Shear wave attenuation in deep soil deposits down to 2.3km in depth was directly and systematically measured at one of the deep-borehole observatories which were constructed for geophysical observations in the Tokyo metropolitan area. Introducing the newly designed SH-wave generator for an S-wave source, the down-hole measurements at 12 different depths were conducted down to 2.3km at intervals of 0.1-0.3km. Attenuation factors determined by the spectral ratio method were linearly inverted to QS-values under an assumption of frequency-independent QS.
The obtained QS-values are 21±7, 51±6, and 48±11 from the top to the bottom of deep soil deposits, respectively, and larger than 150 in the substratum, for the frequency range of 3.5-20Hz. The QS for about 20 of the upper-most soil layer is faily consistent with known values which were estimated in shallower wells at various places. The substratum QS-value of 150 or over is not inconsistent with the crustal QS.
The present measurement in such deep soil deposits is a new contribution to seismological and earthquake engineering studies. A significant effect of the attenuation factors upon the seismic waves is present by computing the amplification characteristics of deep soil deposits. At the present experimental site, the attenuation factor has a rather drastic effect upon the S wave in the period range shorter than 3sec.
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