Source Parameter Scaling for Small Earthquakes Observed at the Western Nagano 800-m-Deep Borehole, Central Japan

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 ((delta)(sigma)s) and apparent stress ((sigma)a) 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. Frequencyindependent quality factors, Q, corner frequencies, f c and the amplitude spectra levels are estimated with the best-fitting Brune (1970) (omega)^2 model. A frequency-dependent attenuation model, Q(f), is calculated by spectra normalization. Static stress drop (delta) (sigma)s 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 (sigma)a calculated from Q has a constant mean value for 1010 < M0 < 1013 N m. Furthermore, (sigma)a calculated using Q(f) has a significant probability that there is a change in (sigma)a values with M0. Using Q(f), 0.02 < (sigma)a < 2 MPa for M0 > 1011 N m, and 0.002 < (sigma)a < 0.2 MPa for M0 < 1011 N m, a narrower range than Q analysis results.