HORIZONTAL-TO-VERTICAL SPECTRUM RATIO OF EARTHQUAKE GROUND MOTION FORSITE CHARACTERIZATION

Nakamura's method, which uses a horizontal-to-vertical Fourier spectru m ratio of microtremor, has become popular to determine the predominan t period and amplification of a site. In this study, this method is ex tended for earthquake ground motion recordings using new strong motion data recorded by JMA-87-type accelerometers. From the analysis of the se accelerograms, horizontal-to-vertical Fourier spectrum ratios of a site for different earthquakes are also found to be stable irrespectiv e of magnitude, distance and depth. To establish this fact, attenuatio n relations of velocity response spectra for horizontal and vertical c omponents are derived for three damping ratios (0, 2 and 5 per cent) u sing the JMA data Then the horizontal-to-vertical ratios of the veloci ty response spectra are obtained. The results show that the horizontal and vertical velocity response spectra are dependent on magnitude, di stance and depth, but that their ratios are almost independent of magn itude, distance and depth. However, since the current data set consist s of mostly intermediate to far field data, this observation should be limited to records of these distance ranges. Introducing station coef ficients, representing site amplification, to this relation yields the value comparable to the horizontal-to-vertical Fourier spectrum ratio at a specific site. The stability of the spectrum ratio is explained by the transfer function between the ground surface and stiff-soil out crop due to S-wave propagation. These results suggest that site amplif ication characteristics can be evaluated by one-point two-component su rface recordings of earthquake ground motion, in a similar manner as p roposed by Nakamura for microtremor. (C) 1997 by John Wiley & Sons, Lt d.