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

Kazuo Yoshimoto,Mariko Okada

doi:10.1186/bf03352957

Abstract

AbstractApparent, scattering, and intrinsicS-wave attenuations (QS−1,Qscat−1andQint−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 ofS-wave attenuation parameters. Scattering attenuation was found to dominate intrinsic attenuation in theS-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 ofQscat−1(∝f−1.5) andQint−1(∝f−0.7) at this frequency. Interestingly,Qscat−1is 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 fluctuationε≈ 10% and a correlation lengtha≈ 2 km are superimposed on the weak background self-similar heterogeneity.

Highlights

The Kanto region of Honshu Island, which is the most densely populated metropolitan area of Japan, has high seismicity associated with the subductions of the PhilippineSea Plate and the Pacific Plate beneath the North American Plate
We propose a new set of time windows for MLTWA in which multiple isotropic scattering is assumed and employ this set to measure the frequency dependence of the attenuation parameters of S-waves in detail
We propose a stochastic structure model that characterizes the random heterogeneities in the upper lithosphere of the Kanto region

Summary

Introduction

The Kanto region of Honshu Island, which is the most densely populated metropolitan area of Japan, has high seismicity associated with the subductions of the PhilippineSea Plate and the Pacific Plate beneath the North American Plate. To improve the accuracy of strong ground motion prediction and prevent the seismic hazards that will be induced by large earthquakes, we have to obtain precise information on the underground structure and seismic wave attenuation in the crust and upper mantle (the upper lithosphere). The apparent attenuation of S-waves in the upper lithosphere has been studied in and around this region using spectral inversion techniques with strong motion data accumulated from array observations (Takemura et al, 1991; Kinoshita, 1994; Kato et al, 1998). The coda normalization method, which can be applied to single-station data from local earthquakes with widespread hypocenters, is another major technique for estimating the apparent attenuation of. Previous studies of S-wave attenuation in this region have revealed that exhibits clear frequency dependence and that it decreases with increasing frequency at frequencies of Hz, with Q−S 1 ≈ 1 × 10−2 at. Despite the fact that many reliable measurements have been performed in relation to the frequency dependence of

Results

Discussion

Conclusion