S Wave Attenuation in the Vicinity of Ground Surface

Abstract

Quality factor, Q_S, 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 Q_S 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 Q_S in the surface layer takes a value from 5 to 20 in the frequency range between 6 and 30Hz.