Scale dependence of apparent stress for earthquakes along the subducting pacific plate in northeastern Honshu, Japan

Apparent stress, which quantifies the ratio of high-frequency spectral amplitude to the direct current level of source displacement spectra, is known to reflect the source rupture process. Whether this parameter depends on the seismic moment has not yet been well established because of the difficulties in the reliable estimation of high-frequency source spectra. To overcome this problem, we estimate S-wave source spectra by using site amplification factors and attenuation factor Q(s)(-1), which are systematically measured by the coda normalization method. Analyzing horizontal components of velocity seismograms recorded at 70-borehole seismic stations of Hi-net (National Research Institute for Earth Science and Disaster Prevention) in northeastern Honshu, Japan, we estimate site factors and Q(s)(-1) in the 0.5-32 Hz frequency band. By using these factors, we evaluate the source spectra of 225 small to moderate earthquakes that occurred in and around the subducting Pacific plate. Site-amplification factors we obtained strongly depend on frequency, and the frequency dependence clearly changes with the lithology and geologic age of rock. Obtained Q(s)(-1) values decrease with frequency in proportion to the reciprocal of frequency. The apparent stresses estimated from S-wave spectra clearly increase from 10(4) to 10(7) Pa with the seismic moment increasing, which cannot be attributed to the limited-frequency band and other artificial causes. The power of scale dependence is estimated as 0.39-0.44 for the seismic moment range from 10(11) to 10(17) N m. Some of the scale dependence of apparent stress is attributed to the scale dependence of Brune's stress drop.