S-Wave Spectra from Strong Motion Seismograms Source Parameters and Site Response

We analyze S-wave spectra from strong motion seismograms observed at the rock and sedimentary sites. The data are horizontal component records from earth· quakes occurring in the southern part of Hokkaido, Japan (the magnitude range is 4 to 7.8 and the depth range, 40 to 130 km). These records were obtained by wide frequency band instruments. The S-wave spectra at the rock site for moderate-size events satisfactorily fit the omega-squared model after correction of spectral decay at high frequencies due to attenuation. Source parameters (seismic moments and corner frequencies) are obtained from the spectra by an objective method. They satisfy the scaling law with an average stress drop of about 200 bars. On the other hand, the S-wave spectra at the sedimentary site have a peak in the frequency range of 0.1 to a few Hz, in contrast with those at the rock site. We attribute the peak to a site response due to the thick sedimentary layer. The site response is estimated by averaging the residuals between the logarithms of the observed spectrum and the model spectrum calculated based on the scaling law