2007 Volume 59 Issue 4 Pages 355-370
We estimated source spectra of the 2005 Miyagi-oki earthquake (MJMA 7.2) and 11 events (4.0< MJMA<5.0) consisting of inter-plate and inland earthquakes occurred around Miyagi region applying the spectral inversion method to seismic records observed at KiK-net stations in Miyagi prefecture. Attenuation and site amplification factors were also estimated. Before applying the method, geometrical spreading factor was estimated as a function of frequency using the twofold spectral ratios. It was successfully estimated at a frequency range from 3 to 20 Hz. The average over this frequency range is represented as 1.12±0.27. Applying this average value to the geometrical spreading factors for all frequency ranges (0.5 to 20 Hz), we performed the spectral inversion for the data recorded at 12 KiK-net stations in Miyagi prefecture. As a result the attenuation factor (1/Qs) of shear waves was successfully estimated at frequency ranges above 3 Hz. The frequency dependent 1/Qs can be approximated by Qs(f)-1=(5.0×10-3)ƒ-0.8 at a frequency range of 3 to 12 Hz. We found that total attenuation effect including geometrical spreading and attenuation factor contribute little to the observed strong acceleration at high frequency range. Short-period spectral level of the 2005 Miyagi-oki earthquake estimated from the source spectrum is roughly two times as large as that expected from the empirical rule for the event with same seismic moment. These results indicate that high level of high frequency source spectrum of the 2005 Miyagi-oki earthquake contributed to the observed strong acceleration at high frequency range in Miyagi region.
