Frequency-dependent attenuation of S-waves in the Kanto region, Japan

Apparent, scattering, and intrinsic S-wave attenuations (Q(S)(-1), Q(scat)(-1), and Q(int)(-1)) of the upper lithosphere in the Kanto region of Japan were measured in the 1- to 32-Hz frequency range using Multiple Lapse Time Window Analysis (MLTWA) for 115 borehole seismograms of local earthquakes. A new set of time windows for MLTWA, in which multiple isotropic scattering is assumed, was proposed and employed to estimate the frequency dependence of S-wave attenuation parameters. Scattering attenuation was found to dominate intrinsic attenuation in the S-wave attenuation mechanism at low frequencies (<2 Hz), whereas the opposite relation was observed at high frequencies. The transition is caused by the different frequency dependences of Q(scat)(-1) (alpha f(-1.5)) and Q(int)(-1)(alpha f(-0.7)) at this frequency. Interestingly, Q(scat)(-1), is almost frequency independent at frequencies >8 Hz, which implies the self-similar nature of short-wavelength heterogeneities in the upper lithosphere. In terms of the upper lithosphere of the Kanto region, these results may indicate that the random heterogeneities characterized by ;the Gaussian autocorrelation function with a fractional fluctuation epsilon approximate to 10% and a correlation length a approximate to 2 km are superimposed on the weak background self-similar heterogeneity.