Abstract

Seismograms from small events in the southern Kanto region have been recorded by a seismometer installed in a 732-m-deep borehole at Chikura observatory, Japan, where mudstone of early Pliocene age is found from the ground surface to the bottom of the borehole. Strong phases occurring 1.5 ∼ 1.6 sec after the S-wave arrival were interpreted as reflections at the ground surface. Ray path directions of the incident S waves were determined from the minimum principal axis of the three-dimensional trajectory ellipsoid, which represents the particle motion for a head wave of S phase. Transverse horizontal SH components were used to estimate the Q−1 value for S waves. Assuming the free surface acted as an 100% reflector for the SH waves and the incident SH phase as an input and the reflected phase as an output, we calculated system functions. The Q−1 value was measured from the transfer characteristics between the incident and the reflected phases in a single seismogram, thus no corrections were necessary for source or site (including instrument) effects. If we applied a power law model, the following relationship was obtained from the regression analyses of 20 events: 
log10(Q1)=(0.52±0.48)log10(f)(1.28±0.22)(1.0<f<5.0Hz),
where f is frequency in Hz and error values are the 95% confidence levels of the regression coefficients.

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