Crustal attenuation characteristics of S-waves beneath the Eastern Tohoku region, Japan

An inversion method was applied to crustal earthquakes dataset to find S-wave attenuation characteristics beneath the Eastern Tohoku region of Japan. Accelerograms from 85 shallow crustal earthquakes up to 25 km depth and magnitude range between 3.5 and 5.5 were analyzed to estimate the seismic quality factor Q_s. A homogeneous attenuation model Q_s for the wave propagation path was evaluated from spectral amplitudes, at 24 different frequencies between 0.5 and 20 Hz by using generalized inversion technique. To do this, non-parametric attenuation functions were calculated to observe spectral amplitude decay with hypocentral distance. Then, these functions were parameterized to estimate Q_s. It was found that in Eastern Tohoku region, the Q_s frequency dependence can be approximated with the function 33 f^1.22 within a frequency range between 0.5 and 20 Hz. However, the frequency dependence of Q_s in the frequency range between 0.5 and 6 Hz is best approximated by Q_s(f) = 36 f^0.94 showing relatively weaker frequency dependence as compared to the relation Q_s(f) = 6 f^2.09 for the frequency range between 6 and 15 Hz. These results could be used to estimate source and site parameters for seismic hazard assessment in the region.