Abstract
Quality factor, QS, of attenuation for shear waves in the vicinity of ground surface is estimated by spectral analysis of body waves from micro-earthquakes observed at Mitsumata in Akita prefecture. The analysis is made for thirty earthquakes, in which S-P times are between 1 and 2sec.Based on some reasonable assumptions, it is found that the value of QS can be easily estimated from the average slope of spectral amplitude ratio between the direct P or S wave and the S or P wave converted at the boundary of the surface layer and the substratum. The result indicates that QS in the surface layer takes a value from 5 to 20 in the frequency range between 6 and 30Hz.
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