσs is self-similar for 1010 < M0 < 1013 N m and values range between 0.01 and 10 MPa. An F distribution test shows there is a 10% probability that σa calculated from Q has a constant mean value for 1010 < M0 < 1013 N m. Furthermore, σa calculated using Q(f) has a significant probability that there is a change in σa values with M0. Using Q(f), 0.02 < σa < 2 MPa for M0 > 1011 N m, and 0.002 < σa < 0.2 MPa for M0 < 1011 N m, a narrower range than Q analysis results.
Limits in recorded frequencies, variations in time window length, and source complexity are not found to significantly affect the calculation of σa. Therefore, the constant scaling of Δσs with M0 and the nonsimilarity and breakdown in σa scaling could be true characteristics of small earthquakes (MW <1.3).