Attenuation of shear-waves in the lithosphere for frequencies from 0.05 to 25 Hz

Physics

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Scientific paper

Q for shear-waves in the crust and upper mantle was determined as a function of frequency in the range 1-25 Hz, using band-pass filtered records of about 900 earthquakes occurring in the central Japan area with focal depths from 0 to 150 km. The data were supplied from two stations in the Kanto region, operated by the Earthquake Research Institute, University of Tokyo. The method is taken from Aki and Chouet (1975) and Rautian and Khalturin (1978), and is based on elimination of the source effect from observed spectra of shear-waves, by taking the ratio of their amplitudes to those of coda-waves (measured at a fixed lapse time). The effect of radiation patterns is removed by averaging the ratio over many events. The experimental procedure uses basically a single-station method, and its validity was confirmed by the agreement between results obtained from station Tsukuba and those from Dodaira. We found that Q shows a strong dependence on frequency: Q increases with frequency f proportionally to fn, where n was found to be 0.8 in the northeastern part of Kanto (area A), and 0.6 in the rest of the region (areas B + C). Previously, a value for n of 0.5 had been determined for the Garm area in central Asia by Rautian and Khalturin. In general, our results agree with those of Fedotov and Boldyrev (1969), who obtained values of n around 0.6 for the Kuril-Islands, using a single-station method based on assumptions more restrictive than those adopted here. Q values for the lithosphere have been estimated by various workers using surface-waves with periods greater than 15 s. If we combine the results from surface-waves with those obtained from the S- to coda-amplitude ratio, we find that the frequency-dependence of Q-1 is similar to that obtained from a simple relaxation model, with the peak somewhere around 0.5 Hz.

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